BCM Code B124: Vehicle Speed Sensor Circuit: Notes

The Vehicle Speed Sensor (VSS) uses a Permanent Magnet (PM) generator to produce an electrical signal representative of vehicle speed. This signal is sent to the BCM on circuit No. 400 (VSS HI) and circuit No. 401 (VSS LO) where the BCM buffer amplifies the signal for its different uses. Output of the PM generator can be seen by using a digital voltmeter (A/C scale) while rotating the generator drive (rotating front wheels).

Code B124 will set when ignition is on and there is no Code B334 or EO28, no input to BCM from VSS, transaxle is in 4th gear (°F status light), and engine speed is greater than 1300 RPM. During a failure, the compressor at idle will be disabled and vehicle speed data parameter BD60 will display actual sensor reading.

NOTE: Test numbers refer to test numbers on diagnostic chart.

Test Number 1) The BCM amplifies and buffers the speed signal before it is sent to the ECM. Code E024 in ECM will be set along with Code B124 in the BCM if PM generator or circuits No. 400 and 401 have a problem. If there is a problem with the vehicle speed sensor circuit, and no Code EO24 is set, there is an internal problem with the BCM.

Test Number 2) BCM parameter BD60 displays MPH. With front wheels turning, a voltage reading indicates that permanent magnet generator and wiring are okay.

Test Number 3) Check for an open in VSS circuit wiring. Check for a ground in VSS circuit wiring prior to replacing sensor.