The Diesel Electronic Control (DEC) system is used the 6.2L Diesel engine with light duty emissions. The DEC system electronically controls EGR system operation, Torque Converter Clutch (TCC) engagement, cold advance, and glow plug system.
The DEC system consists of the following components: Electronic Control Module (ECM), Manifold Absolute Pressure (MAP) sensor, Coolant Temperature Sensor (CTS), "SERVICE ENGINE SOON" light driver module, Exhaust Gas Recirculation (EGR) valve, EGR vent solenoid, EGR solenoid, Exhaust Pressure Regulator (EPR) solenoid, Throttle Position Sensor (TPS), engine speed sensor, Vehicle Speed Sensor (VSS), ALDL connector, Torque Converter Clutch (TCC) solenoid, cold advance relay, cold advance solenoid, electronic controller/glow plug relay, and glow plugs.
ELECTRONIC CONTROL MODULE
The Electronic Control Module (ECM) is located in the passenger compartment, behind glove box. It constantly monitors the information from various sensors to control the EGR, TCC, cold advance, and glow plug systems. The ECM processes input signals from sensors and then sends the necessary electrical responses to control these systems.
The ECM performs the diagnostic function of the DEC system. It can recognize operational problems, alert driver through the "SERVICE ENGINE SOON" light, and store code(s) which identify problem areas to aid technicians in making system repairs.
PROM (Programmable Read Only Memory)
This is factory programmed information, including engine calibration data, transmission, vehicle weight, and rear axle ratio application. The PROM can be removed from the ECM. If battery voltage is removed, PROM information will be retained. The ECM is supplied without a PROM for service and is called a controller. This allows one controller to be used with several different PROM's.
ELECTRONIC CONTROLLER/GLOW PLUG RELAY
The electronic controller/glow plug relay is mounted at rear of left cylinder head. It monitors and controls glow plug operation. Four pins are used by the controller to determine glow plug operating requirements. Pin "B" senses voltage at the starter motor solenoid. Pin "C" senses glow plug voltage. Pin "D" supplies 12 volts, through the cold advance relay, to operate the controller when coolant temperature is below 80°F (27°C). Pin "E" is the controller ground.
A normally operating system works as follows: at room temperature and with ignition on and engine off, the glow plugs come on for 4-6 seconds and then go off for about 4.5 seconds. The glow plugs then cycle on for about 1.5 seconds and off for about 4.5 seconds, for a total start sequence of about 20 seconds. If the engine is cranked during or after start sequence, the glow plugs will cycle on and off for a total of 25 seconds after the ignition switch is returned from the crank position, whether engine starts or not.
COLD ADVANCE CONTROL
The cold advance control circuit is designed to advance injection pump timing about 4 degrees during cold engine operation. This circuit is activated by the ECM, through the cold advance relay, to energize the cold advance solenoid. The ECM opens the circuit when coolant temperature is above 95°F (35°C).
Below the switching point, and with ignition on, the cold advance solenoid is continuously energized without the engine running. Below the switching point and the engine running, injection pump housing pressure is decreased from 10 psi (.70 kg/cm 2 ) to zero, which advances injection pump timing by about 4 degrees. As engine warms-up, the cold advance solenoid is de-energized and the injection pump housing pressure is returned to 10 psi (.70 kg/cm 2 ).
TORQUE CONVERTER CLUTCH
The Torque Converter Clutch (TCC) system uses a solenoid operated valve, in the automatic transmission, to couple the engine flexplate to the output shaft of the transmission through the torque converter.
In order for the TCC to apply, the following conditions must be met: transmission fluid pressure must be correct. The ECM must complete the ground circuit to energize the TCC apply solenoid in the transmission. The solenoid moves a check ball and allows hydraulic pressure to apply the torque converter clutch.
After the torque converter clutch applies, the ECM uses the information from the throttle position sensor to release the clutch when vehicle is accelerating or decelerating at a certain rate. The normally closed brake switch opens when brake pedal is depressed, de-energizing the TCC solenoid.
