DESCRIPTION
The Diesel Electronic Control (DEC) system is used on California 6.2L diesel engines with light duty emissions. The DEC system monitors various engine functions to electronically control the exhaust gas recirculation (EGR) system and torque converter clutch (TCC). The Electronic Control Module (ECM) is the "brain" of the system.
DEC SYSTEM OPERATION
The DEC system is designed to help lower exhaust emissions and improve engine performance. It consists of the following sub-systems: Data Sensors, Electronic Control Module (ECM), EGR System, Torque Converter Clutch (TCC), and Diagnostic System.
DATA SENSORS OPERATION
Each sensor furnishes information to the ECM. Based on sensor information, the ECM determines the amount of EGR flow to lower NOx emissions and improve driveability. The operation of each sensor is as follows
Engine Speed Sensor
This sensor is mounted at the center rear of the engine (on vacuum pump on pickup models). The sensor provides the ECM with engine speed information.
Engine Speed Sensor Location (Pickup Models Only). Scheme 1
Manifold Absolute Pressure (MAP) Sensor
The MAP sensor signal is used by the ECM to determine the amount of vacuum in the EGR vacuum line. This signal is derived by comparing the calculated amount of EGR vacuum and the actual amount of EGR vacuum present. The sensor is mounted on right side of firewall.
MAP Sensor Location. Scheme 2
Throttle Position Sensor (TPS)
The TPS is mounted on the fuel injection pump throttle valve. The TPS provides the ECM with throttle angle information.
Throttle Position Sensor Location. Scheme 3
Vehicle Speed Sensor (VSS)
The VSS is mounted behind the speedometer in the instrument panel. The ECM uses a series of pulses provided by the VSS to determine vehicle speed.
ELECTRONIC CONTROL MODULE (ECM) OPERATION
The ECM is located in the passenger compartment under right side of dash on all pickup models and under driver's seat on all van models. The ECM consists of input/output devices, Central Processing Unit (CPU), power supply and memory.
Input/Output Devices
These devices are an integral part of the ECM. They convert electrical signals, received by the ECM from the various engine sensors, into digital signals for use by the CPU.
Central Processing Unit
Digital signals received by the CPU are used to perform all mathematical computations and logic functions necessary to deliver proper EGR flow. The CPU commands operation of EGR system, torque converter clutch, and the diagnostic system.
Power Supply
The main source of power for the ECM is from the battery, through the ignition circuit.
Memories
Three types of memory are used by the ECM. These are: Read Only Memory, Random Access Memory, and Programmable Read Only Memory.
- Read Only Memory (ROM) - The ROM is programmed information that can only be read by the ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be retained.
- Random Access Memory (RAM) - This memory is the decision making center for the CPU. It works like a calculator. Data sensor input and results of calculations are temporarily stored in the RAM memory. If battery voltage is removed from the ECM, all information stored in the memory is lost.
- Programmable Read Only Memory (PROM) - This memory contains factory programmed information including engine calibration data, transmission type, vehicle weight and rear axle ratio. The PROM can be removed from the ECM. If battery voltage is removed, PROM information will be retained.
EGR SYSTEM
The EGR system is electronically controlled by the ECM and consists of an EGR solenoid, EGR valve, exhaust pressure regulator (EPR) solenoid, EPR valve, EGR vent solenoid valve, and vacuum pump.
The vacuum pump provides vacuum for EPR and EGR valve operation. The EGR and EPR solenoids are mounted at top rear of engine as an assembly.
The ECM controls the EGR solenoid to regulate the amount of vacuum at the EGR valve. The ECM controls on-off time (duty cycle) of the EGR solenoid to regulate EGR flow.
The ECM calculates the amount of EGR flow required based on information supplied by the engine speed and throttle position sensors. When EGR flow should not be recirculated back into the engine, the ECM activates the EGR vent solenoid to rapidly vent vacuum from the EGR system.
During idle situations, the ECM energizes (closes) the EPR solenoid to increase exhaust backpressure. Increasing backpressure increases EGR flow to help reduce emissions during idle.
EGR System Components and Locations. Scheme 4
TORQUE CONVERTER CLUTCH (TCC)
Under proper operating conditions (sufficient vehicle speed), a solenoid valve in the transmission is activated by the ECM. This results in direct coupling of the flywheel to the transmission output shaft through the torque converter, reducing slippage and increasing fuel economy.
When operating conditions indicate that the transmission should function in non-lockup mode, the TCC solenoid is de-energized. This allows the transmission to return to normal automatic operation. Pressing the brake pedal immediately de-energizes the solenoid through the TCC brake switch.
DIAGNOSTIC SYSTEM OPERATION
The ECM of the DEC system is equipped with self-diagnostic capabilities which detect faults in the system (EGR system or Vehicle Speed Sensor). The system does not include a "CHECK ENGINE" light in the dash. Instead, trouble codes are indicated by using the Diesel Diagnostic Check (DDC) tester (J-34750 ).
An Assembly Line Communication Link (ALCL) connecter is used at the factory to check the system prior to shipment. The DDC tester is connected to the ALCL connector and cigar lighter (or BAT terminal in fuse panel) to diagnosis the DEC system. (Scheme 5) Trouble codes are indicated by a "Check Engine" light (CEL) on the DDC tester.
The DDC tester also has selectable diagnostic modes to evaluate a driveability problem. It is the only special tool required to diagnose the DEC system.
