Mother of all wierd idle problems.
 

Got a 91 vert in my garage, that has a wonderful tendency to die when the AC is turned on. Now, before you say "remove your AC", its not my car! Here is a list of things that have been replaced/adjusted.

BAC-Replaced
ECU-Replaced with a Mazda Reman
ACV-Replaced
Water Thermosensor-Replaced
IAT-replaced
TB-replaced
TPS-replaced
Cats-replaced
5thGEAR/Neutral-replaced
Compression tested=good
Coil packs/timing checked

Now here is the deal. No matter what I do, no matter whats replaced, whenever the AC is turned on, the car dies. Only when its warm.

Any ideas?

Thanks in advance!

Jarrett

First thing that comes to mind for me is a bad connection at the BAC.

Do any other electrical things cause the same problem (like the defroster or headlights/brake lights at idle).

when its turned on something is supposed to raise the idle, so something isnt working, dont know what it is though lol guess that makes this a worthless post, but ill see if i can figure it out in a min and repost

Okay, tested that too via the Paul Soakes method. All tests passed.

Nothing else causes the car to die.

what about the ac compressor itself. If you jumper it on while the engine is at idle (and warm) does it try to die then as well?

Not quite sure what that will prove, but I can try it.

remove his AC, kidding, but air bypass solenoid maybe?

Alternator?

If the problem is the compressor (such as it is internally damaged) it could cause the engine to die as well.

The only reason that I ask is that if other electrical devices are not causing the problem, then that rules out the ECU and BAC.

Since you replaced a bunch of other things, I would next be looking at mechanical reasons rather than electrical.

before I start, I am more familiar with the s4, but here it goes anyway

You can look at the voltage at the bac before and after you turn AC on

The DC voltage should go down (this assumes the BAC behaves the same as on a s4), like from 11V to 10.5V.
If this happens, then the ECU is giving proper signal to the BAC.
If this does not happen, then either the ECU is not giving proper signal or the ECU does not receive input that AC has been turned on (from the AC switch).

IF you have PS steering, you can look at what happens when you turn the wheels. The BAC should react similarly to when the AC is turned on.

If everything checks out, then I would follow Mark's advice and look at the AC compressor (it may have a bad bearing or something causing to much stress on engine and causing stall).

Again, this is s4 behavior but s5 should behave same (I think).

Hugues -

Thats where I am at right now... I havent ever heard of the compressor being damaged to the point that it kills the engine. But that doesnt mean its never happened.

hughes, I tested the BAC using the Paul Soaks method, And everything is good. But thanks for the suggestion.

Jarrett

If the compressor's failed internally and much harder to turn than it should be it would probably cause what you describe. With the ignition off you should be able to freely turn the center part of the compressor's clutch. If it's tight or locked that'll be the problem.

my compressor nearly seized on me and whenever i turned it on the motor would stall. check and see if thats it.

Maybe compressor siezed??? Try turning it by hand with the clutch engaged. If siezed, relieve freon for like 5 minutes.

You don't need to engage the clutch, you just turn the centre part of the clutch. It's the outer pulley part that gets magnetised to the inner part. :)

Okay, I am going to check that out tomorrow. I know it engages, but it doesnt really get all that cold. In fact, I really doesnt get cold at all.

Might be worth checking out.

Thanks all!!!

Jarrett

If you over filled the system then it will not get cold due to the compressor stressing out badly (therefore stalling motor). Compressor should not make any wierd noises other than clutch engaging and disengaging.

For like 6 months I thought my compressor was shot. Somone suggested I start dumping the freon out of it (r134 converted). After venting for almost 10 minutes, the A/C worked colder than it had ever. Before it was stalling the car at idle and sounded like a mac truck when cruising.

Hmmm...You know, I havent checked the pressure in the system. Good idea. Thanks guys!