Idle speed control (ISC) function 9.2.98 - GF30.22-P-0002DA: Notes

ENGINE 104.990 in MODEL 140.032 / 033 

Fig 1: Idle Speed Control (ISC) - Communication Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Function 

The functions of idle speed control and electronic throttle control are integrated in the idle speed control (ISC) control module. The ISC control module is supplied with voltage at ignition ON. The position of the set value potentiometer is changed with the accelerator pedal through the throttle control. The signal voltage of the set value potentiometer is passed on to the ISC control module. The throttle valve position is tapped as a voltage at the actual value potentiometer and is supplied to the ISC control module as an input signal. The positioning motor for the throttle valve is operated through an output stage in the ISC control module. The input signals supplied by the set value and actual value potentiometers are compared in the ISC control module. If the two signals differ, the position of the throttle valve is corrected in line with the set value.

Ignition timing is retarded for a short time in order to be able to achieve an immediate reduction in torque.

Throttle control 

The throttle control linkage is linked by the input lever and the internal spring capsule by means of a loose connection to the throttle valve.

Emergency running system 

In the event of a fault in the ISC, the system is switched off. When the accelerator is depressed, the throttle valve is now operated only mechanically, with the throttle valve being able to open to only about 2/3 of its position.

In the even of faults at the safety contacts, engine speed is limited to about 1200 rpm through the safety fuel shutoff. If the accelerator is not depressed, the idle speed safety contact is closed and the slaved safety contact open.

In both cases it is possible to continue driving the vehicle.