Powertrain Controls - ECM/PCM

Diagnostic Chart:

VSS Circuit:

CIRCUIT DESCRIPTION
The Engine Control Module (ECM) provides 5 volts to the Vehicle Speed Sensor (VSS). While the vehicle is moving, the reed switch within the VSS will open and close four times per each revolution of the speedometer cable. As the reed switch opens and closes, the voltage at the ECM is toggled high and low. The ECM then converts this toggled high/low voltage into a vehicle speed that is displayed in kph and mph.

Diagnostic Trouble Code (DTC) 24 will set if the following condition is met for at least 4 seconds: A constant voltage signal (either high or low) detected at the ECM with the engine running and the vehicle moving.

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

1. Checks to see if malfunction is intermittent.
2. If the VSS is not receiving the 5 volts from the ECM, DTC 24 will set. This step checks for an open or short to ground in the YEL/GRN wire or for a faulty ECM.
3. Checks Instrument Panel (I/P) cluster printed circuit for cracks or bad connections, an open in the BLK/BLU wire or for a faulty VSS.

DIAGNOSTIC AIDS
If the speedometer cable is binding, DTC 24 can be set. Make sure that the speedometer cable is free from any restrictions and that it has a secure connection to the Instrument Panel (I/P) Cluster assembly.

Refer to Diagnosis By Symptom / Intermittents. - Intermittent Malfunctions

INTERMITTENT MALFUNCTIONS
An intermittent malfunction may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connections before component replacement.