Vehicle Control Module (VCM)

Engine Control Module:

Fig. 5 ECM Unit (Showing PROM & CALPAK Locations):

Powertrain Control Module (PCM):

PURPOSE
The Vehicle Control Module (VCM) is the control center of the fuel injection system and antilock brakes. It constantly looks at the information from various sensors (inputs) and controls the systems (outputs) that affect engine and antilock operation. The VCM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (MIL) "Service Engine Soon" light on the instrument panel and store a Diagnostic Trouble Code(s) (DTC) in the VCM memory. The DTC identifies the problem areas to aid the technician in performing repairs.

OPERATION
The VCM is an electronic computer designed to process the various input information, and send the necessary electrical response to control fuel delivery, spark control, antilock brakes and other emission control systems. The VCM can control these devices through the use of Quad Driver Modules (QDM). When the VCM is commanding a device or a component "ON," the voltage potential of the output is "LOW" or near zero volts. When the VCM is commanding a device or component "OFF," the voltage potential of the circuit will be "HIGH," or near 12 volts. The primary function of the QDM is to supply the ground for the component being controlled.

The input information has an interrelation between sensor output. If one of the input devices failed, such as the oxygen sensor, this could affect more than one of the systems controlled by the computer.

The VCM in this vehicle is programmable. The only services allowed on the VCM is control module replacement with PROM transferred or PROM only. The PROM contains the operational software, backup fuel calibrations and the up integrated knock sensor circuit. The calibrations are stored in the VCM in the Electronically Erasable Programmable Read Only Memory (EEPROM).
When replacing the VCM, it will be mandatory to program the EEPROM and transfer the PROM to the new VCM.
The four calibrations required for the VCM are engine ABS, VSS buffer and A/C. Each calibration will have its own part number. Determination of the correct calibrations for a particular vehicle will be based on the VIN number of the vehicle.

Learning Ability
The VCM has a "learning" ability which allows it to make corrections for minor variations in the fuel system to improve driveability. If the battery is disconnected, to clear diagnostic trouble codes or for other repairs. the "learning" process resets and begins again. A change may be noted in the vehicle's performance. To "teach" the vehicle, ensure the engine is at operating temperature. The vehicle should be driven at part throttle, with moderate acceleration and idle conditions until normal performance returns.

NOTE The VCM must be maintained at a temperature below 85°C (185°F) at all times. This is most essential if the vehicle is put through a baking process. The VCM will become inoperative if it's temperature exceeds 85°C (185°F). It is recommended that temporary insulation be placed around the VCM during the time the vehicle is in a paint oven or other high temperature processes.

NOTE Since the VCM is located under hood, its connectors are sealed and cannot be backprobed as in previous model years. Do not attempt to backprobe as connector or seal damage could occur.