Sorting out dodgy idle - AFM and ICV (exciting stuff)

[Dr Firefly]

Hey,
Been trying to work out the reasons for my car idling badly, and not catching the revs when I take off the accelerator (sometimes stalls).

Went through my trusty Haynes manual, and tested the AFM - apparently the resistance on the middle two pins (later Motronic model) should increase steadily as the air flap is opened? On mine it went up from 0.569 to 1.06, then down to 0.45. Is this definitely wrong? It strikes me as common sense that if the AFM's giving the ECU the same resistance at two different points, it's going to confuse it, maybe explaining the pulses of power I sometimes get with the throttle on two.

But the situation's complicated by the fact that the wrong voltage seems to be going into the ICV, though. The book said that there should be 10v between the middle and each of the outer pins on the electrical connecter going into the ICV, but on one the voltage was only 3V.

So do you reckon it's the AFM, and the faulty reading from that's causing the wrong signals to the ICV? At a bit of a loss,.

My plan at present is to change the AFM for one on a car my local garage is scrapping, and see if that sorts everything. Sound like the way to go, or is there something much more obvious I should do first?

Cheers, Ian

[Brianmoooore]

AFM should be switched off by the throttle position switch at idle, so check out the TPS.
Correct way to check an AFM is to apply a fixed voltage, of say 5V, across pins 3 and 4 of the AFM, and monitor the voltage across pins 2 and 4 as the flap is opened.
The voltage needs to be monitored with an analogue voltmeter or preferably an oscilloscope, and should be a smooth transition with no spikes.

[Dr Firefly]

So you reckon I won't have gotten any meaningful readings with just a normal circuit tester then checking the resistance? Would failure of the TPS explain the problems do you think? Is it likely to have failed. Can get hold of an ICV, TPS and AFM (all working) off of a 325 - will they work fine? Just thinking of dropping the whole lot in at once and seeing what happens. Though I suppose that's not a good way of diagnosing the actual fault...

Cheers, Ian

[Brianmoooore]

An E30 with a duff TPS will never idle properly. Check out this, first by listening for a click just as the throttle comes off its stop (and again at 2/3 throttle), and then check with a meter (use the forum search function for details).
Your method of testing an AFM will show up a badly duff one, but the method I've mentioned will show up smaller faults.
I take it you've checked the rubber intake hose carefully for splits.

[Dr Firefly]

The rubber intake hose had a massive split in it that had obviously been badly taped up a long time before I bought the car. Putting a new rubber boot in was one of the first jobs I did on it.

I'll check out the throttle switch then. I take it that if the clicks happen that's good? I'll have a trawl through the threads for how to use the meter on it, or see if the Hayne's manual has it in.

Thanks - were you a BMW mechanic? Just u seem to know them pretty well.

Ian