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A recent tune-up had me scratching my head when I could not set the idle speed on my engine. I thought I had it narrowed down to a unit called the "idle speed stabilizer."
This unit appears to be the equivalent to the auxilliary air valve in the K-Jetronic system. It has a hose that attaches before the throttle body on one end, then through the control unit the air moves and a hose feeds it in behind the throttle body.
Assuming it was the same as auxilliary air, I thought it was more or less an electronic version of the old pull choke systems. So I thought it was connected to the coolant temp sensor.
This is not true. This device serves to keep the system at a steady idle. It will compensate when the engine slows and when it speeds up, behaving appropriately based on what the car is being asked to do. For example, if under load, it will open up and allow more air into the system causing the engine to go faster to keep it from stalling.
So finally, understanding this, I was able to determine whether or not it was faulty. I started it up and got it to operating temperature. I attached an RPM meter. Then I removed one of the electrical connections from one of the injectors. The engine speed dropped momentarily. But then it regained its speed. To be sure, I reconnected this, and disconnected a different injector. The same results. My idle-speed stabilizer is very likely in working order.
Just a tip here, to help you to avoid spending a lot of money replacing something that does not need replacing. This is what I almost did.
Note: you must have your A/C off for this. A/C can require higher idle speeds causing the signal to this unit to change, and thus causing it to behave differently. When in park or neutral, my '86 240 DID respond noticeably to the A/C setting.
This is also called: air control valve, idle control valve, idle-speed actuator, idle speed motor (misnomer since it is not a motor, though it looks like one).
Also, I categorized this as applying to model 200, but this applies to anything with LH2 and later models of LH-Jetronic.
/Jarrod
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