Engine Controls - Tests W/codes - 7.4L: Other Chevrolet Suburban K1500

TERMINOLOGY

Due to Federal government requirements, manufacturers may use names and acronyms for systems and components different than those used in previous years. The following table will help eliminate confusion when dealing with these components and systems. Only relevant components and systems whose names have changed from current General Motors Corp. terminology have been listed.

SAE TERMINOLOGY

HARD FAILURES

Most hard failures cause MIL to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause must be found using diagnostic (code) charts. If a sensor fails, control unit will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but driveability can be poor.

INTERMITTENT FAILURES

Intermittent failures may cause MIL to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in control module memory. If related fault does not reoccur within 50 engine restarts, it will be erased from ECM memory. Intermittent failures may be caused by faulty sensor, connector or wiring. See INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

RETRIEVING CODES (NON-SCAN)

Note. The Assembly Line Data Link (ALDL) connector may also be referred to as the Data Link Connector (DLC) in flow charts. This is the same connector.

1. Turn ignition on with engine off. Malfunction Indicator Light (MIL) should glow. Locate Data Link Connector (DLC), attached to control module wiring harness. Most DLCs are located under dash on driver's side of vehicle. For exact location of DLC, see appropriate COMPONENT LOCATIONS illustration in «SYSTEM/COMPONENT TESTS - 7.4L»(ref-20094) article. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of DLC (terminals No. 5 and 6 of 16-terminal connector). (Scheme 369) Turn ignition on. NOTE: Inserting jumper wire into test and ground terminals of DLC with engine running will cause fuel-injected vehicles to enter field service mode and codes will not flash. See FIELD SERVICE MODE in «BASIC TESTING - 7.4L»(ref-20095) article.
2. Malfunction Indicator Light (MIL) should flash codes. Each code is flashed 3 times. If codes DO NOT flash, perform DIAGNOSTIC CIRCUIT CHECK in «BASIC TESTING - 7.4L»(ref-20095) article. To exit diagnostic mode, turn ignition off and remove jumper wire from DLC.

Scheme 369

Scheme 370

Field Service Mode Check

Malfunction Indicator Light (MIL) indicates operational mode of engine if DLC is grounded while engine is running. Light response confirms proper fuel system operation and verifies closed loop operation. Clear codes and perform this test after any repair is completed. Field service mode check can be found by proceeding to FIELD SERVICE MODE CHECK in BASIC TESTING - 7.4L article.

SCAN TESTER USAGE

Note. Before connecting scan tester, check diagnostic system and ensure accurate information is received by scan tester. Perform DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING - 7.4L article. If vehicle does not pass diagnostic circuit check, information received by scan tester may be invalid.

The scan tester is a specialized tester which can diagnose on-board computer control systems by providing access to circuit voltage information without crawling under dash or hood to backprobe sensors and connectors. scan testers reduce diagnostic time by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN TESTER DATA .

Scan testers also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication output signals have been sent to devices by the control module. They do not indicate whether devices respond properly to that signal. This must be verified at output device using a voltmeter or test light.

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.

If trouble codes are not present, a problem may still exist. Driveability-related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Out-of-calibration sensors WILL NOT set a trouble code, but WILL cause driveability problems. A scan tester is the easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See SCAN TESTER DATA .

Note. Information obtained by scan tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off and remove tester. Turn ignition on and ground DLC test terminal. If same codes are not flashed by Malfunction Indicator Light (MIL) as were indicated by scan tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate. If tested vehicle does not provide for grounding diagnostic test terminal, verify tester accuracy by connecting it to a known good vehicle.

SCAN TESTER DATA

Note. Information contained in the following table is typical of readings taken on vehicle with engine idling, upper radiator hose hot, throttle closed, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Not all devices and systems are used on all models. For additional information, see tester owner's manual.

SCAN TESTER DATA

CODE 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.

CODE 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.

CODE 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.

CODE 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

CODE 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.

CODE 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.

CODE 24, VEHICLE SPEED SENSOR - "C" & "K" SERIES

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, 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 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 32, EGR CIRCUIT ERROR BACKPRESSURE EGR WITH SOL

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.

CODE 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.

CODE 32, EGR CIRCUIT ERROR - "P" SERIES WITH LINEAR EGR

Control module controls linear EGR valve using a pulse width modulated signal. Proper operation of EGR valve is determined using 2 separate diagnostic tests. A properly operating EGR system will directly affect fuel integrator counts. With EGR activated, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set. Linear EGR valve also incorporates an EGR pintle position sensor. If actual (feedback) pintle position signal does not match desired pintle position, Code 32 will set.

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

1. Checks status of EGR pintle.
2. Checks for correct operation of linear EGR valve.
3. Checks linear EGR control circuit.
4. Checks power feed to EGR valve connector.
5. Checks EGR control circuit connections and control module.
6. Checks pintle position signal circuit and control module.
7. Checks ground circuit for pintle position sensor and control module.
8. Checks for a grounded EGR control circuit.
9. Checks sensor ground circuit, 5-volt reference circuit and control module.
10. Checks sensor signal and circuit.
11. Checks control module connections, pintle position signal circuit, EGR valve connections, EGR valve and control module.

CODE 32, EGR CIRCUIT ERROR - 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.

CODE 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.

CODE 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.

CODE 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.

