Engine Control Module: Description and Operation

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

DESCRIPTION
The Powertrain Control Module (PCM) is the control center of the vehicle. It controls the following items:

- Fuel metering system.
- Transaxle/transmission 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 Malfunction Indicator Lamp (MIL), and store diagnostic trouble codes which identify the problem areas to aid the technician in making repairs.

The type of PCM used is a PCM 32U. For service, the PCM consists of two parts: a controller (the PCM without the Knock Sensor module) and 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.

The PCM controls output circuits such as the injectors, IAC, cooling fan relays, etc. by controlling the ground or power feed circuit through transistors or a device called a Driver.

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-1 or a Tech-2) containing the correct program and calibration for the vehicle is required to program the PCM.

KS 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. It 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.

3X REFERENCE LOW
The PCM uses this signal, from the ignition control module to calculate engine speed and crankshaft position over 1280 RPM. The PCM also uses the pulses on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, DTC P1374 will set and the PCM will use the 24X reference signal circuit for fuel and ignition control.
This is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the ignition control module. Although this circuit is electrically connected to the PCM, it is not connected to ground at the PCM. The PCM compares voltage pulses on the reference input circuits to pulses on this circuit, ignoring pulses that appear on both. Refer to Electronic Ignition System for further information.