Engine Controls - Tests W/codes - 5.7L: Diagnosis Chevrolet Suburban K1500

DIAGNOSTIC PROCEDURE

Diagnosis of the computerized engine control system should be performed in the following order

1. Ensure all engine systems not related to the computer are operating properly. DO NOT proceed with testing unless all other problems have been repaired. Perform diagnostic circuit check before using trouble code charts. See «BASIC TESTING - 5.7L»(ref-20098) article.
2. If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes may cause Malfunction Indicator Light (MIL) to illuminate continuously with engine running. For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, use DIAGNOSTIC AIDS in appropriate code chart or proceed to INTERMITTENTS in «TESTS W/O CODES - GASOLINE»(ref-20057) article.
3. If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in «TESTS W/O CODES - GASOLINE»(ref-20057) article in this section. From there you will be sent to «SYSTEM/COMPONENT TESTS - 5.7L»(ref-20084) article.
4. After repairs are made, clear trouble codes and perform FIELD SERVICE MODE CHECK in «BASIC TESTING - 5.7L»(ref-20098) article.

READING TROUBLE CODES

The control module stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Read trouble codes by counting Malfunction Indicator Light (MIL) flashes or with diagnostic scan tester connected to the Data Link Connector (DLC). The tester is faster, and capable of reading information which would require testing individual control module and sensor/solenoid connector terminals with a digital voltmeter. Some models REQUIRE a scan tester to pull trouble codes. See SCAN TESTER DATA and SCAN TESTER USAGE .

Note. When using a scan tester, there is a time delay between serial data updates. For instantaneous response, a digital voltmeter must be used.

If scan tester is not available, MIL flashes can be read by grounding DLC terminal with ignition on and engine off. For example, FLASH, FLASH, pause, FLASH, longer pause, indicates Code 21. The first series of flashes are the first digit of trouble code. The second series of flashes are the second digit of trouble code. Trouble codes are displayed starting with the lowest code. Each code is displayed 3 times and will continue as long as DLC is grounded.

Note. Trouble codes will be recorded at various operating times. Some codes require sensor or switch operation for 5 seconds and others may require longer under certain conditions. Some codes may not set in a service bay operational mode.

DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION

Note. On models not using "P" series codes, Code 12 should always exist when DLC test terminal is grounded with key on and engine off, but it may not be indicated by all makes of scan tester.

CLEARING TROUBLE CODES

Turn ignition switch to ON position and ground diagnostic test terminal "B" at DLC. see scheme 1 Turn ignition switch to OFF posit ion and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead. Codes may also be cleared using the General Motors Tech 1 scan tester and is REQUIRED to clear codes on some vehicles.

Diagnostic Aids

Diagnostic aids (located in many trouble code charts) are provided as additional tips to help with diagnosis when inspected circuit is okay and an intermittent condition exists.

SPECIAL TOOLS (DIAGNOSTIC)

Note. A special scan tester, plugged into the DLC, can read trouble codes, check system voltages on the serial data line and save a great deal of time. For additional information, see tester owner's manual. Also, see SCAN TESTER USAGE and SCAN TESTER DATA .

The computerized engine control system is most easily diagnosed using a scan tester. However, other tools may aid in diagnosing problems if a scan tester is unavailable. These tools are a tachometer, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connector at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.

TROUBLE CODE CHARTS

Note. The following mini-schematics are courtesy of General Motors Corp.

DTC 13: OPEN OXYGEN SENSOR CIRCUIT

When exhaust temperature is less than 600°F (316°C), O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not allow system to enter closed loop. On some models, oxygen sensor is equipped with an internal heating unit. This allows sensor to reach operating temperature quicker and maintain closed loop operation even during extended idle. Heating element resistance should be 3.5-14 ohms at 662°F (350°C).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 13 will set at normal operating temperature if at least 2 minutes have passed since engine start, Code 21 or 22 is not present, O2 signal voltage is steady at .35-.55 volt and throttle position sensor signal is greater than idle. All conditions must be met for at least one minute.
2. This determines if fault is in O2 sensor, control module or wiring.
3. Use only a high-impedance Digital Volt-Ohmmeter (DVOM) while checking for continuity in signal and ground circuits. If ground circuit is open, voltage on signal circuit will be greater than .6 volt.

Verify a clean, tight connection for sensor ground. An open circuit at sensor signal terminal or ground terminal will result in a Code 13.

CODE 13 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 13 Schematic (5.7L) Open Oxygen Sensor Circuit. Scheme 1

Code 13 Flow Chart (5.7L) Open Oxygen Sensor Circuit. Scheme 2

DTC 14: COOLANT SENSOR SIGNAL VOLTAGE LOW

Coolant temperature sensor input is used to determining control of fuel delivery, engine timing, idle speed and converter clutch (TCC) application. As engine warms, sensor resistance reduces. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at coolant sensor signal terminal.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This tests if code was set because of a hard failure or intermittent condition.
2. This simulates conditions for a Code 15. If scan tester displays a low temperature, control module and wiring are not at fault.

