Throttle Body: Description and Operation
Throttle Body
CAUTION: Do not attempt to renew or adjust the throttle cable without consulting the relevant procedure. Two procedures apply: reconnection of an original cable or the fitting of a new cable.
The filtered air from the intake duct is drawn downwards through the throttle body to the intake elbow on which the throttle body is mounted. The airflow through the throttle body is varied by the electrically operated throttle valve according to driver demand and engine management system requirements.
The throttle body assembly has the following features:
Conventional cable operated input shaft connected to the accelerator pedal.
Mechanical guard to limit throttle angle should driver demand exceed ECM controlled parameters and a mechanical back-up in the event of a throttle motor failure.
Vacuum actuator for operation of the mechanical guard in cruise control mode.
Throttle valve (blade) to regulate air flow into the engine.
Thermostatic air valve to allow and control air by-pass around the throttle.
DC electric motor to operate the throttle in response to ECM control.
Three sensors (accelerator pedal, mechanical guard and the throttle valve) to relay positional information about the input shaft to the ECM.
Return and control springs fitted to the input shaft, mechanical guard, throttle valve and do motor drive gear.
Throttle Control Modes
There are seven throttle control modes:
1. Normal
2. Cruise
3. Mechanical guard
4. Fixed idle
5. Redundancy
6. Full authority
7. Engine shut-down
Normal mode occurs when the ECM uses the mechanical and monitoring arrangement of the throttle valve to control throttle opening. The ECM does not permit driver demand to be exceeded but it can be restricted to allow for such features as stability/traction control, or engine power limitation.
The ECM determines engine idle speed by controlling the throttle valve motor to vary the blade angle between the non-adjustable preset limits of the mechanical guard and the throttle valve motor.
Cruise mode is engaged as a result of the ECM calculating and controlling the required throttle valve opening via the vacuum system. When the driver releases the throttle pedal, the input shaft disengages from the mechanical guard or the vacuum actuator pulls the guard away from the throttle valve. The throttle pedal will feel light should it be pushed again to accelerate (pressing the pedal further will re-engage the input shaft with the mechanical guard and restore normal feel).
The ECM utilizes sensors to monitor the relative positions of the mechanical guard and throttle valve and adjusts them to maintain the set cruise speed.
Mechanical guard mode permits full mechanical operation of the throttle if the ECM detects that a problem has been encountered with the throttle valve position sensor, do motor, associated harnesses/connectors or the ECM.
Fixed idle mode occurs when any two of the three sensors (two input shaft sensors and the mechanical guard sensor) fail. The ECM will assume values which represent a blade angle of approximately 2.5° and 1200 rpm (unloaded) maximum engine speed.
Redundancy mode occurs when any one of the three sensors (two input shaft sensors and the mechanical guard sensor) fails. The operational pair will be deemed to be safe to continue without intervention, but cruise will be inhibited.
Full authority mode is invoked when a mechanical guard failure occurs which indicates that the guard is stuck fully open. The red warning lamp will be lit and road speed will be limited to 120 kph.
Engine shut-down mode will occur following multiple failures, such as mechanical guard mode following full authority mode (or vice versa) or the throttle blade sticks.