Powertrain Controls - ECM/PCM

*** FOR UPDATED INFORMATION NOT INCLUDED HERE, REFER TO TECHNICAL SERVICE BULLETIN # ATRATB302 DATED JANUARY 2, 1995 ***

Diagnostic Trouble Code (DTC) 72:

VSS Circuit:

CIRCUIT DESCRIPTION
The Vehicle Speed Sensor (VSS) circuit consists of a magnetic induction type sensor and wiring. Gear teeth pressed on the output shaft induce an alternating current in the sensor.

The Powertrain Control Module (PCM) compensates for various axle ratios and converts the signal into a square wave for use by the speedometer, cruise control, antilock brakes, and Traction Control (TC) system.

DIAGNOSTIC TROUBLE CODE (DTC) 72 WILL SET WHEN
Not in P/N, transmission output speed changes greater than 1000 RPM, or in P/N, transmission output speed change greater than 2048 RPM. No DTC 28, and engine speed greater than 200 RPM. All conditions met for 2 seconds.

ACTION TAKEN (PCM DEFAULT)
A delayed (soft) landing to third gear. DTC 72 will be stored in the PCM memory but will not turn "ON" the Malfunction Indicator Lamp (MIL).

DTC 72 WILL CLEAR WHEN
The fault condition(s) no longer exist, and the ignition switch is cycled "OFF" then "ON."

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

1. The TC system must be disabled when performing this step. Whenever the ignition key is cycled to the "OFF" position and then cycled back "ON," the TC system will default "ON."

2. Be sure harness is routed away from hot exhaust components.

3. If software/calibration is correct and DTC 72 sets, replace PCM.

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