After engine is started, temperature should rise steadily to about 194°F (90°C), then stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature sensor and intake air temperature sensor (if equipped) should read close to each other, when measured with a scan tester.

CODE 14 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 14 Schematic (5.7L) Coolant Sensor Signal Voltage Low. Scheme 3

Code 14 Flow Chart (5.7L) Coolant Sensor Signal Voltage Low. Scheme 4

DTC 15: COOLANT SENSOR SIGNAL VOLTAGE HIGH

As engine warms, sensor resistance reduces and voltage drops. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at control module coolant sensor signal terminal. If sensor signal circuit opens, control module will see -56°F (-49°C) and deliver fuel for this temperature.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if code was set as a result of a hard failure or intermittent condition.
2. This simulates conditions for a Code 14. If control module recognizes grounded circuit and displays a high temperature, control module and wiring are okay.
3. This determines if problem is control module or wiring. There should be 5 volts present at sensor when measured with a DVOM.

After engine starts, temperature should rise steadily to about 194°F (90°C) and stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature sensor and intake air temperature sensor (if equipped) should read close to each other when measured with a scan tester. Code 15 will also set if sensor signal or ground circuit is open.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

CODE 15 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 15 Flow Chart (5.7L) Coolant Sensor Signal Voltage High. Scheme 5

DTC 16: VSS BUFFER FAULT

Note. Test number refers to number on diagnostic chart.

1. Checks for battery voltage at VSS buffer.
2. Tests for proper ground path for VSS buffer
3. Tests for vss buffer signal to control module
4. Tests for faulty connections and a faulty VSS buffer

Check for poor connections or damaged harness.

Code 16 Schematic ("C" & "K" Series A/T) VSS Buffer Fault. Scheme 6

Code 16 Schematic ("G" Series) VSS Buffer Fault. Scheme 7

Code 16 Flow Chart (All Models) VSS Buffer Fault. Scheme 8

DTC 21: TPS SIGNAL VOLTAGE HIGH

Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from about .50 volt at idle to 4 volts at wide open throttle. Each time TPS voltage drops to less than 1.25 volts and stops, control module assumes this is zero degrees throttle angle and measures throttle percentage angle from this point.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 21 and checks if fault is a hard failure or an intermittent condition.
2. This test simulates conditions for Code 22. If control module recognizes low voltage signal and sets Code 22, control module and power and signal circuits are not at fault.
3. This step isolates a faulty sensor, control module or an open ground circuit.

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 21 will also result if ground circuit is open or TPS signal circuit is shorted to voltage.

CODE 21 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 21 Schematic (5.7L) TPS Signal Voltage High. Scheme 9

Code 21 Flow Chart (5.7L) TPS Signal Voltage High. Scheme 10

DTC 22: TPS SIGNAL VOLTAGE LOW

Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from less than about .50 volt at idle to 4 volts at wide open throttle.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 22 and tests if fault is a hard failure or an intermittent condition.
2. This simulates Code 21. If control module recognizes a high voltage signal and sets Code 21, control module and wiring are not at fault. Replace TPS.
3. This simulates a high voltage signal to check for on open TPS signal circuit.

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.0 volt. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.

CODE 22 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 22 Flow Chart (5.7L) TPS Signal Voltage Low. Scheme 11

DTC 24: VSS - "C" & "K" SERIES EXCEPT 5.7L M/T

VSS output sensor is a magnetic induction type. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning. Since vehicle speed is taken from transfer case on 4WD vehicles, output speed sensor signal on these units goes directly to control module. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at least 3000 RPM and output speed is less than 250 RPM for at least 1.5 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies VSS voltage at control module.
2. Test checks VSS circuit at buffer module.
3. Test verifies VSS signal at sensor.

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. While Code 24 is set, scan tester will display an RPM derived from input speed. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

Code 24 Flow Chart ("C" & "K" Series Except 5.7L M/T) Vehicle Speed Sensor. Scheme 12

DTC 24: VSS TRANSMISSION OUTPUT SIGNAL - A/T

Vehicle Speed Sensor (VSS) output sensor is a magnetic induction type. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning. Since vehicle speed is taken from transfer case on 4WD vehicles, output speed sensor signal on these units goes directly to control module. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at least 3000 RPM and output speed is less than 250 RPM for at least 1.5 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies voltage at buffer module.
2. Test checks VSS buffer ground circuit.
3. Test checks VSS circuit at buffer module.
4. Test verifies VSS signal at from module.

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. While Code 24 is set, scan tester will display an RPM derived from input speed. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

Code 24 Flow Chart (All Models) VSS Transmission Output Signal. Scheme 13

DTC 24: VSS - M/T

Note. The VSS buffer is an internal part of the Digital Ratio Adapter Controller (DRAC) and the terms are used interchangeably.

