With 4L80 Automatic Transmission

Code 24. 1991 Four wheel drive models:

Circuit Description:
The output sensor is of a magnetic induction type. Gear teeth pressed in 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 four wheel drive vehicles, the transmission output sensor signal on these units goes directly to the PCM.

Code 24 will set if engine is at least 3000 RPM and output speed sensor speed is below 200 RPM for 1.5 seconds, park/neutral not selected.

If the input sensor is not operational at start up, this can cause the output sensor to read zero.

Code 24. 1991 Four wheel drive models:

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. The first test checks the output speed sensor signal at the PCM/TCM.

2. The second test directly checks the output speed sensor operation.


Diagnostic Aids:
Check all connections, especially at the transmission pass-thru connector.

Check input sensor reading. A faulty input sensor during start up will cause the PCM/TCM to read output at 0 RPM.