Throttle Position Sensor: Description and Operation

THROTTLE ACTUATOR CNTRL (TAC) MODULE W/ INTEGRAL TP SENSORS

Throttle Actuator Control (TAC) Module With Integral Throttle Position Sensors:

The L81 engine does not use a traditional throttle cable found on most other vehicles. The throttle actuator control (TAC) module, which is part of the throttle body assembly, contains a direct current actuator motor controlled by the ECM used to move the throttle plate (in either direction) and two throttle position sensors to determine throttle plate position. The throttle plate is held at a 7% rest position to a mechanical stop by a constant force return spring. This spring will hold the throttle plate to the stop when there is no current flowing to the actuator motor. There is another return spring, which creates constant force on the throttle plate only when the throttle plate is moved towards the full closed position.

Each time the ignition is turned On, the ECM will perform a closed throttle test. During this test, it will close the throttle from the 7% rest position to the full closed position and allow it to return. This is performed to ensure the springs have enough force to close the throttle in case of an actuator motor failure.

The ECM will also go through a throttle body relearn procedure 29 seconds after the ignition is turned On with the engine Off as long as certain conditions have been met. Refer to DTC P1526 for these conditions. During this learn procedure, the ECM will move the throttle from the rest to full closed position, then to around 14%. During this period, the TP sensors lower limits, rest position of the TP sensors, the actuator motor force to overcome both spring pressures as well as the return rates of both springs are learned. If any of these parameters are out of range, a DTC will be set. The Scan tool contains the parameter TAC LEARN COUNTER, which can be monitored during the learn procedure. The counter should start at 0 then count up to 9 counts during the 3 second relearn procedure. If the counter starts at 0 and goes to 9, the sensors, springs and motor are OK. A relearn procedure is performed only once per ignition cycle. IMPORTANT: Anytime a throttle body or ECM is replaced, a relearn procedure MUST be performed. Refer to DTC P1526 for this procedure. P1526