Engine Control Module: Description and Operation

Figure C1-1 - Powertrain Control Module (PCM):

Description
The Powertrain Control Module (PCM) is located underhood in front of the right strut tower near the engine coolant reservoir. It is the control center of the fuel injection system and constantly looks at the information from various sensors and controls the systems that affect vehicle performance. It controls the following items:

^ Fuel metering system.
^ Transaxle shifting.
^ Ignition timing.
^ On-board diagnostics for Powertrain functions.

It constantly looks at the information from various sensors, and controls the systems that affect vehicle performance. The PCM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the MIL (Service Engine Soon), and store diagnostic trouble codes which identify the problem areas to aid the technician in making repairs. Refer to Introduction in Engine Controls for more information on using the diagnostic function of the PCM.

For service, the PCM consists of two parts:
^ The controller (the PCM without the Knock Sensor module).
^ the Knock Sensor module.

PCM Function
The PCM supplies either 5 or 12 volts to power various sensors or switches. This is done through resistances in the PCM which are so high in value that a test light will not light when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low. Therefore, a digital voltmeter with at least 10 megohms input impedance is required to ensure accurate voltage readings. Tool J 39200 meets this requirement.

EEPROM
The Electrically Erasable Programmable Read Only Memory (EEPROM) is a permanent memory that is physically soldered within the PCM. The EEPROM contains program and calibration information that the PCM needs to control powertrain operation. Unlike the PROM used in certain past applications, the EEPROM is not replaceable. If the PCM is replaced, the new PCM will need to be programmed Techline equipment (Tech-2) containing the correct program and calibration for the vehicle is required to program the PCM.

Knock Sensor Module
The PCM contains a replaceable Knock Sensor (KS) module. The KS module contains the circuitry that allows the PCM to utilize the KS signal and diagnose the KS sensor and circuitry. If the PCM is replaced, the KS module needs to be transferred from the original PCM. If the KS module is missing or faulty causing a continuous knock condition to be indicated, the PCM will set DTC P0325.

Information Sensors
All of the sensors and input switches can be diagnosed using a scan tool. Following is a short description of how the sensors and switches can be diagnosed by using a scan tool. The scan tool can also be used to compare the values for a normal running engine with the engine you are diagnosing.

Password Learn Procedure
In order for a theft deterrent vehicle to run, a password is communicated between the instrument Panel Cluster (IPC) and the Powertrain Control Module (PCM). If a PCM is replaced, the new PCM needs to learn the correct password of the vehicle. When the new PCM is installed, the EEPROM calibration is flashed into the new PCM and the vehicle will learn the new password upon initial ignition On. If the IPC is replaced, the PCM needs to learn the new password from the IPC. The Password learn procedure is as follows:

1. Attempt to start vehicle, then leave the ignition On. The Theft System telltale will flash for 10 minutes.
2. When the Theft System telltale stops flashing, start the vehicle. Once the vehicle is running, the password is learned.