VEHICLE SPEED SENSOR (VSS) AFFECT ON PERFORMANCE

TO: ALL CHEVROLET DEALERS

When servicing a vehicle speed sensor or speedometer head, it is necessary to repair or replace the unit as soon as possible. The vehicle should not be operated without a vehicle speed sensor. The VSS input to the EFI system affects three (3) different functions of the ECM: idle air control, torque converter clutch and canister purge.

If the vehicle is driven without a vehicle speed sensor or speedometer, the torque converter clutch will not operate and gasoline fumes will not be removed from the charcoal canister.

Rough idle, improper idle speeds (high or low) and surging can also be caused by an inoperative or disconnected vehicle speed sensor. The ECM determines the position of the idle air control conical valve by seating the IAC fully into the throttle body housing at approximately 30-40 mph. Without a vehicle speed sensor, the ECM cannot seat the valve for reference. Once this reference point is lost, the ECM can no longer control the IAC accurately.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.