Powertrain Controls - ECM/PCM

DTC 24 (4WD).:

VSS Circuit (4WD).:

Circuit Description:
The speed sensor circuit consists of a magnetic induction type sensor, a vehicle speed sensor buffer module, 4WD Low switch and wiring. Gear teeth pressed onto the output shaft induce an alternating current in the sensor. This signal is transmitted to the buffer. The buffer compensates for various axle ratios and converts the signal into a square wave for use by the speedometer, cruise control, antilock brakes and PCM. The buffer sends two different signals to the PCM. The 1716 circuit relays the vehicle speed which is used to control engine operating functions and DTC 16. The 437 circuit relays the transfer case output speed to the PCM. PCM uses both the transfer case output speed signal and the 4WD Lo switch to determine the transmission output shaft speed. The PCM uses transmission output shaft speed for shift points, line pressure control, TCC control, DTC 24 and DTC 72.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.
These conditions are met during a road load deceleration.

DTC 24 will set if:
^ Not in P/N.
^ CKT 437 voltage is constant.
^ Engine speed is greater than 3000 RPM.
^ Output speed is less than 250 RPM.
^ MAP is between 100 kPa and 255 kPa.
^ TP is between 10% and 100%.
^ All conditions must be met for 6.4 seconds on diesel engines or 3 seconds on gasoline engines.

1. This test checks the vehicle speed sensor signal at the PCM.
2. This test checks the vehicle speed sensor at the buffer module.
3. This test checks the vehicle speed sensor directly.

Diagnostic Aids:
Check all connections at the transmission pass-thru connector. Refer to "Diagnosis By Symptom." Testing and Inspection