EXHAUST GAS RECIRCULATION SYSTEM
See DIESEL EGR article in EXHAUST CONTROL SYSTEMS section.
COOLANT TEMPERATURE SENSOR
The Coolant Temperature Sensor (CTS) is a thermistor (a resistor that changes its resistance value based on temperature). A coolant temperature of -40°F (-40°C) produces a high resistance (100,000 ohms), while a coolant temperature of 266°F (130°C) produces a low resistance (70 ohms).
The ECM supplies a 5-volt reference signal to the CTS, through a resistor in the ECM and measures the return voltage. Voltage will be high when coolant temperature is low, and low when coolant temperature is hot. By measuring voltage, the ECM knows the engine coolant temperature. Engine coolant temperature affects the cold advance and glow plug systems.
MANIFOLD ABSOLUTE PRESSURE SENSOR
The Manifold Absolute Pressure (MAP) sensor is mounted on left side of cowl and is used to monitor the amount of vacuum in the EGR system. It senses the actual vacuum in the EGR vacuum line and sends a signal back to the ECM.
The signal is compared to the EGR duty cycle calculated by the ECM. If there is a difference in the vacuum value sensed and the ECM command, the ECM makes minor correction adjustments. When a major difference is sensed, the ECM recognizes a fault and sends a full EGR signal.
THROTTLE POSITION SENSOR
The Throttle Position Sensor (TPS), mounted on the injection pump, is a variable resistor that monitors the degree of throttle opening for the ECM. The sensor is connected to a 5-volt reference signal and has a high resistance value when throttle is closed. At wide open throttle, the TPS resistance value is low and output to the ECM will be near 5 volts.
ENGINE SPEED SENSOR
The engine speed sensor is a camshaft driven pick-up and is mounted at center, rear of engine. The sensor receives a 5-volt reference signal and allows the ECM to measure engine RPM by the number of times reference voltage is pulsed. The engine speed sensor pulses 4 times per revolution.
VEHICLE SPEED SENSOR
The Vehicle Speed Sensor (VSS) is installed on the transmission. The ECM will calculate vehicle speed based on a series of pulses provided by the VSS. This calculation is used to control TCC engagement.
Removal
- Disconnect negative battery cable. Unplug connectors from ECM (behind glove box). (Scheme 3) Remove ECM from vehicle. To remove PROM, position ECM so that bottom cover is facing upward. Remove PROM access cover.
- Note location of reference notch in PROM and ECM for reassembly reference. Engage end of PROM carrier with hook end of rocker-type PROM remover. Press on vertical bar end of remover and rock engaged end of PROM carrier up as far as possible.
- Engage opposite end of PROM carrier in same manner and rock this end up as far as possible. Repeat process until PROM carrier and PROM are free of PROM socket. PROM carrier and PROM should lift off of PROM socket easily.
Note. PROM carrier should ONLY be removed with PROM remover. Use of any other method may damage PROM or PROM socket.
Scheme 3
Installation
- To install PROM, check that new PROM has same service number as the one being replaced. Place new PROM in PROM carrier. Position PROM carrier squarely over ECM PROM socket. Press on PROM carrier until PROM is firmly seated in ECM. NOTE: Ensure that reference notch in both ECM and PROM are properly aligned. If PROM is installed backwards and key is turned on, PROM will be destroyed.
- Install PROM access cover on ECM. To install ECM, reverse removal procedure. Turn ignition on and enter diagnostics. See ENTERING OR EXITING DIAGNOSTIC MODE in this article. Code 12 should flash 4 times to indicate that PROM is installed properly. No other codes should be present.
- If Code 51 is displayed, PROM is not fully seated in ECM, is installed backwards, has bent pins, or is defective. If pins are bent and crack when straightened, PROM must be replaced. If PROM is installed backwards or is defective, it must be replaced.
Remove air cleaner assembly and related hoses. Disconnect TPS connector. Remove 2 screws and throttle position sensor from injection pump.