Control module applies and monitors 12 volts on VSS signal circuit. Circuit is connected to VSS buffer which alternately grounds this circuit when it is receiving voltage pulses from Vehicle Speed Sensor (VSS). Scan tester reading should closely match speedometer reading.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks to see if code is set as a result of a hard failure or an intermittent condition.
2. This determines if VSS buffer is receiving AC signal from VSS.
3. This test monitors VSS buffer voltage on VSS signal circuit. With wheels turning, pulsing voltage should be present. Voltage variation will be greater at a low speed to an average of 4-6 volts at about 20 MPH.

Scan tester reading should closely match speedometer reading with drive wheels turning. If vehicle is equipped with an automatic transmission, check park/neutral switch adjustment. If no problem is found while using flow chart, proceed to the INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 24 Schematic ("C" & "K" Series 5.7L M/T) Vehicle Speed Sensor. Scheme 14

Code 24 Schematic ("P" Series 5.7L M/T) Vehicle Speed Sensor. Scheme 15

Code 24 Flow Chart (All Models) Vehicle Speed Sensor. Scheme 16

DTC 28: TRANSMISSION RANGE PRESSURE SWITCH

Transmission range pressure switch assembly consists of 5 pressure switches (2 normally closed, 3 normally open), and a fluid temperature sensor combined into one unit and mounted on valve body. Control module supplies voltage to each range signal. By grounding one or more of these circuits through various combinations of pressure switches, control module detects which manual valve position has been selected. With ignition on and engine off, Park/Neutral will be indicated. Code 28 will set when range signals "A" and "C" are both zero volts (on) for 2 seconds.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks indicated range signal to valve position signal actually selected.
2. Checks for correct voltage from control module to transmission external connector.
3. Checks for short to ground in any one of 3 valve position circuits.

Code will set if control module detects one of 2 illegal combinations. Check all connections for good contact. An intermittent may be caused by a poor connection, chaffed wire insulation or a broken wire. Monitor voltage of each terminal while moving related harness connectors. If failure is induced, voltage reading will change.

VALID PSM COMBINATION (4L60-E)

VALID PSM COMBINATION (4L80-E)

Code 28 Schematic ("C", "K" & "P" Series) Transmission Range Pressure Switch. Scheme 17

Code 28 Schematic ("G" Series) Transmission Range Pressure Switch. Scheme 18

Code 28 Flow Chart (All Models) Transmission Range Pressure Switch. Scheme 19

DTC 31: ENGINE SPEED GOVERNOR - "G" SERIES

Control module controls engine speed control governor based upon engine speed (RPM). Control module sends a pulse width modulation signal to engine speed control governor module to drive engine speed control governor motor. Depending upon amount of governing required, module translates control module data into a voltage signal which it sends to motor. Motor then pulls back throttle to correct overspeed condition. A Code 31 will set if a fault occurs in governor module, motor, linkage, wiring or control module. If Code 31 is set, control module will limit engine speed by momentarily reducing fuel delivery to injectors.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Determines if engine speed governor module has proper voltage.
2. Applying voltage to motor will determine motor, wiring and linkage integrity. DO NOT hold current to motor for longer than 30 seconds or damage to motor will result.
3. This will command full governing to determine if control module or engine speed governor is at fault.

Before replacing control module, use ohmmeter to check resistance of each control module-controlled relay and solenoid. Replace any relay or solenoid where resistance is less than 20 ohms.

Code 31 Schematic (5.7L "G" Series) Engine Speed Governor. Scheme 20

Code 31 Flow Chart (5.7L "G" Series) Engine Speed Governor. Scheme 21

DTC 32: EGR CIRCUIT ERROR BACKPRESSURE EGR WITH SOLENOID

Control module controls a solenoid regulating vacuum to EGR valve. Normally closed solenoid prevents vacuum from passing until it is energized by control module. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.

Note. Test numbers refer to test numbers on diagnostic chart.

1. EGR valve should open when vacuum is applied to solenoid. Vacuum should hold.
2. When Tech 1 energizes EGR solenoid, vacuum to EGR valve should bleed through a vent in solenoid and EGR valve should close. Vacuum gauge may or may not bleed off vacuum. However, this does not indicate a problem.
3. Determines if fault lies in electrical control part of system, connector or solenoid.
4. This system uses a negative backpressure EGR valve. Valve should hold vacuum with engine off.
5. When engine is started, backpressure should cause vacuum to bleed off and valve should fully close.

Prior to replacing control module, check resistance of all control module-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.

Code 32 Schematic ("C" & "K" Series - 5.7L) EGR Circuit Error Backpressure EGR With Solenoid. Scheme 22

Code 32 Schematic ("G" Series - 5.7L) EGR Circuit Error Backpressure EGR With Solenoid. Scheme 23

Code 32 Schematic ("P" Series - 5.7L) EGR Circuit Error Backpressure EGR With Solenoid. Scheme 24

Code 32 Flow Chart (All Models) EGR Circuit Error Backpressure EGR With Solenoid. Scheme 25

DTC 32: EGR CIRCUIT ERROR WITH EVRV SOLENOID

Control module controls a solenoid that regulates vacuum to EGR valve. The normally closed solenoid prevents vacuum from passing until it is energized by control module. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.

