Powertrain Controls - ECM/PCM

Diagnostic Trouble Code (DTC) 24:

VSS Circuit:

CIRCUIT DESCRIPTION
Vehicle speed information is provided to the Powertrain Control Module (PCM) by the Vehicle Speed Sensor (VSS), which is a Permanent Magnet (PM) generator located in the transmission. The PM generator produces a pulsing Alternating Current (AC) voltage. The AC voltage level and the number of pulses increases as the speed of the vehicle increases. The PCM then converts the pulsing voltage to vehicle speed which is used for calculations. The vehicle speed can be displayed with a Tech 1 scanner.

PCM SPEED SIGNAL
The PCM supplies a signal, on CKT 817, to the Central Control Module (CCM), the instrument cluster for operating the speedometer and the odometer, the cruise control module, the chime module, and may be sent to the radio control head, so that the volume of the radio will increase as the vehicle speed increases.

DIAGNOSTIC TROUBLE CODE (DTC) 24 WILL SET WHEN
Transmission output speed is less than 250 RPM when engine speed is greater than 3000 RPM, not in park or neutral, and DTCs 21, 22, 28, 33 and 34 are not set. All conditions met for 3 seconds.

ACTION TAKEN (PCM DEFAULT)
The Malfunction Indicator Lamp (MIL) will illuminate, transmission will default to third gear, high transmission line pressure.

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

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

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

3. If software/calibration is correct and DTC 24 sets, replace PCM.

DIAGNOSTIC AIDS
The Tech 1 scanner should indicate a vehicle speed whenever the drive wheels are turning.

Check CKTs 400 and 401 for proper connections to be sure they are clean and tight and the harness is routed correctly.