Powertrain Controls - ECM/PCM

Chart 72:

Vehicle Speed Sensor Wiring Schematic:

CIRCUIT DESCRIPTION
A vehicle speed signal is provided to the Powertrain Control Module (PCM) by the Vehicle Speed Sensor (VSS). This sensor produces a pulsing waveform voltage whenever vehicle speed is greater than 2 mph. The voltage level or amplitude and the frequency of pulses increases with vehicle speed. The PCM conditions this signal and calculates the vehicle speed. The PCM will then supply the signal to the instrument panel (4000 pulses per mile).

DIAGNOSTIC TROUBLE CODE (DTC) 72 WILL SET WHEN
^ Transaxle not in park or neutral, no DTC 28, engine running and vehicle speed changes more than 33 mph in 2 seconds.
OR
^ Transaxle in park or neutral, no DTC 28, engine running and vehicle speed changes more than 45 mph in 2 seconds.

ACTION TAKEN (PCM) WILL DEFAULT TO
The Malfunction Indicator Lamp (MIL) will not become illuminated, the cruise control will become inoperative.

DTC 72 WILL CLEAR WHEN
A current DTC 72 will clear when the PCM begins receiving VSS pulses. A History DTC 72 will clear after 50 consecutive ignition key cycles without a current DTC 72 being stored.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. The sensor will produce a signal and vehicle speed only if the wheels are turning faster than 3 mph.
2. if circuits 400, 401 and 817 are OK, and if the speedometer works correctly, DTC 24 is being caused by a faulty PCM or an incorrect program.

DIAGNOSTIC AIDS
Tech 1 scan tool should indicate a vehicle speed whenever the drive wheels are turning at greater than 3 mph. A problem in circuit 817 will not affect the VSS input or the readings on a Tech 1 scan tool.
Check circuit 400 and circuit 401 for proper connections to be sure they are clean and tight, and the harness is routed correctly.
Refer to Computers and Controls / System Diagnosis / Procedures / Diagnosis by Symptom / Intermittents. - Intermittent Malfunctions

NOTE: DTC 72 is set when the PCM detects a loss of the VSS signal as the vehicle is moving at road speeds. This would typically indicate a hardware or circuit failure rather than a software or calibration error.