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I have been following this discussion with much interest. I am attempting to get my '89 760 idle set properly. The problem I have is that grouding the test point does nothing. I have cleaned the throttle body and IAC valve. Prior to this, the idle would occasionally drop so low that it would stall - usually when creeping along in traffic.
I have been playing with the IAC and trying to adjust the idle. Grounding the test point does nothing and adjusting the idle screw with the IAC in place seems useless. I tried the 'feel' test to see if the IAC is alternating between open and close, and could feel nothing which surprised me. I know it is working because if I disconnect it (engine off) it ends up in some random position and the engine idle is random upon restart. I took my stethascope and listened to the IAC and heard it humming, estimate 60 to 100 hZ. This is easiest to hear with the engine not running with key in position two. This isn't what I was expecting. I was hoping to hear the armature clicking stop to stop.
My theory at the moment is that the valve is not opening and closing at this speed, as it is not likely it could oscilate that fast and would make a louder racket. I believe that maybe one coil is fully grounded and the other is being pulsed with a varying duty cycle which presents an opposing magnetic force of a variable magnitude. This would give full 'analog' control to the position of the armature. (Could be both coils are receiving complimentary duty cycle pulses.)
I have been trying to collect bits and pieces defining the output pins of the ecu. (LH 2.2) This may allow me to verify the above 'theory' as I am not a big fan of piercing insulation of wires.
When I had the throttle body off for cleaning and adjustment, I forgot to verify that the idle position switch contacts are working. I have the click set as per the FAQ, I just don't know if the insides are working. I suspect that if the ecu must see "idle on" if it is going to close the IAC for base idle adjustment. Any comments? Note: I used my listening test as above with the engine off and the hum wes present without regard to the open/closed position of the throttle body and without regard to grounding the test point. I did not try WOT.
One additional note: In this car, the jumper is installed on the two pin "test" connector adjacent to the ecu. I pulled one from another car at the local Pick-n-pull and the jumper is not installed. Makes me wonder what is correct. I might just need to buy a box of donuts and visit my local mechanic one morning.
In the throttle body, I too can see a halo of light around the throttle plate when fully closed (stop screw fully retracted). Is this normal?
Thanks,
Dan
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