PeachParts Mercedes-Benz Forum - View Single Post - 91 300SEL

stevebfl · #3 09-05-2003, 09:27 AM

The idle control system is more intricate than the service info implies. Let me tell you a bit of its characteristics.

You can read that there are an idle switch and a decel switch involved. They let the controller know when to control. But! There also is involvment of the hall speed sensor (on the speedo) and the airflow potentiometer.

I'm not sure how much the hall sensor plays in the idle control decision, it is involved with decel cut-off. The airflow potentiometer does seem to play a significant part in the decision to "Control".

"Control" can be measuered with a tach and the same arrangement used to measure EHA current. The readings will be in the 600-900ma range. Control will mean that a motor running at 650rpm at 600ma will see a 50ma increase when the AC is turned on. It will do a feedback control of engine speed under varying load (trans in gear, PS turning) by varying the current up and down to maintain engine speed.

If "control" is not initiated then the system works in a fixed state, usually around 6-700ma. In this state the idle valve is positioned for a fixed amount of air. This is the state the thing is in while driving down the road. Remember if the idle tried to maintain "control" as the car drove off the throttle (in the valve)would be held shut and a sudden stop would stall the motor.

So, your condition seems to be either one of no "control" or one totally dead (these are not the same condition although they give similar results). If the idle valve is not powered at all on the 103 cars it will assume a "limp home" position. This position is slightly greater airflow than a warm idling motor would see unloaded and in "Control". This will give a slightly greater idle speed unloaded but will of course not compensate for for load. Adding AC and placing it in gear might stall the motor if the steering wheel were then quickly turned.

The initiation of control while needing the idle switch and decel switch relies on the airflow pot for the real decision. It seems that condition will not happen till the pot voltage gets below .7v.

Tons of threads on this.