Control module checks EGR operation when engine speed is greater than 1600 RPM, MAP sensor signal indicates cruise condition and throttle position are constant.

Note. Test numbers refer to test numbers on diagnostic chart.

1. With ignition on and engine off, solenoid should not be energized or allow vacuum to pass to EGR valve. When Tech 1 energizes solenoid, vacuum should pass through solenoid to EGR valve. Vacuum should hold.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
3. Vehicle must be driven during this test to produce sufficient load to operate EGR. Lightly accelerating (about 1/4 throttle) will produce a large and stable enough reading to determine if control module is commanding system on.

Prior to replacing control module, check resistance of all control module-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms. EVRV solenoid cannot be checked with a DVOM due to solid state circuitry.

Code 32 Schematic ("C", "G", "K" & "P" Series A/T) EGR Circuit Error With EVRV Solenoid. Scheme 26

Code 32 Schematic ("P" Series M/T) EGR Circuit Error With EVRV Solenoid. Scheme 27

Code 32 Flow Chart (All Models) EGR Circuit Error With EVRV Solenoid. Scheme 28

DTC 32: EGR CKT ERROR - "C", "G", "K" SERIES W/ LINEAR EGR

Control module regulates linear EGR valve to control exhaust gas recirculation by providing a ground control for internal pintle (solenoid). Pintle will pull away from its seat when energized. Control module controls linear EGR valve based upon coolant temperature and throttle position.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks pintle's ability to be commanded to desired position.
2. Checks for voltage to linear EGR valve to verify if problem is in ignition feed circuit.
3. Checks control module control circuit by jumpering across harness terminals with a test light and energizing EGR valve.

Before replacing control module, use an ohmmeter and check EGR valve resistances. Resistance between terminals "A" and "E" should be 7.8-8.6 ohms. Resistance between terminals "B" and "D" should be greater than 3000 ohms. Resistance between terminals "D" and "C" should start at .7 ohm and increase to about 4000 ohms as pintle is slowly moved inward.

Code 32 Schematic ("C" & "K" Series) EGR Circuit Error. Scheme 29

Code 32 Flow Chart ("C" & "K" Series) EGR Circuit Error. Scheme 30

DTC 33: MAP SENSOR SIGNAL VOLTAGE HIGH

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If MAP sensor fails, control module will substitute a fixed MAP value and use TPS input to control fuel delivery.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 33 and determines if it is a hard failure or an intermittent condition. Code 33 will set when voltage signal reading is too high and TPS voltage indicates throttle is closed.
2. This step simulates conditions for a Code 34. If control module recognizes and indicates low MAP signal, control module and 5-volt reference and MAP signal circuits are not at fault.

With ignition switch in ON position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage is high. Comparing BARO readings from a known good vehicle using same sensor is a good way to check accuracy of suspected sensor. Readings should be within .4 volt of each other. Code 33 will also result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.

CODE 33 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 33 Schematic (5.7L) MAP Sensor Signal Voltage High. Scheme 31

Code 33 Flow Chart (5.7L) MAP Sensor Signal Voltage High. Scheme 32

DTC 34: MAP SENSOR SIGNAL VOLTAGE LOW

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If MAP sensor fails, control module will substitute a fixed MAP value and use TPS input to control fuel delivery.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This confirms Code 34 and determines if code was a hard failure or an intermittent condition. Code 34 will set when ignition is on and MAP signal voltage is low. On some systems, engine must be running to set code.
2. Jumpering harness 5-volt reference circuit and MAP signal circuit terminals will determine if problem is sensor, control module or wiring. If control module recognizes and indicates high MAP signal, control module and wiring are okay.
3. Scan tester may not display 12 volts. The important thing is that control module recognizes voltage as greater than 4 volts (high MAP voltage signal), indicating control module and MAP signal circuit are not at fault.

With ignition switch in ON position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage will be high. Comparing BARO readings with a known good vehicle using the same sensor is a good way to check accuracy of suspected sensor. Readings should be within .4 volt of each other. A Code 34 will also result if 5-volt reference and MAP signal circuits are open or shorted to ground. If 5-volt reference circuit is not shorted to ground and a Code 22 is stored, check MAP signal circuit for short to ground.

CODE 34 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 34 Flow Chart (5.7L) MAP Sensor Signal Voltage Low. Scheme 33

DTC 35: IDLE SPEED ERROR

Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. IAC driver is used to extend and retract IAC valve. Movement is verified by changing engine speed. If no engine speed change occurs, valve can be retested when removed from throttle body.
2. Step checks IAC circuits. Each light on node light should flash Red and Green, while IAC valve is cycled. While color sequence is not important, if either light is off or does not flash Red and Green, check circuits beginning with poor terminal contacts.

A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. Scan counts will be greater than 60 if too low, and zero counts if too high. If idle is too high, stop engine. With ignition on, ground ALDL test terminal "B". Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is greater than 800 RPM, inspect vehicle for vacuum leaks.

