Powertrain Controls - ECM/PCM

DTC 24 Chart:

Vehicle Speed Sensor Circuit:

Circuit Description:
Vehicle speed information is provided to the Powertrain Control Module (PCM) by the Vehicle Speed Sensor (VSS), which is a Permanent Magnet (PM) generator mounted in the transmission. The PM generator produces a pulsing voltage whenever vehicle speed is over about 3 mph. The AC voltage level and the number of pulses increases with vehicle speed. The PCM then converts the pulsing voltage to mph, which is used for calculations. The vehicle speed can be displayed with a Tech 1 scan tool.

The PCM supplies a signal, on CKT 817, to operate the speedometer and the odometer. The VSS signal is also sent to the cruise control module.

DTC 24 Will Set When:
Engine speed is between 2200 and 4400 RPM, TP sensor voltage less than 290, low load condition (low MAP voltage), high manifold vacuum, transaxle not in park or neutral, VSS less than or equal to 3 mph. All conditions met for 4 seconds.

Action Taken (PCM will default to):
The vehicle will default to 3rd gear, the transmission will have maximum line pressure and the MIL will become illuminated.

DTC 24 Will Clear When:
A current DTC 24 will clear after each ignition key cycle. A history DTC 24 will clear after 50 consecutive ignition key cycles without a current DTC 24 being stored.

Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. Disregard DTC 24 that sets when drive wheels are not turning. This can be caused by a faulty PNP switch circuit. The sensor only produces a signal if drive wheels are turning faster than 3 mph.

2. If CKTs 400, 401 and 817 are OK, and if the speedometer works properly, DTC 24 is being caused by a faulty PCM, or an incorrect program. If the PCM is faulty and must be replaced, the new PCM must be programmed, also, note that the ESC module will have to be transferred when replacing the PCM.

Diagnostic Aids:
Tech 1 scan tool should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph.

A problem in CKT 817 will not affect the VSS input or the readings on a Tech 1 scan tool.

Check CKT 400 and CKT 401 for proper connections to be sure they are clean and tight, and that the harness is routed correctly. Refer to "Intermittents" in "Diagnosis By Symptom." - Intermittent Malfunctions

- A faulty or misadjusted Park/Neutral Position (PNP) switch can result in a false DTC 24. Use a scan tool to check for the proper signal while in a drive range. Refer to CHART C-8A for the PNP switch check. Chart C-8A Trans Range Pressure Switch Circuit Check