Output Speed Sensor Circuit Check (4WD Vehicles): Notes

WARNING: This page does not describe the selected car, but rather 23 other vehicles, including the 1991 GMC Vandura Special, 1991 GMC Vandura, 1991 GMC Value Van, 1991 GMC Suburban, and 1991 GMC Rally Camper Special. However, it is still accessible from the selected car via links, so may be relevant.

The output sensor is a magnetic induction type. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor. Since vehicle speed is taken from the transfer case on 4WD drive vehicles, the transmission output sensor signal on these units goes directly to the PCM/TCM. If input sensor is not operational at start up, it can cause the output sensor to read zero.

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

This test checks for a speed signal at the PCM/TCM.
This test checks the output sensor directly.