Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Vehicle Speed Sensor (VSS) Check
Vehicle speed is sensed by a Vehicle Speed Sensor (VSS). The VSS is supplied with 12 volts from ignition and ECM. Output from the VSS is a "toggling" of voltage. The faster the vehicle speed, the more rapidly the signal "toggles." VSS input is used to calculate the amount of EGR and also TCC engagement.
The VSS will turn "ON" a CEL only if the ECM does not receive a VSS signal for at least 26 seconds under an MVS load of 25-50%. The CEL will turn "OFF" as load falls below 25%. CEL will turn back "ON" if load exceeds 25% again for at least 10 seconds. A VSS generated CEL is load sensitive and will go "ON" and "OFF" with load.
- This step checks for a VSS signal. Voltage will vary from less than 1 to more than 10 volts as wheel is turned slowly. The faster wheel is rotated, the less variation.
- This step checks for battery voltage to terminal "C" of I.P. harness connector.
- This step checks for ground at I.P. harness connector.
- This step checks for VSS output signal from ECM. Voltage will be more than 10 volts, if circuit and ECM are okay.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.