| CAUTION | If ECM replacement is instructed in following testing, always ensure ECM connectors and ground circuit are okay. If either are suspect, repair and repeat testing to confirm ECM malfunction. If ECM is replaced, ECM must be programmed with proper ignition key code for engine immobilizer system. For programming procedures, see appropriate ENGINE IMMOBILIZER SYSTEMS article in ACCESSORIES & EQUIPMENT. |
Note. For component location, see appropriate illustration in THEORY & OPERATION article.
Circuit Description
Anti-Lock Brake System (ABS) Vehicle Speed Sensor (VSS) detects wheel speed and sends signals to ABS ECU. The ABS ECU converts signals into a 4-pulse signal and outputs signal to instrument cluster. Instrument cluster converts signal to a more precise waveform and outputs signal to ECM. ECM determines vehicle speed based on frequency of these pulse signals. DTC is set when ECM does not detect any VSS signal while vehicle is in motion. Possible causes are
- Defective vehicle speed sensor.
- VSS circuit is open or shorted.
- Defective instrument cluster.
- Defective ECM.
Diagnostic Aids
Using scan tool, read freeze frame data. Freeze frame records engine conditions when malfunction is detected.