A/C causes very high RPM

[gvilkelis]

Hello,

My car is e34 525 m20 '88.
When I drive and switch A/C on I notice an increase in vehicle speed even if I keep acceleration pedal steady. Speed dicreases when I turn A/C off.

Yesterday I have done the following tests:
1. If I switch A/C on when idling, this produces only a slight increase in rpm (up to 900) for a second, and the rpm goes back to 750-800. Which I think is fine?
2. But if I depress gas pedal to keep, for example, steady 1500 rpm and then switch A/C on, the rpm rises up to 4000! It rises slowly - in about 2-3 seconds. Then it stays there for a few seconds and drops back to 1500. if I turn A/C off rpm drops back to 1500 also.

Any ideas whout could cause this behaviour?

[SRR2]

Two things to check first: the Idle Control Valve and the Throttle Position Sensor. The ICV can be removed and cleaned with carb cleaner, Goof-Off, or something similar. They can accumulate crud and fail to "flutter" like they're supposed to. The moving valve part in the ICV should be free to click audibly when you twist it back and forth on its axis. The ICV is a cylindrical object, painted black, about 5" long and 1.5" dia located just forward of the throttle body, with two approx 1" dia hoses connected at right angles to its business end. There's a connector at the other end with three wires in it.

The TPS is a switch on the end of the throttle plate shaft that's supposed to send a signal to the ECU at full open and full closed throttle. If it isn't closing properly, or closing intermittently at the idle end, you can get bizarre engine speeds even with your foot off the accelerator. You can test the TPS with an ohmmeter, however, be advised that they can be intermittent when they're bad. Ask me how I know that. Heh heh... Your TPS is a real pain to get to since it's underneath the throttle body. There are three terminals 2, 3, and 18. No, I'm not joking. 2, 3, and 18. 18 is common. You should have continuity to 2 at idle and 3 at WOT. When the microswitch at the idle end of the TPS ages it can become intermittent and temperature dependent. When I had trouble with the one in my 535 it would test out OK after it had set a while or if it was cold. When it got warm it would fail to close and the idle speed would vary all over the place, up to 3000+ RPM. On the other hand, this behavior should be independent of A/C operation, so it's possible that none of this will help you out. But, these are pretty easy to check and are often the cause of idle issues, so if I were you, I'd start here.

BTW, there is no idle speed adjustment. Don't even try. Speed is set by the ECU electronically through the drive to the ICV.

[gvilkelis]

hi,

thanx for your answer.
However I have a feeling (I'll test it), that ICV and throtle position sensor are working. Because this rpm increase with A/C on only happens when accelerator pedal is depressed. When on idle, engaging A/C does not produce big rpm increase.
So I would assume, that when throtle position sensor detects pressed accelerator pedal, ICM passes control to Motronic, and Motronic fails to keep rpm when A/C is on? Are my assumptions correct? Does idle control manager functions only on idle, or does it anything when accelerator pedal is depressed?

Another question - when throttle position is fixed, and ICM is disconnected, what could cause rpm to go higher?

[gvilkelis]

did one more test.
Disconnected idle control valve and tried to reproduce high rmps with accelerator pedal depressed a little and A/C on. When idle control valve is disconnected, rpm does not go high, it even gets a little bit lower.

So I guess I've isoleted the problem to idle control system.
By the way, when I disconnect idle control valve, the idle rpm gets lower - to 500-600 rpm, but it does not get higher nor fluctates. I am planning to check idle control valve, idle control manager and wiring, as well as TPS. Hopefullly I'll find the cause :)