Powertrain Controls - ECM/PCM

Code 24 Chart:

Vehicle Speed Sensor Wiring Diagram:

CIRCUIT DESCRIPTION:

Vehicle speed information is provided to the Electronic Control Module (ECM) by the Vehicle Speed Sensor (VSS) which is a Permanent Magnet (PM) Generator and it is mounted in the transaxle. 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 ECM then converts the pulsing voltage to mph which is used for calculations, and the mph can be displayed with a "Scan" tool. The function of VSS buffer used in past model years has been incorporated into the ECM. The ECM then supplies the necessary signal for the instrument panel (4000 pulses per mile) for operating the speedometer and the odometer. If the vehicle is equipped with cruise control the ECM also provides a signal (2000 pulses per mile) to the cruise control module

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 24 will set if vehicle speed equals 0 mph when:

- Coolant temperature is greater than 85~C.
- Engine speed is between 2200 and 4400 rpm.
- Throttle Position Sensor (TPS) is less than 2%.
- Low load condition (Low Air Flow).
- Not in park or neutral.
- All conditions met for 3 seconds These conditions are met. during a road load deceleration. Disregard Code 24 that sets when drive wheels are not turning.
- The PM Generator only produces a signal if drive wheels are turning greater than 3 mph.

2. CKTs 400, 401 and 381 are OK if the speedometer works properly. Code 24 is being caused by a faulty ECM, faulty Mem-Cal or an incorrect Mem-Cal.

DIAGNOSTIC AIDS:

- "Scan" should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph. A problem in CKT 437 or 381 will not affect the VSS input or the readings on a "Scan".

- Check CKT 400 and 401 for proper connections to be sure there clean and tight and the harness is routed correctly.

- A faulty or misadjusted park/neutral switch can result in a false Code 24. Use a "Scan" and check for proper signal while in drive (3L40) or overdrive (4L60). Chart C-1A Park/Neutral Switch Diagnosis