Diagnostic Aids

Perform this test when directed by QUICK TEST  . This CIRCUIT TEST is intended to diagnose:

• PSOM output to PCM.
• PSOM (+) and PSOM (-) wiring harness circuits.
• PCM.

To prevent replacement of good components, be aware following non-EEC related areas and components may be cause of problem:

• Cruise control system.
• Rear anti-lock brake system.
• Ring gear inside differential.
• Instrumentation system.

Fig 1: Programmable Speedometer/Odometer Module (PSOM) Circuit Logic

Fig 2: Programmable Speedometer/Odometer Module (PSOM) Circuit & Connector Terminals

1. Step 1) Continuous Memory Code 452  Code 452 indicates PCM detected incorrect output from VSS sometime during vehicle operation. Following are possible causes of code:
   • PSOM Output To PCM
   • PSOM (+) And PSOM (-) Wiring Harness Circuits
   • PCM

   Turn ignition off. Remove PCM 60-pin connector, and inspect for damaged pins, corrosion and loose wires. Repair as necessary. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between test pins No. 3 and 6 at breakout box. If reading is 21,000-55,000 ohms, go to step 4). If resistance is not 21,000-55,000 ohms, go to next step.

2. Step 2) Check PSOM Circuit Continuity  Ensure ignition is off. Disconnect PSOM wiring harness connector. Disconnect cruise control module. Measure resistance between test pin No. 3 at breakout box and PSOM (+) circuit at PSOM wiring harness connector. Also, measure resistance between test pin No. 6 at breakout box and PSOM (-) circuit at PSOM wiring harness connector. If any reading is 5 ohms or more, repair open circuit and repeat QUICK TEST  . If both readings are less than 5 ohms or less, go to next step.
3. Step 3) Check For Short Circuit  Ensure ignition is off. Measure resistance between test pin No. 3 and test pins No. 6, 37 and 40 at breakout box. If any reading is 10,000 ohms or less, repair short circuit and repeat QUICK TEST  . If all readings are more than 10,000 ohms, go to next step.
4. Step 4) Check Rear Anti-Lock Brake Sensor (RABS)  Turn ignition off. Disconnect RABS connector. Measure resistance between sensor terminals. If resistance is not 1300-1550 ohms, replace RABS and repeat QUICK TEST  . If resistance is 1300-1550 ohms, go to next step.
5. Step 5) Check PSOM Output Voltage  Turn ignition off. Connect PCM to breakout box. Connect wiring harness connector to PSOM. Set DVOM on 20-volt AC scale. Start engine, and warm it to normal operating temperature. Using an assistant, measure voltage between test pins No. 3 and 6 while gradually increasing vehicle speed to 50 MPH. If maximum voltage is 4 volts or less, fault is in instrument cluster. If maximum voltage is more than 4 volts, replace PCM.