Powertrain Controls - ECM/PCM

DTC 24 (2WD):

VSS Circuit:

Circuit Description:
The speed sensor circuit consists of a magnetic induction type sensor, a vehicle speed sensor buffer module and wiring. Gear teeth pressed on 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 437 circuit relays the transmission output speed which is used to control shift points, line pressure, TCC, DTC 24 and DTC 72. The 1697 circuit relays the vehicle speed which is used to control engine operating functions and DTC 16.

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 i00kPa 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 PCM voltage on CKT 437. With the wheels rotating, the pulsing action will vary voltage. The variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 mph (32 km/H).
2. A voltage of less than 1 volt at the PCM connector indicates that the CKT 437 wire is shorted to ground. Disconnect CKT 437 at the buffer module. If voltage now displays greater than 10 volts, the VSS buffer module is faulty. If voltage remains less than 10 volts, then CKT 437 wire is grounded. If CKT 437 is not grounded, check for a faulty PCM connector or faulty PCM.
3. A steady 8-12 volts at the PCM connector indicates CKT 437 is open or a VSS buffer module fault.


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