Vehicle Speed Sensor: Description and Operation

Code B124 - Vehicle Speed Sensor Circuit:

Wiring Diagram for Code B124 - Vehicle Speed Sensor Circuit Problem:

CODE B124 - VEHICLE SPEED SENSOR CIRCUIT PROBLEM

Circuit Description:

The vehicle speed sensor uses a permanent magnet generator to generate an electrical signal representative of vehicle speed. This signal is brought to the BCM by circuits 400 (VSS hi) & 401 (VSS lo) where the BCM buffers and amplifies the signal for its prospective uses. Output of the generator can be checked by using a digital voltmeter on the AC while rotating the generator drive (rotate front wheels).

Code B124 will set under the following conditions:

^ Ignition "ON"
^ No code E029
^ No code B334
^ Transaxle in 4th gear (EI 82 "LO")
^ No input to the BCM from the VSS
^ Engine RPM greater than 500

During the time the failure is present the continuous compressor at idle will be disabled, however vehicle speed reading BD60 will display actual sensor reading.


Test Description:

1. Since the BCM amplifies and buffers the speed signal before it is sent to the ECM, code E024 in the ECM will set along with code B124 in the BCM if the P.M. generator or CKTS 400 & 401 have a problem. If code B124 ever sets without setting a code E024 there is an internal problem in the BCM.

2. BCM data value BD60 displays MPH.

3. With the wheels turning, a voltage reading indicates that the P.M. generator and wiring are okay.

4. Check for open in VSS circuit wiring.

5. Check for ground in VSS circuit wiring prior to replacing sensor.



Note on Intermittents:

If an intermittent code B124 is being set, manipulate the related wiring while observing BCM data parameter BD60. If the failure is induced, the reading will jump from a normal value to 0 mph.