Powertrain Controls - ECM/PCM

DTC 72 Vehicle Speed Sensor (VSS) Circuit Diagnostic Chart:

Vehicle Speed Sensor (VSS) Circuit Wiring Schematic:

The speed sensor circuit consists of a magnetic induction type sensor, a Vehicle Speed Sensor (VSS) buffer module, 4WD low switch (when needed), and wiring. Gear teeth pressed onto the output shaft induces an alternating voltage into the sensor. This signal is transmitted to the VSS buffer module. The VSS buffer module compensates for various final drive ratios. The VSS buffer module will also convert the Alternating Current (AC) VSS signal into a 40 Pulse Per Revolution (PPR) Direct Current (DC) signal on CKT 437 to indicate transmission output speed. On 4WD vehicles, the 4WD low signal will also be used for adjustment of the 40 PPR signal to the Powertrain Control Module (PCM).




DTC 72 WILL SET WHEN
^ Not in P/N.
- Transmission output speed change is greater than 1000 RPM.
- Engine speed is greater than 200 RPM.
- Conditions met for 2 seconds.
- No DTC 28 set.
^ In P/N.
- Transmission output speed change is greater than 2050 RPM.
- Engine speed is greater than 200 RPM.
- Conditions met for 2 seconds.
- No DTC 28 set.

ACTION TAKEN (PCM WILL DEFAULT TO)
A (soft) delayed landing to second gear and second gear starts.

DTC 72 WILL CLEAR WHEN
The ignition is cycled "OFF" then "ON."

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This test checks for voltage to the VSS buffer module.
2. This test checks the ground circuit to the VSS buffer module.
3. This test checks the Vehicle Speed Sensor (VSS) circuit at the VSS buffer module.
4. This test checks for an output speed signal from the VSS buffer module.

DIAGNOSTIC AIDS
^ DTC 24 will set when no vehicle speed is detected at start off. Transmission Controls With PCM
^ DTC 72 will set when vehicle speed has been detected and is lost.