Scheme 4
- Install throttle position sensor on injection pump and lightly tighten screws. Make jumper wires using Terminals No. 1214836 and 12014837. Install jumper wires between TPS and harness. (Scheme 4)
- Turn ignition on, DO NOT start engine. Install.646" (16.41 mm) thick Gauge Block (J-33043-2) between gauge boss on injection pump and wide open throttle valve stop screw on throttle lever (switch on position).
- Rotate throttle shaft and hold the wide open throttle stop screw against gauge block. Using DVOM, measure reference voltage across terminals "A" (Black wire) and "C" (Gray wire). Record voltage reading. Now measure and record TPS return voltage between terminals "B" (Dark Blue wire) and "C" (Gray wire).
- Divide TPS return voltage by the reference voltage. For example: a TPS return voltage of 3.50 volts divided by a 5.01 reference voltage equals a.70 TPS voltage ratio. See «TPS ADJUSTMENT SPECIFICATIONS»(/gmc/cab-chassis-v3500/1987-1991/remont/theory-operation/#62l-cec-descriptionoperation) table. The TPS voltage ratio should be within +/-.03 volts. TPS ADJUSTMENT SPECIFICATIONS Application TPS Voltage Ratio Automatic Transmission (1).70 Manual Transmission (2).62 (1) Some models require a.78 TPS voltage ratio, see emissions label. (2) Some models require a.69 TPS voltage ratio, see emissions label.
- To adjust TPS, loosen attaching screws and rotate TPS until a correct TPS voltage ratio is obtained. If adjustment is okay, tighten attaching screws to 53 INCH lbs. (6 N.m)
- Release throttle lever and allow it to return to the idle stop position. Check TPS voltage between terminals "B" (Dark Blue wire) and "C" (Gray wire). Now return lever against gauge block and note voltage reading.
- Voltage should be less than 2.2 volts at closed throttle and return to within +/-.03 volts of the adjusted voltage when throttle is against gauge block.
- If reading does not return to adjusted voltage, repeat adjustment procedure. If reading is still incorrect, replace TPS. Also, if voltage is not less than 2.2 volts, with throttle closed, replace TPS.
- After TPS adjustment, remove gauge block. Turn ignition off. Remove jumper wires and reconnect TPS harness connector. Install air cleaner assembly and related hoses.
Disconnect negative battery cable. Drain cooling system below level of sensor. Disconnect coolant temperature sensor and remove. To install, reverse removal procedure.
Disconnect electrical lead and vacuum hose from manifold absolute pressure sensor. Remove sensor from vehicle. To install, reverse removal procedure.
TESTING & DIAGNOSIS
Note. For information not covered in this article, see 6.2L DIESEL FUEL INJECTION article in FUEL SYSTEMS section.
TEST EQUIPMENT
A tachometer, test light, Digital Volt/Ohmmeter (DVOM) with 10 megohms impedance, a vacuum gauge, and jumper wires are required to test and diagnose DEC system. A "SCAN" tester may be used. Tester will supply a visual reading of most inputs to the ECM and some outputs.
Preliminary Inspection
- Check all vacuum hoses for correct routing, restrictions, cuts, or other damage. Be sure to inspect hoses that are difficult to see beneath the air cleaner assembly and other engine components.
- Inspect all wires in engine compartment for correct and good connections. Also check wires for pinched or chaffed spots, as well as contact with sharp edges or exhaust manifolds.
- The preliminary inspection is very important and should be performed CAREFULLY and THOROUGHLY as it can often lead to fixing a problem without further diagnosis.
Diagnostic Procedure
- Make sure that all engine systems not related to the Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all other non-DEC system related problems have been repaired.
- The diagnosis of the DEC system should ALWAYS START with the "DIESEL DIAGNOSTIC CIRCUIT CHECK" chart. This will determine if the DEC system and ECM are working properly. If trouble codes are displayed (other than Code 12), determine if codes are "hard" or "intermittent" trouble codes.
- A "hard" code is one which is present when working on vehicle and the condition still exists. "Hard" codes will cause the "SERVICE ENGINE SOON" light to come on.
