Powertrain Controls - ECM/PCM

DTC 24 Vehicle Speed Sensor (VSS) Circuit Low:

Vehicle Speed Sensor (VSS) Circuit:

CIRCUIT DESCRIPTION
The Vehicle Speed Sensor (VSS) circuit consists of a magnetic induction type sensor, Vehicle Speed Signal (VSS) buffer and wiring. Gear teeth pressed on the outside diameter of the output shaft assembly induce an alternating current in the sensor, as the shaft rotates current is transmitted to the vehicle speed signal buffer where it is passed on to the Powertrain Control Module (PCM). The VSS buffer compensates for various axle ratios and converts the signal to a square wave for use by the speedometer, cruise control and antilock brake system. Since vehicle speed is taken from the transfer case on four wheel drive vehicles, the transmission vehicle speed sensor signal on these units goes directly to the PCM.

DTC 24 WILL SET WHEN
Vehicle speed is less than 250 RPM for 3 seconds while engine speed is greater than 3000 RPM. Park/Neutral Position (PNP) not selected.

If the input speed sensor is not operational at start-up, this can cause the vehicle speed sensor to display zero.

ACTION TAKEN (PCM WILL DEFAULT TO)
The PCM will turn "ON" the MIL (Malfunction Indicator Lamp).

DTC 24 WILL CLEAR WHEN
Conditions for fault are no longer present and Tech 1 "Clear DTCs" function is used or 50 ignition switch keycycles have passed with no further faults or ignition switch is turned "OFF" fuse #9 is removed for 10 seconds.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Verifies power and ground to VSS buffer module.
2. The first test checks the vehicle speed sensor signal to the PCM.
3. Test two checks the vehicle speed sensor signal to the digital ratio adapter.
4. Test three directly verifies the vehicle speed sensor signal.

DIAGNOSTIC AIDS
Check all connections especially at the transmission pass-thru connector. Refer to Intermittents. - Intermittent Malfunctions