Powertrain Controls - ECM/PCM

DTC 24 Chart:

Vehicle Speed Sensor (VSS) 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 generator that is mounted in the transaxle. The permanent magnet generator produces a pulsing AC voltage signal whenever vehicle speed is over 5 km/h (3 mph). The AC voltage level and the number of pulses increases with vehicle speed. The PCM then converts the pulsing voltage to mph. The VSS output can also be measured by using a digital voltmeter on the AC scale while rotating the drive wheels.

The PCM then supplies the necessary signal to the instrument panel to operate the speedometer, odometer, and the cruise control module.

DTC 24 WILL SET WHEN
- Vehicle speed is less than 3 km/h (2 mph).
- Engine speed is between 1700 and 3600 RPM.
- TP sensor is less than 2%.
- Not in park or neutral (3T40).
- DTC 21, 22, 33, or 34 is not set.
- Engine vacuum is between 72-80 kPa (21-24 in Hg).
- All conditions met for 5 seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
The Malfunction Indicator Lamp (MIL) will illuminate, and the cooling fan will enable.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. The permanent magnet generator only produces a signal if drive wheels are turning greater than 3 mph (5 km/h). This step checks for a hard or intermittent condition.
2. Check EPROM for correct application before replacing PCM.
3. Check CKT 400 and CKT 401 for proper connections. Make sure they are clean, tight and the harness is routed correctly.

DIAGNOSTIC AIDS
- A Tech 1 scan tool should indicate a vehicle speed whenever the drive wheels are turning greater than 5 km/h (3 mph).
- (A/T) A faulty or misadjusted Park/Neutral Position PNP) switch can result in a false DTC 24. Use a Tech 1 scan tool and check for proper signal while in drive.
- Refer to Chart C-1A Park/Neutral Position Switch Diagnosis. Chart C-1A Park/Neutral Switch Diagnosis
- DTC 24 can be falsely set if engine is "brake-torqued" in gear for 20 seconds.