Idle with A/C on

[FreedS4]

What controls the idle speed when the A/C is turned on? I'm trying to trace the cause for my low idle when the A/C is on. Once warm, my car ('89 325i) will idle at 700-800 all day long, under no load. As soon as I turn the A/C on, or headlights for that matter, the idle drops to around 600 and sometimes acts like it is going to stall out when I'm coming to a stop. Also, when the car is cold, it seems to idle a little rough, almost like it has a loupy cam. Once the car gets to a quarter of the way up the stock gauge, 150* on my VDO gauge, it idles perfectly in the 700-800 range. What gives? Thanks in advance.

edit: I've swapped out 3 different ICVs and smoke tested for vaccum leaks, nothing seems to change. Oh, and I've searched.

[Sagaris]

Check for voltage at the ICV plug. check it with A/C off and with A/C on. On another car I had the wires get corroded and break inside the rubber boot.

[jlevie]

On this car the idle speed is controlled by the DME. When presented with a signal pin 40 of the DME connector (from the A/C switch) the DME will raise the idle speed. For the DME to control the idle speed the idle switch in the TPS mush be good and closing when the throttle is closed, the ICV must be operating properly, and there can't be any intake leaks.

[FreedS4]

Check for voltage at the ICV plug. check it with A/C off and with A/C on. On another car I had the wires get corroded and break inside the rubber boot.

Haven't checked it with the A/C on, didn't even think of doing that. I'll let you know what I find. Voltage checked good without the A/C on though.

On this car the idle speed is controlled by the DME. When presented with a signal pin 40 of the DME connector (from the A/C switch) the DME will raise the idle speed. For the DME to control the idle speed the idle switch in the TPS mush be good and closing when the throttle is closed, the ICV must be operating properly, and there can't be any intake leaks.

No intake leaks, as noted in the lack of any found during a smoke test. I've adjusted the TPS according to the Bentley manual. Any other ideas?

[jlevie]

Was the adjustment of the TPS done using a test light or meter, or by feeling for the click? If the latter the idle switch could still be bad.

Have you checked to see if power is applied to DME pin 40 when A/c is engaged?

[FreedS4]

Was the adjustment of the TPS done using a test light or meter, or by feeling for the click? If the latter the idle switch could still be bad.

Have you checked to see if power is applied to DME pin 40 when A/c is engaged?

Adjusted with a meter and it was a brand new TPS, so I'm assuming it's good. I haven't checked pin 40, but I probably should check there. Someone also mentioned checking to make sure my alternator was working properly. I'll check both of them later today.

I noticed on my way to work this morning (lights on, no A/C), that my idle was about 50-100 RPM lower than it normally is with no accessories on. Does that change what my "problem" could be?

[jlevie]

Adjusted with a meter and it was a brand new TPS, so I'm assuming it's good. I haven't checked pin 40, but I probably should check there. Someone also mentioned checking to make sure my alternator was working properly. I'll check both of them later today.

I noticed on my way to work this morning (lights on, no A/C), that my idle was about 50-100 RPM lower than it normally is with no accessories on. Does that change what my "problem" could be?

In that case I think it safe to say that the TPS is working correctly. That leaves the ICV or no A/C signal to the DME as the prime suspects. The idle drop w/o AC tends to implicate the ICV.

[FreedS4]

In that case I think it safe to say that the TPS is working correctly. That leaves the ICV or no A/C signal to the DME as the prime suspects. The idle drop w/o AC tends to implicate the ICV.

I'm leaning towards that direction as well. Which is a shame because I'm now on my 4th ICV. I guess I just can't seem to find a good used one.

[xwill112x]

my 318 idles higher? wtf?

[FreedS4]

Took the car to get the alternator tested and it tested below 30amps, which is about 2/3 too low. So I got a reman alternator that I'll be installing today. Hopefully this will take care of the issue. I also cleaned out the ICV yesterday. When I buttoned everything back together I noticed that my idle didn't change whether I had the ICV plugged in or not. This probably means the ICV is bad, or the wiring to the ICV is bad, correct?

[FreedS4]

Tested the TPS according to the Bentley and everything tested fine. So just to recap, when the car is at operating temperature (150*F or more) it idles right atr 750. As soon as I turn on the A/C, the idle drops to below 600ish and stumbles a little bit. This drop in idle does not occur when I introduce any other electrical load (radio, headlights, etc...). The TPS is a brand new unit, properly adjusted. Voltage to the ICV tests good and the ICV test in the Bentley manual shows that the ICV is good. What else could be causing this drastic drop in idle. My car just started doing this. Up until a few weeks/months ago, whenever I turned on the A/C, the idle jumped up to 900ish, like it should.

[Mike]

Maybe pull the a/c belt and spin the compressor by hand?

[jlevie]

Have you checked to see if power is being applied to the DME A/C On inputs (pins 40 & 41)?

[FreedS4]

Have you checked to see if power is being applied to the DME A/C On inputs (pins 40 & 41)?

I've got to check that still. Dumb question, where exactly would one go about checking that? At the DME?

[FreedS4]

Anyone?