PCM/System Description

The Powertrain Control Module (PCM) monitors many different circuits in the fuel injection, ignition, emission and engine systems. This monitoring is called "on-board diagnostics". The PCM compares input signal voltages from each input device with specifications (the established high and low limits of the range) that are programmed into it for that device. If the input voltage is not within specifications and other trouble code criteria are met, a trouble code will be stored in the PCM memory.

If the code applies to a non-emissions related component or system, and the problem is repaired or ceases to exist, the PCM cancels the code after 40 warm-up cycles.

Diagnostic trouble codes that affect vehicle emissions illuminate the Malfunction Indicator Lamp (MIL).

Certain criteria must be met before the PCM stores a DTC in memory. The criteria may be a specific range of engine RPM, engine temperature, and/or input voltage to the PCM.

The PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of the DTC criteria for the circuit has not been met. For example, assume the diagnostic trouble code criteria requires the PCM to monitor the circuit only when the engine operates between 750 and 2,000 RPM.

Suppose the sensor's output circuit shorts to ground when engine operates above 2,400 RPM (resulting in 0 volt input to the PCM). Because the condition happens at an engine speed above the maximum threshold (2,000 rpm), the PCM will not store a DTC.

There are several operating conditions for which the PCM monitors and sets DTC's. Refer to Monitored Systems, Components, and Non-Monitored Circuits.

NOTE: Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For instance, pulling a spark plug wire to perform a spark test may set the misfire code. When a repair is completed and verified, use the DRB or scan tool to erase all DTC's and extinguish the MIL.

Technicians can display stored DTC's by two different methods:
- Through the MIL
- Using a scan tool

Monitored Circuits
The PCM is able to monitor and identify most driveability related trouble conditions. Some circuits are directly monitored through PCM feedback circuitry. In addition, the PCM monitors the voltage state of some circuits and compares those states with expected values. Other systems are monitored indirectly when the PCM conducts some type of a rationality test to identify problems.

Although most subsystems of the powertrain control module are either directly or indirectly monitored, there may be occasions when diagnostic trouble codes are not immediately identified.

For a trouble code to set, specific conditions must be met and unless these conditions are encountered, a code will not set. For example, the Speed Control Solenoid Circuits trouble code will not set unless the system is active (speed control is turned on, the brake is off, and mph > 35).

When a trouble code identifying a problem in an emission related system is set, the Check Engine lamp in the dash will turn on. Conversely, any other trouble code will set without illuminating the Check Engine lamp.

Electronic Pinion Factor
The VSS supplies distance pulses to the powertrain control module (PCM), which are used to calculate speed and mileage. A pinion factor is stored in the transmission control module (TCM) in order to provide the appropriate distance pulses for the vehicle. The pinion factor is programmed into the TCM at the assembly plant.