Powertrain Control Module

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NOTE: All these descriptions have been for OBD1 Modules


The powertrain control module (PCM) uses signals from various sensors in order to optimize the control of the engine, transaxle, and electrical variable orifice power steering system. The subsystem also provides information to other subsystems and to service equipment.

The PCM is a microprocessor based computer that is the main component of the powertrain control system.

There are four different versions of the PCM:

1. Throttle body injection with manual transaxle.
2. Throttle body injection with automatic transaxle.
3. Multi-port fuel injection with manual transaxle.
4. Multi-port fuel injection with automatic transaxle. 

Each powertrain control module (PCM) contains either one or two separate controllers located in one assembly. Manual transaxle versions contain only the engine controller (EC), the automatic transaxle versions contain an engine controller (EC) and a transaxle controller (TC). The physical size of the PCM will be the same, regardless of the number of controllers it contains.


A listing of the pin assignments for OBDI PCMs is maintained on the following page:

PCM connectors


The PCM has an EPROM id and an EEPROM id that is viewable from the Datastream. You can also get an idea of what your PCM came out of by the Service Number printed on the label, but you will not be able to determine what program of that year is on the PCM. Check out the PCM Identification page for identifying your PCM.

PCM Operation

The PCM operates in two basic modes:

1. Normal operation
 * Computer and outputs operating properly and system voltage is between 6 and 16 volts.
 * Computer is operating properly, the outputs are non-functional and the system voltage is greater than 16 volts.
2. Backup operation
 * When system voltage is between 4.5 and 6 volts.
 * When system voltage is between 16 and 24 volts.
 * When the PCM is not operating properly. 

NOTE: When in backup opeartion the PCM will light the SES telltale. keep the fuel pump and electric engine cooling fan energized.

Fuel Control

The PCM controls fuel by receiving input signals from various sensors. It then takes this information and calculates the amount of fuel required to achieve a specific air/fuel ratio. The PCM controls the fuel flow into the engine by controlling four fuel injectors on the multi-port fuel injection (MFI) system. The PCM pulses the injectors at least once every crankshaft revolution.

Fuel Backup Control

If the PCM should become non-operational, the PCM will deliver a set amount of fuel, based on inputs received from the manifold absolute pressure (MAP) sensor and coolant temperature sensor (CTS).

Spark Timing Control

The PCM receives reference signals from an electronic ignition module so that it can calculate the spark output signal. The spark output signal controls the timing of the primary of the ignition coils. The PCM also provides spark retard when knock conditions are detected by a knock sensor mounted in the engine block.

Engine Cooling Fan Control

The PCM controls operation of the engine cooling fan based on coolant temperature, air conditioning state and vehicle speed. If the coolant rises above 105°C (220°F) or if the air conditioning compressor is turned "ON". the PCM will supply a ground to the engine coolant fan relay, energizing the relay and engine cooling fan.

The PCM will shut off the cooling fan under the following conditions:

1. Vehicle exceeds 97 kph (70 mph) for three minutes.
2. When the PCM stops receiving an A/C request signal, it will run the cooling fan for three minutes, then de-energize the relay.
turning "OFF" the cooling fan. The A/C request signal can be removed by the operator turning the A/C switch to "OFF", or by the
A/C high or low pressure switches opening the A/C request circuit from the A/C control head to the PCM

NOTE: If the PCM is non-operational, the cooling fan will be turned "ON", as long as the engine is running.

Open and Closed Loop Operation

NOTE: 1992 vehicles will not go into closed loop until a change in throttle position is seen.

The PCM operates in two different fuel control modes: Open Loop and Closed Loop. Whenever the vehicle is first started the PCM operates in Open Loop fuel control. When the PCM determines that the coolant temperature sensor (CTS) has reached at least 20°C (68°F), and the oxygen sensor has reached operating temperature of 318°C (600°F), it will go into Closed Loop fuel control operation. Close Loop fuel control operation will be maintained as long as certain parameters are met.

In Close Loop fuel control, the PCM varies the fuel to the engine according to signals received from the oxygen sensor, located in the exhaust manifold. The oxygen sensor varies a voltage signal to the PCM indicating the oxygen content of exhaust gases. If the oxygen sensor signals that the air/fuel mixture is lean, the PCM will increase the amount of fuel to the engine. If the oxygen sensor signals that the air/fuel mixture is rich, the PCM will decrease the amount of fuel to the engine. During Closed Loop fuel control operation, the PCM is constantly adjusting the amount of fuel to the engine, according to signals received from the oxygen sensor, to try and obtain a 14.7:1 air fuel ratio.

If there are any oxygen sensor related PCM/EC codes or if the oxygen sensor does not switch between rich and lean, or if the CTS does not obtain 20°C (68°F), the PCM will not go into Closed Loop fuel control operation.

Field Service Mode

If the diagnostic terminal is grounded with the engine running, the system will enter the Field Service Mode. In this mode, the SERVICE ENGINE SOON telltale will indicate whether the system is in Open Loop or Closed Loop.

In Open Loop the SERVICE ENGINE SOON telltale lamp flashes two and one-half times per second.

In Closed Loop, the lamp flashes once per second. Also, in Closed Loop, the telltale lamp will stay "OFF" most of the time if the system is running lean. It will stay "ON" most of the time if the system is running rich.

While the system is in the Field Service Mode and the Closed Loop timer is bypassed, new trouble codes cannot be stored, but information flags can be stored in the PCM.


This section has been moved to its own page


This section has been moved to its own page

Transmission Control Module

The transmission control module (TCM) is contained within the PCM on vehicles with automatic transmissions.