I thought I had a problem with the IAC but I am not so sure now. I grounded the base idle test point and the RPM would do what I expect. I repeated it many times.

The problem has to do with when I turn the air conditioner OFF. It is happy when I turn the A/C on, but when I turn it off the rpm drops significantly and then goes back up - even with the IAC connector unplugged. I can't figure out why the RPM would drop as the A/C relay and clutch are disengaging.

I am baffled. I would not worry about it but my engine shut down yesterday in traffic when I turned the A/C off. (I guess I will leave it on all the time)

If any of you have any other ideas I would appreciate it.

Thanks,

R Duke

The IAC on your car is a three wire connection where the center wire is powered via the fuel system relay and the outer two terminals are alternately grounded for varying duration by the ECU. One terminal when grounded closes the IAC, the other terminal opens the IAC. One of the outer wires is always grounded, and thd duty cycle is continuous. If one wire is grounded 42% of the time, the other wire will be grounded 58% of the time.

Unplugging the IAC (with engine running) usually doesn't change idle speed as the IAC will usually stay where it was when unplugged and stay in that postition, giving you an idle speed between 700 and 2000 RPM.

I would check and set the Throttle Position Switch. The IAC will not be operated in idle mode by the ECU unless the Throttle Switch is closed.
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john

John,

OK thanks, now I understand how the IAC motor works. That explains why the door flops open and closed so easily when it is removed. I can check the duty cycle and signal with an o-scope, but I don't think that is what the problem is. I just checked the engine timing and it is around 11 degrees at idle with the a/c on or off (750 or 900 RPM). The thing I can't figure out is why the engine RPM drops so much when the a/c clutch releases. Apparently the ECU tries to compensate which would explain why it surges to about 1500 right after the dip in RPM. Then it goes back to normal. I have checked the throttle switch adjustment, but I will go ahead and trace the signal down to be sure it is working correctly. I don't suppose the clutch could be temporarily binding as it is released ? It is working fine when engaged, and I am getting 40-42 degree air.

Thanks,

R Duke

How is your base ignition timing setting? Turning the A/C on causes the ignition ECU to advance the timing a few degrees which increases idle speed when the throttle is closed. Turning off the A/C (at idle) retards the timing and reduces the idle speed. A base timing setting that is too retarded may cause a stall when A/C is turned off.

HTH
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bagle 87 745 64 Vespa 150

I will go check it again. It had been set to 12 degrees, but I have not checked it since this problem developed. I will check it out.

Thanks,
R Duke