System Too Lean

If air/fuel ratio is too lean, idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary and disconnecting IAC may not help. Scan tester and/or digital voltmeter (10 megohm) will read an oxygen sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in fuel.

System Too Rich

If air/fuel ratio is too rich, idle speed will be too low and scan tester counts will usually be greater than 80. The system may be obviously rich with Black smoke from tailpipe. Scan tester and/or voltmeter will read an oxygen sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure or leaking/sticky injectors. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging bore. A silicone-contaminated oxygen sensor will produce lean air/fuel mixture. Oxygen sensor output would be fixed greater than 800 mv (.8 volt). This may also set Code 45.

Throttle Body

Remove IAC and inspect bore for evidence of IAC valve dragging.

IAC Valve Connections

Carefully inspect connections for looseness or corrosion.

PCV Valve

The wrong PCV valve may cause incorrect idle speed.

CODE 35 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 35 Schematic (All Models) Idle Speed Error. Scheme 371

Code 35 Flow Chart (All Models) Idle Speed Error. Scheme 372

CODE 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.

CODES 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.

CODE 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.

CODE 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.

CODE 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.

CODE 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.

CODE 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 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".

O2 Sensor Wire

Wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.

Fuel Contamination

Water, even small amounts, near in-tank fuel pump inlet can reach fuel injector, causing a lean exhaust and setting Code 44.

Fuel Pressure

System will be lean if fuel pressure is low. It may be necessary to monitor fuel pressure while driving vehicle. For fuel pressure checking procedure, see BASIC TESTING - 7.4L article.

Exhaust Leaks

If exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into system and past O2 sensor. Vacuum or crankcase leaks can also cause a lean condition. If Code 44 is intermittent, proceed to the INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

CODE 44 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 44 Flow Chart (All Models) Lean Exhaust Indication. Scheme 373

CODE 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.

Fuel Pressure High

If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see BASIC TESTING - 7.4L article. The control module can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.

Ignition Ground

If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses picked up by control module on IC harness reference line. Additional pulses result in a higher than actual engine speed signal. The control module will increase injector pulse width ("on" time) to match increased RPM signal. Scan tester will show higher than actual RPM, which can help diagnose problem.

Fuel Canister

Charcoal canister fuel saturation will cause a rich air/fuel ratio. If full of fuel, check canister control and hoses.

MAP Sensor

If control module senses higher than normal manifold pressure (low vacuum) system can go rich. Disconnecting MAP sensor allows control module to substitute a fixed value for MAP sensor. If rich condition disappears, replace MAP sensor and continue testing.

TPS

An intermittent TPS output will cause system to operate rich due to a false indication of engine acceleration.

O2 Sensor Contamination

O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a White-powdery coating to cover O2 sensor. The false high signal voltage produced (or low oxygen content sensed) is interpreted by control module as a rich mixture, causing control module to set Code 45.

EGR Problem

EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. If Code 45 is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

CODE 45 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 45 Flow Chart (All Models) Rich Exhaust Indication. Scheme 374

CODE 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.

CODE 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.

CODE 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.

CODE 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.

CODE 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 375

Code 54 Schematic ("G" Series) Fuel Pump Circuit. Scheme 376

Code 54 Schematic ("P" Series) Fuel Pump Circuit. Scheme 377

Code 54 Flow Chart (All Models) Fuel Pump Circuit. Scheme 378

CODE 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.

CODE 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.

CODE 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.

CODE 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 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.

CODE 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.

CODE 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.

CODE 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.

CODE 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.

CODE 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.

CODE 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 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.

CODE 87, HIGH 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.

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

SUMMARY

If hard fault codes are not present and driveability symptoms or intermittent codes exist, proceed to TESTS W/O CODES - GASOLINE article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.), or intermittent diagnostic procedures.

Wiring Diagram ("C" & "K" Series Pickup, Sierra & Suburban - 7.4L With A/T - 1 Of 3). Scheme 379

Wiring Diagram ("C" & "K" Series Pickup, Sierra & Suburban - 7.4L With A/T - 2 Of 3). Scheme 380

Wiring Diagram ("C" & "K" Series Pickup, Sierra & Suburban - 7.4L With A/T - 3 Of 3). Scheme 381

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 7.4L With M/T - 1 Of 3). Scheme 382

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 7.4L With M/T - 2 Of 3). Scheme 383

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 7.4L With M/T - 3 Of 3). Scheme 384

Wiring Diagram ("P" Series - Commercial & Motorhome - 7.4L With 4L80-E - 1 Of 3). Scheme 385

Wiring Diagram ("P" Series - Commercial & Motorhome - 7.4L With 4L80-E - 2 Of 3). Scheme 386

Wiring Diagram ("P" Series - Commercial & Motorhome - 7.4L With 4L80-E - 3 Of 3). Scheme 387

Wiring Diagram ("P" Series - Cutaway - 7.4L - 1 Of 3). Scheme 388

Wiring Diagram ("P" Series - Cutaway - 7.4L - 2 Of 3). Scheme 389

Wiring Diagram ("P" Series - Cutaway - 7.4L - 3 Of 3). Scheme 390

Wiring Diagram (Van - 7.4L With 4L80-E - 1 Of 3). Scheme 391

Wiring Diagram (Van - 7.4L With 4L80-E - 2 Of 3). Scheme 392

Wiring Diagram (Van - 7.4L With 4L80-E - 3 Of 3). Scheme 393