Code 24 - Output Speed Sensor Low (1991 2WD - "C" & "K" Series): Notes

Fig 1: Output Speed Sensor Component Diagram (1991 2WD - "C" & "K" Series)

Courtesy of GENERAL MOTORS CORP.

The speed sensor, which is a Permanent Magnet (PM) generator, provides Digital Ratio Adapter Control (DRAC) with vehicle speed information. PM generator, mounted in transmission, produces a pulsing voltage signal whenever vehicle speed is more than 3 MPH. Voltage level and pulse increase with vehicle speed. DRAC converts PM generator signal to a pulsing signal by grounding circuit 437 (Brown wire) 2000 times per mile. Control module uses this signal in calculations to determine vehicle adjustments.

Code 24 will set under following conditions: vehicle speed is less than 20 MPH, transmission is not in Park or Neutral, and engine speed is more than 3000 RPM. All of these conditions must be met for 1.5 seconds. PM generator only produces voltage signal if drive wheels are turning faster than 3 MPH.

1. This step tests for voltage variation while wheels are turning. Voltage will be low at low wheel speeds (4-6 volts at 20 MPH).
2. This step tests operation of output speed sensor.
3. This step checks for faulty DRAC, or open circuits 821 and 822.