DTC 36: IDLE SPEED ACTUATOR FAULT - A/T

Control module grounds solenoid circuit when coolant temperature is within range to enable solenoid, allowing vacuum to retract solenoid actuator. This lowers idle speed to controlled IAC idle specification.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks if vacuum source is present.
2. Checks to see if actuator is commanded on.
3. Checks for power, ground and proper connections at actuator solenoid.
4. Checks for open ground circuit.
5. Checks for open driver circuit or faulty control module.

If BARO is low, a 96 second reset will result. If Code 36 is set, Code 35 will usually also be set. Repair actuator circuit first. If IAC counts are zero, idle speed control actuator will disengage and raise idle speed.

Code 36 Schematic (5.7L - A/T) Idle Speed Actuator Fault. Scheme 34

Code 36 Flow Chart (5.7L - A/T) Idle Speed Actuator Fault. Scheme 35

DTC 37/38: TCC BRAKE SWITCH ON/OFF

The normally closed brake switch supplies battery voltage to control module. Signal voltage will drop to zero volts when brake pedal is applied.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks if voltage is available at brake switch.
2. Checks brake switch function.
3. Checks circuit from brake switch to control module.

If code is intermittent, check customer driving habits (driving with foot resting on brake pedal) or for unusual traffic conditions (stop and go expressway traffic).

Codes 37/38 Schematic (5.7L) TCC Brake Switch On/Off. Scheme 36

Codes 37/38 Flow Chart (5.7L) TCC Brake Switch On/Off. Scheme 37

DTC 39: TCC STUCK OFF - 4L80-E TRANSMISSION

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks mechanical status of TCC. When control module commands TCC solenoid off, TCC slip speed should increase.

If TCC is mechanically stuck on, vehicle speed is zero MPH, brakes are applied and D2 is selected, TCC fluid will mechanically apply TCC causing an engine stall.

Code 39 Schematic (5.7L W/ 4L80-E Transmission) TCC Stuck Off. Scheme 38

Code 39 Flow Chart (5.7L W/ 4L80-E Transmission) TCC Stuck Off. Scheme 39

DTC 42: IGNITION CONTROL

Code 42 indicates control module has seen an open or short to ground in Ignition Control (IC) or by-pass circuits.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
2. This tests for a normal IC ground path through ignition module. If circuit is shorted to ground, reading will be less than 500 ohms.
3. As test light voltage touches by-pass circuit, module should switch. This causes ohmmeter to "over-range" with meter in 100-200 ohm range. A higher ohm range will indicate over 5000 ohms. This test assures module switched.
4. If module did not switch, this step tests for a short in IC circuit, an open in by-pass circuit and a faulty ignition module connection or module.
5. This step confirms Code 42 is a faulty control module and not an intermittent problem in IC and by-pass circuits.

The scan tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 42 Schematic ("C", "K" & "P" Series) Ignition Control. Scheme 40

Code 42 Schematic ("G" Series) Ignition Control. Scheme 41

Code 42 Flow Chart (All Models) Ignition Control. Scheme 42

DTC 43: KNOCK SENSOR WITHOUT SPARK MODULE (DUAL SENSORS)

Note. Test numbers refer to test numbers on diagnostic chart.

1. Tests for 5-volt signal applied by control module.
2. An improperly installed sensor can prevent knock sensor from grounding to block.

Control module applies and monitors a 5-volt DC signal to knock sensors. Internal knock sensor circuitry pulls this DC signal down to about 2.5 volts. When knock sensor detects detonation, it generates an AC signal which rides back on DC signal to control module. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic (5.7L) Knock Sensor Without Spark Module (Dual Sensors). Scheme 43

Code 43 Flow Chart (5.7L) Knock Sensor Without Spark Module (Dual Sensors). Scheme 44

DTC 43: KNOCK SENSOR WITHOUT SPARK MODULE (SINGLE SENSOR)

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 43 will set when vehicle reaches normal operating temperature (but not overheating), high engine load is indicated by MAP sensor and voltage on sensor signal circuit is greater than 3.5 volts DC or less than 1.5 volts DC. This step determines if system is functioning properly at current time.
2. This step determines state of 5-volt reference signal applied to sensor.
3. Checks knock sensor internal resistance.

Control module applies and monitors a 5-volt DC signal to knock sensor. Internal knock sensor circuitry pulls this DC signal down to about 2.5 volts. When knock sensor detects detonation, it generates an AC signal which rides back on DC signal to control module. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic (5.7L) Knock Sensor Without Spark Module (Single Sensor). Scheme 45

Code 43 Flow Chart (5.7L) Knock Sensor Without Spark Module (Single Sensor). Scheme 46

DTC 43: KNOCK SENSOR WITH SPARK CONTROL MODULE

Note. Test numbers refer to test numbers on diagnostic chart.

