Engine Control Module: Description and Operation
Typical Inputs And Outputs:
Powertrain Control Module
The PCM is the control center of the vehicle. It controls the following:
^ Fuel metering system.
^ Transmission shifting.
^ Ignition timing.
^ On-board diagnostics for powertrain functions.
The PCM 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 and alert the driver through the Malfunction Indicator Lamp (MIL) (Service Engine Soon). When the PCM detects a malfunction, it stores a Diagnostic Trouble Code (DTC) which will identify the problem areas and aid the technician in making repairs.
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. The resistance is 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 (J 39200) with at least 10 megohms input impedance is required to ensure accurate voltage readings.
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 an "Output Driver Module."
EEPROM
The Electronically 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 past applications, the EEPROM is not replaceable. If the PCM is replaced, the new PCM will need to be programmed. Factory GM Techline equipment, 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 sensors 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, the PCM will set DTC P0325.
Short Term Fuel Trim
The short term fuel trim is a PCM erasable memory register. The neutral value for the short term fuel trim is 0%. Any deviation from 0% indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 0%. The short term fuel trim is rich when the scan tool indicates -99%. The short fuel trim is lean when the scan tool indicates 99%. The short term fuel trim changes the pulse width by varying the Closed Loop factor of the base pulse width equation. As the PCM monitors the oxygen sensor's input, it is constantly varying the short term fuel trim value. The value is updated very quickly, therefore, the short term fuel trim only corrects for short term mixture trends. The correction of long term mixture trends is the function of long term fuel trim.
When the PCM determines that the Short Term Fuel Trim is out of the operating range, the following DTCs will set:
^ DTC P0171 Bank 1 Too Lean.
^ DTC P0172 Bank 1 Too Rich.
^ DTC P0174 Bank 2 Too Lean.
^ DTC P0175 Bank 2 Too Rich.
Long Term Fuel Trim
The long term fuel trim is a matrix of cells arranged by RPM and MAP. Each cell of the long term fuel trim is a register like the short term fuel trim. As the engine operating conditions change, the PCM will switch from cell to cell to determine what "long term fuel trim" factor to use in the base pulse width equation.
While in any given cell, the PCM also monitors the short term fuel trim. If the short term fuel trim is far enough from 0%, the PCM will change the long term fuel trim value. Once the long term fuel trim value is changed, it should force the short term fuel trim back toward 0%. If the mixture is still not correct (as judged by the HO2S), the short term fuel trim will continue to have a large deviation from the ideal 0%. In this case, the long term fuel trim value will continue to change until the short term fuel trim becomes balanced. Both the short term fuel trim and long term fuel trim have limits which vary by calibration. If the mixture is off enough so that long term fuel trim reaches the limit of its control and still cannot correct the condition, the short term fuel trim would also go to its limit of control in the same direction. If the mixture is still not corrected by both short term fuel trim and long term fuel trim at their extreme values, a Fuel Trim Diagnostic Trouble Code (DTC) will likely result. When the PCM determines that the Long Term Fuel Trim is out of the operating range, the following DTCs will set:
^ DTC P0171 Bank 1 Too Lean.
^ DTC P0172 Bank 1 Too Rich.
^ DTC P0174 Bank 2 Too Lean.
^ DTC P0175 Bank 2 Too Rich.
Under the conditions of power enrichment, the PCM sets the short term fuel trim to 0% and freezes it there until power enrichment is no longer in effect. This is done so the Closed Loop factor and the long term fuel trim will not try to correct for the commanded richness of power enrichment.