Does the AIC turn off completely when throttle is opened (activating TPS switch ) ? assuming the TPS switch is adjusted correctly ( click on/off ). Or does it always run and just adjusts the rate when signal given from ECU ?
Thanks , Schumi 1

With engine running, my 93 940 IAC valve is open all the time and if duty cycle measurments correct opens more at higher RPM.

Finally found 93 940 Craigslist car on used car lot and checked it out yesterday. IAC valve is open all the time though didn't have opportunity to check its duty cycle with dwell meter. My valve makes loud drone noise and this car didn't!

Quick way to check if valve open after TPS idle switch clicks: pull IAC inlet hose off airfilter box nipple, accelerate engine to increasing rpm and check with thumb over open hose end for vacuum. My drone noise also much loader with hose pulled off.

Art B. had great idea to check valve's duty cycle.

If you have dwell meter check your valve's duty cycle and we can compare notes. Quick way to check DC: Fold boot down on IAC harness connection. Bl-y is battery voltage from Regina ECU pin 9. R-SB wire is cycled ground connection from ECU pin 33. Hook up meter, turn ignition on, and if you have old analog dial guage meter like mine, zero meter. Start car and read dwell readings. Should be less than full scale. Duty Cycle is actual reading divided by number at full sale x 100 to make it %. My readings increased to half scale at 3500rpm or 50% DC.

IAC valve uses 3 wire connector but there's no middle pin. I found it easier to hook dwell meter B+ to battery and then only had to work with the ecu controlled ground on the Bl-y wire in the harness connector.

Good luck!

Steve in Charlotte

Nice work Steve! The dwell or duty cycle should be roughly proportionate to the valve's opening. Now that you've confirmed the "drone" is different in your car, it raises my suspicion the frequency is not right. The frequency of the signal to a mechanical actuator being adjusted by pulse width has to be matched to the inertial characteristic of the moving mass, i.e. resonant. The sound may be saying yours isn't.

Of course, you can't measure that with the dwell meter, but you can maybe hear it and feel it without a scope or a counter. Did the hum seem to be a lower pitch than the throb of the working example? Sorry for the hijack; I'll see if I can locate an update in your thread. Last I heard you were looking into a Rex swap.


--
Art Benstein near Baltimore

You spend the first two years of their life teaching them to walk and talk. Then you spend the next sixteen telling them to sit down and shut up.

Art, Still need to locate Rex swap. There's a couple affordable on ebay but one seller insists on sending me out of focus label pictures and I want to see/read both labels. May trip to Columbia, SC this weekend to dig around at Pull-A-Part lot.

Used car lot's car's engine was filthy. IAC valve was greasy lump so didn't investigate much. Pulled valve inlet hose off filter box and determined that valve always open even at high rpm. Did not put finger on valve or hold open inlet hose to ear to sense vibration. TPS switch clicked so assume that part of circuit ok and there was no big jump in rpm with click.

Suspect my duty cycle readings are off. My valve vibrates with the ignition on! Can barely feel it touching valve and can hear it if open end of inlet hose held close to ear; but, turns to loud drone when engine starts. Seems all the same frequency regardless of only ignition on or engine RPM.

With valve vibrating faintly with ignition on, I zeroed dwell meter full scale with pulsating ground rather than engine ground so readings are higher than actual. Will redo. But valve is still open regardless of rpm!

Seems like IAC valve should park when TPS switch clicks but Regina/Rex valve doesn't. Regina/Rex Greenbook says duty cycle supposed to be less than 25% at 2500rpm. Have driven Bosch system 240s since 81, never had a problem and never checked IAC duty cycle or if valve parks at some engine RPM.

Need to copy all this and transfer to my earlier thread re: IAC duty cycle.

Will also report DC with correctly zeroed meter.

Steve

Hello there,
just wanted to know if you found a new AIC and swapped it? Did it solve your problem?
Schumi 1

Hi there, didn't get around to much testing my AIC valve to help you out. My AIC does not "hum" , it does vibrate at ignition on position though . I remember reading a post a while back about someone's alternator wire touching the AIC and causing faulty readings and causing problems. They are sealed units as far as I can tell and you just have to replace them unfortunately. I know a brick shop in Long Island NY that has a lot of used parts for pre 1996 bricks. I once swapped out my AIC with a used one he had in his shop when diagnosing the health of mine so know he has a good used Regina one he would probably sell. hope this helps

the throttle switch basically turns the valve on at idle. there's no need for it off idle. good luck, chuck.

Thanks for the reply, but I am still a bit confused. In all my service manuals it states that "the AIC ONLY operates when throttle switch is closed."
So the question is : When the throttle is OPEN (TPS switch engaged) is it a fault somewhere if the AIC is still running at all let alone too "high" ? (testing with the vehicle is in the "Park" position). I think my AIC is not releasing enough vacuum on my FPR hose, keeping the pressure low on the rail when I give it throttle.

Art , if reading this, I updated my thread about the fuel pressure results..I replied to my original post , the "reply to" feature through email is not working, best to post replies the normal way per each threaded message... thanks, Schumi 1