m103 low idle and stalling when air conditioner on?
 

Hi,

My car idles "perfectly fine" at around 700rpm in gear when the a/c is off but with the a/c is turned on, the engine exhibits erratic idle and rpms jumps between 500 and around 800. I find that the engine often stalls when stuck in traffic and have to put the transmission in neutral to avoid this from happening.

I'm not sure where to start looking for a solution and hope that there is a simple one! :) So far, I have cleaned the ICV, replaced all vacuum hoses, changed O2 sensor, changed the ignition coil, and resoldered the OVP. Is there something else that I should be looking at that I may be overlooking? Thanks for any input you might have and have a great weekend!

It is most likely that the idle system is either not functioning or is in a set position. It takes a set position when the car isn't at idle. It understands the car to be idling when a number of things happen. It looks for the idle switch to be closed, the decel switch to be closed, and maybe most importantly it looks to the airflow potentiometer voltage to be less than .7v.

* It sounds like you've checked or replaced quite a few relevant things. If the voltage to the idle air slide is correct, while the AC is on and the idle is hunting, a possible problem could be a compressor that's gotten very hard to turn. With a good belt and good belt tensioner, the extra drag would slow down an idling 5.0L v8, let alone a 3.0L six. This is a long shot, but if the electronics checks out . . . .

Mr. Brotherton: Thanks for the explanation on how the idle system works. I'm just going off information that I've found in the archives here and have no manuals! I will search for info on how to check the items you mentioned and where the heck they're located!

Mr. Squires: Thanks for the suggestion on checking the voltage. Is the "idle air slide" the same thing as the "idle control valve?" If so, I haven't checked the voltage going to that part yet and will do a search on the correct voltages. I haven't thought about the compressor itself being the culprit and think that would be the job of the local independent tech. :)

* Yes, idle air slide, idle air valve, idle control valve are all names for the same CIS-E component. Hey, why use model and engine numbers, Mercedes nomenclature and locater numbers and be absolutely clear when you can introduce some confusion (sorry, a little sarcasm).
* Measure the voltage at the 2-pole connector (without disconnecting the connector from the air valve). Note the reading when the engine's idling smoothly with the AC off, and then note it again when you switch on the AC.
* Before you start the engine, you ought to be able to reach the AC compressor pulley (inner part) and turn it by hand. It should be fairly easy to turn.

* How does this story end? Does the engine still stall, or do you just drive with the AC off? Every good mystery needs a conclusion.

I would try pulling the plug on the airflow meter potentiometer, then see if the idle smooths out with the air on. Note that this will create other driveability problems, but if idle is smoother, then the pot likely has a worn spot right at the airflow position of the meter for 'air on' idle requirements.

Steve