PCM Outputs Diagnosis

PCM Outputs Diagnosis:

Circuit Description
The Powertrain Control Module (PCM) controls most components with electronic switches which complete a ground circuit when turned on. These switches are arranged in groups of 4 or groups of 7, called either a surface mounted Quad Driver Module, which can independently control up to 4 output (PCM) terminals or Output driver Modules which can independently control up to 7 outputs. Not all outputs are always used. When an output is on, the terminal is grounded and the terminal voltage will normally be low. When an output is off, the terminal voltage will normally be high.

Drivers are fault protected. If a relay or solenoid is shorted, having very low or zero resistance, or if the control side of the circuit is shorted to voltage, too much current flows into the driver. The driver senses this and the output is turned off or the internal resistance increases in order to limit the current flow and protect the drivers. The result is a high output terminal voltage when the voltage should be low. If the circuit for B+ or the component is open or the control side of the circuit is shorted to ground, the terminal voltage will be low, even when the output is commanded off. Either of these conditions is considered to be a driver fault.

The drivers also have a fault line in order to indicate the presence of a current fault to the PCM. A scan tool displays the status of the driver fault lines as 0=OK, 1=Fault.

Diagnostic Aids
The scan tool has the ability to command certain components and functions to turn on and off. If a component or function does not have this capability, operate the vehicle during its normal function criteria in order to check for an open or shorted circuit.

An open or short to ground will appear in the open positions only when it is not commanded by the PCM. A short to voltage will appear in the short position only while the component is being commanded by the PCM.

Test Description
The numbers below refer to the step numbers in the diagnostic table.

1. The Powertrain OBD System Check prompts the technician to store the Freeze Frame Data and the Failure records on the scan tool if applicable. This creates an electronic copy of the freeze frame data taken when the fault occurred that can be referred to later.
2. A 1 in any of the positions indicates that an open or short is present in the corresponding circuit for that position.
3. Refer to the Scan Tool Display art in order to determine which circuit corresponds to which position of the display.
4. An open or short to ground appears in the open positions only when it is not commanded by the PCM while a short to voltage will appear in the short positions only while PCM commands the component.
5. Proper operation of any component or function without a 1 in any of the positions indicates that the system operation is normal at this time.
6. A component or function that failed to operate at this point indicates that the fault is not on the PCM side of the circuitry.
9. When the scan tool indicates a fault after disconnecting the component electrical connector and verifying that no open or short is present in the circuit, then the PCM is faulty.
10. The 1 going away after disconnecting the component electrical connector indicates that the component or component circuitry is at fault. Refer to Knock Sensor System Diagnosis. System Diagnosis - Knock Sensor (KS)