The vehicle speed sensor uses a permanent magnet alternator in order to produce an electrical signal of 24000 pulses per mile. The signal value is representative of vehicle speed. The signal is sent to the PCM through CKT 400 (VSS HI) and CKT 401 (VSS LO). The PCM buffers the signal and sends the IPC a pulse train of 4000 pulses per mile through CKT 817. The IPC supplies system voltage to CKT 817. The PCM pulses the circuit to ground in order to transmit the vehicle speed. The IPC monitors the circuit and uses the data in order to calculate and display the vehicle speed.

1. The PCM filters the vehicle speed signal and transmits the signal to the IPC. If PCM DTC P0500 is set, this indicates that the PCM is not correctly able to read the vehicle speed signal. Diagnose DTC P0500 first. Refer to Engine Controls.
2. IPD03 transmits the vehicle speed to the IPC. If IPD03 reads zero and DTC P0500 is not set, the malfunction is between the IPC and the PCM. Testing the voltage at IPC connector C2 terminal C4 determines if CKT 817 is being pulsed.
3. If IPD03 is displaying the correct vehicle speed, the IPC is receiving and processing the vehicle speed data, but it is not displaying the data correctly. Inspect the IPC connectors for proper contact. If the IPC connectors are OK, replace the IPC.
4. IF IPD03 fluctuates with the gauge, complete the following steps:

4.1.	Inspect the terminal contact at all electrical connections.

4.2.	Attempt to duplicate the conditions under which the malfunction was detected, while manipulating the related wiring.

4.3.	If intermittent fluctuations occur, test for the proper routing of CKT 817.

5. If the vehicle speed always reads 8% faster than the actual speed, the IPC may be operating in the export mode.
6. A loose or intermittent PCM ground can result in erratic speedometer and tachometer displays. Test the PCM and engine ground connection for good contact.