Speedometer Inoperative / Improper

Circuit Description:
The Vehicle Speed Sensor uses a permanent magnet Alternator to produce an Electrical signal of 24000 pulses per mile representative of vehicle speed. This signal is sent to the PCM via CKTs 400 (VSS HI) & 401 (VSS LO) where the PCM buffers the signal, and feeds the IPC a pulse train of 4000 pulses per mile via CKT 817. The IPC supplies system voltage to CKT 817, and the PCM pulses the circuit to ground to transmit the vehicle speed. The IPC uses this data to Calculate and display vehicle speed.

Test Description:
1. The PCM filters the vehicle speed signal and transmits it to the IPC. If DTC PC 0500 is set, this indicates that the PCM is not correctly Able to read the vehicle speed signal. Diagnose DTC PC 0500 first.
2. If ID03 (vehicle speed to IPC) reads zero, and DTC PC 0500 is not set, the Malfunction is between the IPC and the PCM. Checking voltage at "C4" will determine if CKT 817 is being pulsed.
3. Since D03 is correct vehicle speed, the IPC is receiving and processing vehicle speed data, but is not displaying correctly. Therefore the IPC should be replaced.
4. If vehicle speed Always reads 8% faster than Actual speed, the IPC may be operating in export mode.

Troubleshooting hints:
If D03 is correct while gage fluctuates, replace IPC. If IPD03 fluctuates with the gage, ensure good terminal contact at all Electrical connection. Attempt to duplicate conditions under which failure was noted, while manipulating related wiring. If intermittent fluctuations occur, ensure proper routing of CKT 817.