1. If conditions for a Code 43 exist, scan tester will display YES. A knock signal should exist at idle unless an internal or system problem exists.
2. Determines if system is functioning. Usually, a knock signal can be made by tapping on exhaust manifold. If knock signal is not made, try tapping on engine block near sensor. On models with automatic transmission, it may be necessary to place gear selector lever in Drive.
3. Because Code 43 sets when signal voltage on spark retard line remains low, this test should cause signal on that line to go high. The 12-volt signal should be seen by control module as a "no knock" signal if control module and wiring are okay.
4. This test determines if knock signal is detected on sensor-to-controller line or if ESC module is at fault.
5. If sensor line is routed too close to secondary ignition wires, ESC module may see interference as a knock signal.
6. This checks ground circuit to module. An open ground will cause voltage on monitored line to remain constant at about 12 volts. This would cause Code 43 functional test to fail.
7. This should generate a knock signal to controller. This determines if ESC controller is operating correctly.

Code 43 can be caused by a faulty knock sensor connection at ESC module or engine control module. Also, check controller-to-control module signal line for an open or short to ground.

Code 43 Schematic ("C" & "K" Series) Knock Sensor With Spark Control Module. Scheme 47

Code 43 Schematic ("P" Series) Knock Sensor With Spark Control Module. Scheme 48

Code 43 Flow Chart (All Models) Knock Sensor With Spark Control Module. Scheme 49

DTC 44: LEAN EXHAUST INDICATION

Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600°F (316°C). An open sensor circuit or cold sensor causes "open loop" operation.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 44 sets when O2 sensor signal remains low for a precalibrated period and system is operating in "closed loop".

Using scan tester, observe Block Learn Memory (BLM) value at different RPMs. If Code 44 conditions exist, block learn value will be around 150-172.

DTC 45: RICH EXHAUST INDICATION

Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600°F (316°C). An open sensor circuit or cold sensor causes "open loop" operation. Code 45 indicates a rich exhaust and diagnosis should begin with: fuel pressure, leaking injector, HEI shielding, canister purge saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Tests if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is greater than 5 percent, O2 sensor signal at control module is greater than .75 volt for 60 seconds or more.

Code 45, rich exhaust, is most likely caused by one of the following

DTC 51: PROM ERROR (FAULTY OR INCORRECT PROM)

Ensure all pins are fully inserted in socket. If okay, replace PROM/MEM-CAL, clear memory and recheck. If Code 51 reappears, replace control module.

DTC 52: FAULTY CALPAK

Ensure all pins are fully inserted in socket. If okay, replace CALPAK, clear memory and recheck. If Code 51 reappears, replace control module.

DTC 52/53: SYSTEM VOLTAGE HIGH

Code 53 will set when ignition is on and control module 12-volt battery feed voltage is greater than 19.5 volts for about 2 seconds. During the time failure is present, force motor is turned off, transmission immediately shifts to 2nd gear, and TCC operation is inhibited. The setting of additional codes may result.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Normal voltage is 9-15 volts.
2. Checks if alternator is faulty under load.

Jump-starting engine or charging battery with a battery charger may set code. If code is set when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link.

Code 52/53 Schematic ("C","K" & "P" Series) System Voltage High. Scheme 50

Code 52/53 Schematic ("G" Series) System Voltage High. Scheme 51

Code 52/53 Flow Chart (All Models) System Voltage High. Scheme 52

DTC 55: ECM/PCM ERROR

Ensure control module grounds are good and MEM-CAL is properly latched. If okay, replace control module. Clear codes and confirm closed loop operation. Check operation of SERVICE ENGINE SOON light.

DTC 54: FUEL PUMP CIRCUIT

The status of fuel pump signal is monitored by control module and is used to compensate fuel delivery based on system voltage. Signal is also used to store Code 54 if fuel pump relay is defective or if relay voltage is lost after engine is running. Voltage should be present at fuel pump signal terminal of control module the first 2 seconds after ignition is turned on and anytime reference (RPM) pulses are being received by control module.

Code 54 Schematic ("C" & "K" Series) Fuel Pump Circuit. Scheme 53

Code 54 Schematic ("G" Series) Fuel Pump Circuit. Scheme 54

Code 54 Schematic ("P" Series) Fuel Pump Circuit. Scheme 55

Code 54 Flow Chart (All Models) Fuel Pump Circuit. Scheme 56

DTC 58: TRANSMISSION FLUID TEMPERATURE HIGH

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors voltage to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid warms, sensor resistance and voltage will drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Code 58 will set if signal voltage indicates a transmission fluid temperature greater than 305°F (151°C) for one second.
2. This test determines if circuit is shorted to ground, which will result in conditions for Code 58.

