Powertrain Controls - ECM/PCM

DTC 24.:

Vehicle Speed Sensor 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 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 (or equivalent) Scanner.
The PCM supplies a signal, on CKT 817, to the instrument cluster for operating the speedometer and the odometer, the cruise control module, the chime module, and the power steering control module.

DTC 24 WILL SET WHEN:
Transmission output speed is less than 250 RPM when engine speed is greater then 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 WILL DEFAULT TO)
The Malfunction indicator Lamp (MIL) will illuminate, transmission will default to third gear, high transmission line pressure.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. If vehicle speed is indicated at this point, DTC 24 is intermittent. Check harness routing and connections at VSS and the PCM.
2. The rear of the transmission will have to be raised to gain access to the vehicle speed sensor connector.
3. Be sure harness is routed away from hot exhaust components.
If software/calibration is correct and DTC 24 sets, replace PCM.

DIAGNOSTIC AIDS
The Tech 1 (or equivalent Scan tool) should indicate a vehicle speed whenever the drive wheels are turning.
Check CKTs 400 and 401 for proper connections to be sure they're clean and tight and the harness is routed correctly.