DESCRIPTION
The throttle positioner (TP) system controls exhaust emissions during deceleration by holding the throttle plate at an above-idle position during "OFF" throttle operation. This prevents the fuel mixture from being starved of air and helps control HC and CO emissions. The controlling unit is a vacuum operated TP diaphragm.
The system consists of a throttle positioner diaphragm, a vacuum transmitting valve (VTV), and a thermostatic vacuum switching (TVS) valve.
TP System (Corolla - FWD). Scheme 21
OPERATION
The TP system is vacuum controlled. When the engine is cold, the TVS valve is open and holds the positioner in a set position. When the engine is warm, the throttle open and vacuum at TP port is low, the positioner is in a set position. When the throttle is released, as during deceleration, the positioner keeps it from closing completely until vacuum bleeds through the VTV. The high vacuum present at the TP port will then pull the positioner back and allow the throttle to close completely.
TP System (RWD). Scheme 22
THROTTLE POSITIONER (TP)
- Warm up engine and check idling speed. Check throttle positioner setting speed. To adjust speed disconnect vacuum hose from throttle positioner diaphragm and plug hose end.
- Throttle positioner setting speed is 1400 RPM. If not at specified speed, adjust with throttle positioner screw.
- Check operation of VTV. Reconnect vacuum hose to TP diaphragm, and check that engine returns to idle speed within 2-6 seconds.
VACUUM TRANSMITTING VALVE (VTV)
Blow air through each side of valve. Air should pass freely from side B to side A, but air should pass with difficulty from side A to B. (Scheme 23)
Testing Vacuum Transmitting Valve. Scheme 23
Scheme 24
- Drain coolant from radiator and remove TVS valve. Place threaded end of valve in container of ice water with thermometer.
- With temperature below 45°F (7°C), check that air flows from pipe A to pipe B and E and from pipe C to D. (Scheme 24) (Scheme 24): Testing TVS Valve
- Heat water to 63-122°F (17-50°C). Check that air flows from pipe C to D and E and from pipe A to B.
- Continue heating water to above 154°F (68°C). Check that air flows from pipes C to B and E and does not flow from pipe J to any other pipe. If a problem is found, replace the TVS valve.