Check harness routing for a potential short to ground in TFT signal circuit. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, temperature display should rise steadily to about 100°C then stabilize. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 58 Schematic ("C", "K" & "P" Series) Transmission Fluid Temperature High. Scheme 57

Code 58 Schematic ("G" Series) Transmission Fluid Temperature High. Scheme 58

Code 58 Flow Chart (All Models) Transmission Fluid Temperature High. Scheme 59

DTC 59: TRANSMISSION FLUID TEMPERATURE LOW

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors 5 volts to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid temperature warms, sensor resistance and voltage drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Code 59 will set if signal voltage indicates a transmission fluid temperature less than -34°F (-37°C) for one second.
2. This test simulates Code 58. If control module recognizes low signal voltage (high temperature) and scan tester reads 305°F (151°C) or greater, control module and wiring are okay.
3. This test determines if signal circuit is open. There should be 5 volts present at sensor connector if measuring with a DVOM.

Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, displayed temperature should rise steadily to about 100°C then stabilize. A faulty connection or an open in ground circuit or signal circuit will result in a Code 59. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in firm shifts or TCC enabled complaint. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 59 Flow Chart (All Models) Transmission Fluid Temperature Low. Scheme 60

DTC 66: 3-2 CONTROL SOLENOID CKT FAULT 4L60-E TRANSMISSION

The 3-2 control solenoid hydraulically coordinates apply rate of 2-4 band with hydraulic release of 3-4 clutch during a 3-2 downshift. The 3-2 circuit duty cycle is continually monitored by control module depending one command state of circuit. When transmission is in 1st gear, duty cycle of solenoid is equal to zero. When transmission is in 2nd gear or higher, duty cycle of solenoid will be about 90 percent. When transmission downshifts 3-2, duty cycle of solenoid will be about 20 percent.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks 3-2 control solenoid and internal transmission harness for short circuits.
2. This test checks for power, from ignition through fuse, to 3-2 control solenoid.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. Some slight TCC slippage is normal. The 3-2 control solenoid feedback normally oscillates on/off when duty cycle is applied.

Code 66 Schematic (5.7L) 3-2 Control Solenoid Circuit Fault 4l60-E Transmission. Scheme 61

Code 66 Flow Chart (5.7L) 3-2 Control Solenoid Circuit Fault 4l60-E Transmission. Scheme 62

DTC 67: TCC SOLENOID CIRCUIT CHECK - 4L60-E TRANSMISSION

TCC solenoid is a normally open exhaust valve. Control module will engage solenoid by grounding circuit with an internal quad-driver.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks ability of control module to control solenoid.
2. This test checks for power, from ignition through fuse, to TCC solenoid.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. Some slight TCC slippage is normal.

Code 67 Schematic (5.7L) TCC Solenoid Circuit Check - 4L60-E Transmission. Scheme 63

Code 67 Flow Chart (5.7L) TCC Solenoid Circuit Check - 4L60-E Transmission. Scheme 64

DTC 68: OVERDRIVE RATIO ERROR - 4L80-E TRANSMISSION

Control module monitors the difference in engine RPM and input shaft RPM. With transmission in Drive, scan tester reading should show engine speed closely matching input speed.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks indicated range signal to selected range. A faulty switch could set this code.
2. Checks TCC for slippage while in a commanded lock-up state.

Check for spread connectors at pass-thru connector. Code 68 will set when going to default (2nd gear). If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 68 Flow Chart (5.7L) Overdrive Ratio Error - 4L80-E Transmission. Scheme 65

If TCC is mechanically stuck on, TCC fluid will apply TCC, causing an engine stall.

Code 69 Schematic (5.7L) Torque Converter Clutch Stuck On 4L80-E Transmission. Scheme 66

Code 69 Flow Chart (5.7L) Torque Converter Clutch Stuck On 4L80-E Transmission. Scheme 67

If TCC is mechanically stuck on, TCC fluid will apply TCC, causing an engine stall.

Code 69 Flow Chart (5.7L) Torque Converter Clutch Stuck On 4L80-E Transmission. Scheme 68

DTC 72: VSS CIRCUIT LOSS TRANSMISSION OUTPUT SIGNAL

VSS output sensor is a magnetic induction type. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies VSS voltage at control module.
2. Test checks VSS buffer ground circuit.
3. Test checks VSS circuit at buffer module.
4. Test verifies VSS signal at sensor.

Code 72 will set when VSS signal is present and is lost. Code 24 will set when no vehicle speed is detected at vehicle start off. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 72 Schematic ("C", "G" & "K" Series) VSS Circuit Loss Transmission Output Signal. Scheme 69

Code 72 Flow Chart (All Models) VSS Circuit Loss Transmission Output Signal. Scheme 70

DTC 73: PRESSURE CONTROL SOLENOID (CURRENT ERROR)

Note. This flow chart requires the use of a bidirectional (Tech 1) scan tester.

Pressure control solenoid is controlled by control module to regulate transmission line pressure. Control module looks at TPS voltage, engine RPM and other inputs to determine appropriate line pressure for a given load, then regulates pressure by applying a varying amperage. Applied amperage can vary from 1 to 1.1 amps. Control module then monitors amperage at return line.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks ability of control module to command solenoid.
2. Checks internal transmission harness and solenoid for incorrect resistance.

Check for poor connections at control module and transmission pass-thru connector.

