Diagnostic Aids

WARNING: This page does not describe the selected car, but rather 26 other vehicles, including the 1999 Mercury Tracer, 1999 Mercury Sable, 1999 Mercury Mystique, 1999 Mercury Mountaineer, and 1999 Mercury Grand Marquis. However, it is still accessible from the selected car via links, so may be relevant.

Perform this test when directed by QUICK TEST or other test procedures. This test is intended to diagnose the following:

PSP sensor.
PSP, SIG RTN and VREF wiring harness circuits.
Powertrain Control Module (PCM).

1) Verify Operator Interaction

DTC P0552 indicates PSP sensor circuit is open or shorted to ground. DTC P0553 indicates PSP sensor circuit is shorted to power. DTC P1550 indicates PSP sensor circuit is out of self-test range. Possible causes for this fault are:
- Damaged PSP sensor.
- Open or shorted SIG RTN circuit.
- Open or shorted VREF circuit.
- Open or shorted PSP sensor signal circuit.
- Faulty Powertrain Control Module (PCM).
Ensure steering wheel was turned at least 1/2 turn within 30 seconds of starting KOER ON-DEMAND SELF-TEST . If steering wheel was turned, go to next step. If steering wheel was not turned, repeat QUICK TEST .

2) Check VREF Circuit Voltage

Turn ignition off. Disconnect PSP sensor connector. Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Turn ignition on. Measure voltage between VREF terminal and SIG RTN terminal at PSP sensor wiring harness connector. See Fig 1. If voltage is 4-6 volts, go to next step. If voltage is not 4-6 volts, go to TEST C .

Fig 1: Identifying PSP Sensor Circuits & Connector Terminals (Escort & Tracer)
Courtesy of FORD MOTOR CO.

3) Check PSP Sensor Circuit Resistance

Turn ignition off. Disconnect PCM 104-pin connector. Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Ensure PSP sensor is disconnected. Perform the following procedures:
- Measure resistance of PSP sensor circuit between PSP sensor wiring harness connector and PCM connector pin No. 31.
- Measure resistance of VREF circuit between PSP sensor wiring harness connector and PCM connector pin No. 90.
- Measure resistance of SIG RTN circuit between PSP sensor wiring harness connector and PCM connector pin No. 91.
If all resistance readings are less than 5 ohms, go to next step. If any resistance reading is 5 ohms or more, repair open circuit.

4) Check For Short To Power Or Ground

Measure resistance between chassis ground and PCM connector pins No. 31, 90 and 91. If all resistance readings are more than 10,000 ohms, go to next step. If any resistance reading is 10,000 ohms or less, repair short circuit.

5) Check PSP Sensor Operation

Reconnect PSP connector. Reconnect PCM connector. Start engine and allow it to idle. Measure voltage between PCM connector pins No. 31 and 91 while turning steering wheel all the way left then all the way right. If voltage changes from .5 volt to 4.5 volts, PSP circuit is functioning properly. Replace PCM. If voltage does not change from .5 volt to 4.5 volts, replace PSP sensor.