Speed Sensor '05 Allison

[Rhoward225]

I am a trailer transporter, and picked up my new

[madmatt]

diagnoising these sensors in pretty in depth and should really be done by a tech. also knowing if any codes were set would help. if you feel like you'd still want to diag it yourself this will help explain how the VSS circuit works and how to diag it.

The signal from the VSS to the TCM is an A/C signal that produces approximately 34 HZ per 1 mile per hour. This signal can be checked using a Fluke 87 (DVOM) set on the A/C voltage scale with hertz function activated and connecting leads to Circuit 821 and 822.

The TCM converts the VSS signal from an A/C sign wave to a digital wave. The TCM sends out 4.5 volts on Circuit 400 to the PCM/ECM. With the vehicle stopped, the Fluke 87 will read 4.5 volts on circuit 400, but once the vehicle begins to move the voltage will drop to near 0 volts. To check the voltage with the Fluke 87 with the vehicle moving, the min/max function should be set to 1 ms, the max should be 4.5 volt and the minimum should be -4.5 volts. The reason the reading goes negative is because the TCM converts the A/C signal from the VSS to the exact digital wave, therefore when checking the HZ signal on circuit 400 the fluke meter can be set on the A/C or D/C signal with hertz mode active. Circuit 400 can be checked by backing out the terminal at the ECM/PCM connector and placing the red lead of the meter to Circuit 400 and the black lead to ground.