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.

[Kershaw]

have you tested the alternator?

[FreedS4]

have you tested the alternator?

I haven't but I was beginning to suspect that.

[b*saint]

dme should control that?

[blueapplesoda]

isnt there a switch that signals to the dme when you pressurize the ac lines (turning the pump on by pushing the ac button) maybe thats bad?

[FreedS4]

isnt there a switch that signals to the dme when you pressurize the ac lines (turning the pump on by pushing the ac button) maybe thats bad?

I'll have to look further into this...

[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?

[jlevie]

Or that the idle switch in the TPS isn't closing (bad switch, mis-adjusted throttle stop, or mis-adjusted TPS).

[FreedS4]

Or that the idle switch in the TPS isn't closing (bad switch, mis-adjusted throttle stop, or mis-adjusted TPS).

I'll re-adjust the TPS while I'm instaling the alternator today.

[jlevie]

When you check the TPS, use an ohmmeter or test light. Feeling the TPS click doesn't tell whether the switch is working.

If the TPS has failed (a fairly common ocurrence), drill a 3/16" hole in the bottom cover of the new one to allow the inevitable oil an escape path.

[FreedS4]

When you check the TPS, use an ohmmeter or test light. Feeling the TPS click doesn't tell whether the switch is working.

If the TPS has failed (a fairly common ocurrence), drill a 3/16" hole in the bottom cover of the new one to allow the inevitable oil an escape path.

I'm assuming the write-up in the Bentley is the best way to go about testing the TPS. If not, what's the best way?

[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.

[FreedS4]

Any other ideas?