Programmable Speedometer/Odometer Module (PSOM)

1996 PCED OBDI

SECTION 18: Diesel Diagnosis — 7.3L DI Engine

J: Programmable Speedometer/Odometer Module (PSOM)

J: Introduction

J1 DIAGNOSTIC TROUBLE CODE (DTC) 0500: CHECK PSOM RESISTANCE

Possible error in the PSOM output signal.
Possible causes:

Damaged rear anti-lock brake system (RABS) sensor
Damaged PSOM
Damaged harness circuits
Damaged PCM

Key off.
Disconnect PCM. Inspect for damaged or pushed-out pins, corrosion, loose wires, etc. Service as necessary.
Install breakout box. Leave PCM disconnected.
Measure resistance between Test Pin 58 and Test Pin 33 at the breakout box.

Is resistance between 21,000 and 55,000 ohms?

Yes	No
GO to J4.	GO to J2.

J2 CHECK CONTINUITY OF THE HARNESS CIRCUITS

Key off.
Breakout box installed, PCM disconnected.
Disconnect PSOM.
Measure resistance between Test Pin 58 at the breakout box and PSOM (+) circuit at the PSOM vehicle harness connector.
Measure resistance between Test Pin 33 at the breakout box and PSOM (-) circuit at the PSOM vehicle harness connector.

Is each resistance less than 5 ohms?

Yes	No
GO to J3.	SERVICE open circuit. REMOVE breakout box. RECONNECT all components. RERUN Scan Tool Diagnostic Test.

J3 CHECK HARNESS CIRCUITS FOR SHORTS TO POWER AND GROUND

Key off.
Breakout box installed, PCM disconnected.
PSOM disconnected.
Measure resistance between Test Pin 58 and Test Pins 33, 71 and 51 at the breakout box.

Is each resistance greater than 10,000 ohms?

Yes	No
GO to J4.	SERVICE short circuit. REMOVE breakout box. RECONNECT all components. RERUN Scan Tool Diagnostic Test.

J4 CHECK RABS SENSOR RESISTANCE

Key off.
RABS sensor disconnected.
Measure the resistance of the RABS sensor.

Is resistance between 1300 and 1550 ohms?

Yes	No
GO to J5.	REPLACE the RABS sensor. REMOVE breakout box. RECONNECT all components. RERUN Scan Tool Diagnostic Test.

J5 CHECK PSOM OUTPUT VOLTAGE

WARNING: THIS TEST STEP REQUIRES ANOTHER PERSON TO ACCOMPANY THE DRIVER TO MAKE MEASUREMENTS AND RECORD DATA. ALL APPLICABLE SAFETY PROCEDURES AND TRAFFIC LAWS MUST BE FOLLOWED.

Key off.
Breakout box installed.
Reconnect PCM to breakout box.
Reconnect PSOM.
Set NGS Tester for frequency measurement with a 4 volt DC scale.
Attach NGS Tester probes across Test Pins 33 and 58.
Drive vehicle at 40 mph or 60 mph.

Does frequency read 90 Hz at 40 mph or 130 Hz at 60 mph (±2 Hz)?

Yes	No
REPLACE PCM. REMOVE breakout box. RERUN Scan Tool Diagnostic Test.	REMOVE breakout box. RECONNECT components. REFER to the Instrument Cluster, Speed Control or Brake Group in the Service Manual for further diagnosis.