Throttle Housing

Fig. 4 Throttle Housing Components:

The throttle housing Fig. 4, controls the amount of air supplied to the engine, but the position of the throttle butterfly is controlled by means of a pulse width modulated motor.
A throttle cable is connected in parallel, for use in emergency limp-home mode.

Normal Mode

Refer to Fig. 4, for component locations.
When the engine is started, a vacuum is raised in the vacuum unit (1). This causes the diaphragm (2) and lever assembly to move upwards, which tensions the spring (3). This action also causes the throttle cable attachment lever (4) to turn, slackening the cable and allowing the throttle butterfly (5) to be controlled over idling to full throttle range by the motor (6). The motor actuates the throttle spindle (7) through a link rod (8) attached at either end to levers on the motor and spindle.

Limp-Home Mode

If a fault should occur anywhere in the system, a safety valve on the bulkhead immediately exhausts the vacuum in the vacuum unit.
The diaphragm rod and lever assembly moves down, tensioning the spring, to bring full spring force to bear on the throttle spindle. Since the motor is not powerful enough to overcome the force of the spring to operate the throttle butterfly, the throttle can only be operated by the throttle cable.
The car is now in the limp-home mode, which is clearly evident by both the additionally force required to move the accelerator and the fact that the TCS CTRL light has come on.