Idle Speed/Throttle Actuator - Mechanical: Description and Operation

TBI Unit 295:

Engine Speed Governor Diagram:

DESCRIPTION
The DC engine speed control governor motor is electronically actuated and controlled by the Powertrain Control Module (PCM) through the governor control module (governor motor driver circuit). Governed operation prevents excessive engine speed under light loads, yet allows complete throttle opening for full power under heavy loads.

The components of the engine speed control governor system are the PCM the engine speed control governor control module, and the engine speed control governor throttle drive assembly. The throttle body assembly also contains throttle and governor parts that interface with the engine speed control governor throttle drive assembly.

OPERATION
The model TBI 295 throttle body assembly is unique because the throttle lever cam is not attached directly to the throttle shaft. Instead, the throttle lever cam is attached to its own shaft with a throttle drive lever on the opposite end that engages a fixed lever permanently attached on the main throttle shaft. This split arrangement allows the throttle lever cam to close the throttle valves, but not to open them directly. The throttle lever cam drive only limits the maximum opening of the throttle valves. Based on engine speed and load, the engine speed control governor controls the actual position of the throttle valves, within that range of possible opening.

A network of engine sensors, such as the Throttle Position (TP) sensor, Manifold Absolute Pressure (MAP) sensor. and distributor reference pulses (RPM) provides continuous feedback information on engine operating conditions to the Powertrain Control Module (PCM). The PCM uses the feedback information on throttle valve position and engine RPM to determine whether or not governed operation is needed.

At low engine speeds, below approximately 3800 RPM, the engine speed control governor drive motor is not energized and the governor drive motor lever returns, by spring force, to the parked position. That position is determined by contact between the governor motor drive lever and the shock absorber stop. This allows the throttle valves to cycle with the throttle lever cam position, and no governing action takes place.