Speedometer Inoperative or Inaccurate

Diagnostic Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION

The Vehicle Speed Sensor (VSS)uses a permanent magnet generator to produce an electrical signal 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 818. The IPC supplies system voltage to CKT 818, and the PCM pulses the circuit to ground to transmit the vehicle speed. The IPC uses this data to calculate and display vehicle speed.

DIAGNOSIS

TEST DESCRIPTION

Test numbers refer to circled numbers on the diagnostic chart.

1. The PCM filters the vehicle speed signal and transmits it to the IPC. If code E024 is set, this indicates that the PCM is not correctly able to read the vehicle speed signal. Diagnose Code E024 first.

2. If ID60 (vehicle speed to IPC) reads zero, and code E024 is not set, the problem is between the IPC and the PCM. Checking voltage at "C4" will determine if CKT 818 is being pulsed.

3. Since ID60 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 7% faster than actual speed, the IPC may be operating in export mode. For diagnosis refer to "Speed Display Higher Than Actual Speed". Speed Displayed Higher Than Actual Speed

NOTES ON INTERMITTENTS

If ID60 is correct while gage fluctuates, replace IPC. If ID60 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 818.