Code 73 Schematic (5.7L) Pressure Control Solenoid (Current Error). Scheme 71

Code 73 Flow Chart (5.7L) Pressure Control Solenoid (Current Error). Scheme 72

DTC 74: TRANSMISSION INPUT SPEED ERROR 4L80-E TRANSMISSION

Sensor is a permanent magnet type. When rotating trigger teeth pass sensor, an AC voltage signal is produced. Signal voltage and frequency varies with forward rotational speed.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks entire circuit for continuity.
2. Checks output of input speed sensor.

Check for poor connections in related circuits.

Code 74 Schematic (5.7L) Transmission Input Speed Error 4L80-E Transmission. Scheme 73

Code 74 Flow Chart (5.7L) Transmission Input Speed Error 4L80-E Transmission. Scheme 74

DTC 75: SYSTEM VOLTAGE LOW

Code 75 will set when ignition is on and control module 12-volt battery feed voltage is less than 9 volts for about 4 seconds. During time failure is present, force motor, also referred to as pressure control solenoid, is turned off, maintaining only 2nd gear and inhibiting TCC operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for normal battery voltage.
2. Checks if low voltage display is due to generator, battery voltage input circuit or control module.

Charging battery with a charger and jump starting engine may set this code. If code sets when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link. Minimum voltage allowed for Code 75 to set is on a graduated scale and changes with temperature from a low of 7.3 volts at -40°F (-40°C) to a voltage of 11.7 volts at 304°F (150°C).

Code 75 Schematic (5.7L) System Voltage Low. Scheme 75

Code 75 Flow Chart (5.7L) System Voltage Low. Scheme 76

DTC 79: TRANSMISSION FLUID TEMPERATURE HIGH

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors voltage to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid warms, sensor resistance and voltage will drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Code 79 will set if signal voltage indicates a transmission fluid temperature greater than 151°C for one second.
2. This test determines if circuit is shorted to ground, which will result in conditions for Code 79.

Check harness routing for a potential short to ground in signal circuit. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, temperature display should rise steadily to about 100°C then stabilize. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 79 Flow Chart (All Models) Transmission Fluid Temperature High. Scheme 77

DTC 81: TRANSMISSION 2-3 ERROR

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks function of 2-3 shift solenoid and internal transmission wiring.
2. Checks for power to 2-3 shift solenoid from ignition.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 81 Schematic (5.7L) Transmission 2-3 Error. Scheme 78

Code 81 Flow Chart (5.7L) Transmission 2-3 Error. Scheme 79

DTC 82: TRANSMISSION 1-2 ERROR

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks 1-2 shift solenoid and internal transmission wiring harness for short circuits.
2. This test checks for power, from ignition through fuse, to shift solenoid.

Check all connections, especially at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 82 Schematic (5.7L) Transmission 1-2 Error. Scheme 80

Code 82 Flow Chart (5.7L) Transmission 1-2 Error. Scheme 81

DTC 83: TCC SOLENOID CIRCUIT FAULT

Control module continually monitors voltage on each circuit connected to quad-driver for either low or high voltage, depending on commanded state of device connected to it. Code 83 will set if control module detects an inappropriate reading on TCC circuit. For example, if TCC duty cycle is zero, but voltage on TCC circuit drops as if solenoid were on, then Code 83 will set. TCC solenoid, because of its large current draw, is connected to 2 terminals of a single quad-driver.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks if control module is commanding TCC solenoid on.
2. This test checks for voltage to solenoid.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 83 Schematic (5.7L) TCC Solenoid Circuit Fault. Scheme 82

DTC 85: UNDEFINED GEAR RATIO

While in each gear, control module calculates actual gear ratio from input and output speed readings, also referred to as vehicle speed, then compares these to what gear ratio should be, taking into consideration selected gear range. This monitor includes reverse gear, but does not include overdrive gear.

Note. Test numbers refer to numbers on diagnostic chart.

1. An out-of-calibration transmission range pressure switch could falsely set Code 85.
2. This test verifies proper ratio.

Code will set when an unknown gear ratio is detected for any gear but 4th. Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. Compare scan tester gear ratio reading to specifications in GEAR RATIO table.

GEAR RATIO

Code 85 Schematic (5.7L) Undefined Gear Ratio. Scheme 83

Code 85 Flow Chart (5.7L) Undefined Gear Ratio. Scheme 84

DTC 86: LOW GEAR RATIO ERROR

Control module calculates ratio based on transmission input speed and output speed sensor readings. Control module compares known ratio to calculated ratio.

Note. Test numbers refer to numbers on diagnostic chart.

1. An out-of-calibration transmission range pressure switch could falsely indicate actual transmission range.
2. This test verifies proper ratio.

If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 86 Flow Chart (5.7L) Low Gear Ratio Error. Scheme 85

Check all connections, especially those at transmission pass-thru connector. Fault may be an internal transmission problem.

Code 87 Flow Chart (5.7L) High Gear Ratio Error. Scheme 86