- An "intermittent" code is one which does not reset itself and is not present while working on vehicle. An intermittent code is often caused by loose connections. The "SERVICE ENGINE SOON" light will go out 10 seconds after fault goes away.
ENTERING OR EXITING DIAGNOSTIC MODE
- With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). (Scheme 5) The Diesel Electronic Control (DEC) system will enter the diagnostic mode.
- In this mode, the ECM will display Code 12, by flashing the "SERVICE ENGINE SOON" light. Code 12 consists of one (1) flash, followed by a short pause, then 2 flashes in quick succession.
- Code 12 will be displayed 3 times. If no other codes are stored, Code 12 will continue to flash until the diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
Scheme 5
TROUBLE CODE IDENTIFICATION
- The DEC system codes indicate a failure of a specific sensor and/or circuit. Sensor/circuit diagnosis may indicate replacement of the ECM. Code 51 indicates that PROM is improperly installed or has failed.
- Code 52 indicates that the ECM has failed and must be replaced. If the ECM is replaced and the condition was not corrected, the following may be the cause: An incorrect ECM or PROM application may cause a malfunction and may, or may not, set a code. The ECM connector may be the problem. Connector terminals may have to be removed from the connector to properly check them. PROM failure. Although the PROM rarely fails, it could be the cause of the problem. Replacement ECM may be faulty. Intermittent problem. Make a careful physical inspection of affected sensor/circuit. A shorted solenoid, coil relay, or harness may cause an ECM to fail and a replacement ECM to fail when it is installed. Use Short Circuit Tester (J-34636 ), to check for short circuits.
| Code & (Affected Circuit) | Probable Cause |
|---|---|
| 12 (Engine Speed Sensor) | No engine speed sensor reference pulse to ECM. |
| 14 (CTS High Temp. Indication) | Sets if sensor or signal line becomes grounded for 5 minutes. |
| 15 (CTS Low Temp. Indication) | Sets if sensor, connections, or wires open for 5 minutes. |
| 21 (TPS Signal Voltage High) | Open circuit or misadjusted TPS. Engine must run for 30 seconds at curb idle, before code will set. |
| 22 (TPS Signal Voltage Low) | TPS circuit grounded. Engine must run for 2 minutes at 1250 RPM or higher before code will set. |
| 23 (TPS Not Calibrated) | Voltage not between .25-1.3 volts at curb idle. Engine must run for 30 seconds at curb idle, before code will set. |
| 24 (Vehicle Speed Sensor) | VSS circuit open or grounded. Vehicle must operate at road speed for 10 seconds before code will set. |
| 31 (MAP Sensor Signal Voltage Low) | Engine must run at curb idle for 10 seconds before code will set. |
| 32 (EGR Loop Error) | EGR vacuum circuit has improper vacum signal. Vehicle must operate at 30 MPH for 10 seconds before code will set. |
| 33 (MAP Sensor Signal Voltage High) | Engine must run at curb idle for 10 seconds before code will set. |
| 51 (PROM Fault) | Improperly installed or faulty PROM. 10 seconds must elapse before code will set. |
| 52 (ECM Fault) | Fault in ECM circuit. 10 seconds must elapse before code will set. |
| 53 (5-Volt Reference Overload) | Grounded 5-volt reference signal. 10 seconds must elapse before code will set. |
TROUBLE CODE IDENTIFICATION
CLEARING TROUBLE CODES
Trouble codes should be cleared after repairs have been completed. Also, some diagnostic charts require that codes be cleared before using diagnostic chart. To clear codes, remove ECM memory battery voltage for 30 seconds.
Scheme 6
Diesel Electronic Control System Wiring Diagram ("C/K" Series). Scheme 7
Diesel Electronic Control System Wiring Diagram ("G" Series). Scheme 8
Diesel Elect. Control System Wiring Diagram ("R/V" Series). Scheme 9
See also:
• TPS ADJUSTMENT SPECIFICATIONS