krahmlow

I've heard that the ignition control module can cause some running problems (shuddering, rough idle, hesitation on acceleration, etc.) when it's failing, so in my ongoing (epic, two-year) attempt to get this car running correctly I replaced the ICU. Unfortunately, the car won't run at all now. When I put the old unit back in, it restarts. I used an aftermarket ICU, so just to be sure, I've ordered a Bosch unit, and it's on the way. But I seriously doubt that's the problem. If the Huco unit weren't compatible, people wouldn't be selling it. Anyway, I'm wondering if anyone has an idea what this could mean? This seems to imply that my problem is related to ignition/timing/engine management. I haven't replaced the Hall sensor, but I can do that. I have to say that I'm beginning to suspect the DME, but testing that thing is kind of beyond my capabilty as a DIY-er (besides, I don't have an oscilloscope). I do know that the previous owner "jump-started" the car with a high-amperage battery charger when it went dead one winter, which the manual warns against.

nickg

did you disconnect the battery when you installed it? it just may have been DOA...it happens

krahmlow

Yeah, it could have been DOA. The Bosche unit will be here on Fri., so I'll know more then.

I'll add that I've already done the vacuum lines, hoses, and seals, idle control valve, "retracked" and tested the AFM, replaced the 02 sensor, had the injectors cleaned and tested, replaced the fuel filter, checked the fuel pressure, replaced the DME temp sensor, done a compression test (but not a leakdown), rebuilt the cylinder head, replaced an anti-knock sensor that was corroded, cleaned all the grounding points that I could find, replaced the speed sensor on the clutch housing, replaced cap and rotor, plug wires, plugs.

944bucky

How about a little background on this ICU you speak of? Where is it, what's it do...that sort of thing.

krahmlow

It's only on the S and S2, as I understand it. It's a small plug-in unit on the inside of the driver's side fender in the engine bay, just over the AC coolant nozzle. It mounts onto a 3X3 flat piece of metal (a heat sink). To be honest, I have no idea what it does, exactly. I wouldn't mind learning either. Maybe that would help the diagnosis.

Jfrahm

It switches the coil current on 16v 944s and 32v 928s.

944bucky

Please tell me more Frahm, is it a resistor? I though that part was called the ignition final stage or something. Really I'm not positive about that. I tested a friends last weekend on my osciliscope, the readout was in accordance with the 944S DME Test manual, but I never figured out what it's purpose is or how it works.
-Krahmlow, I do remember that it required a heat condive gel on its mating surface. I'll look the bosch part number up when I get home tonight and let you know.

nickg

same as a vw btw...like $15 at mosty p[arts stores

greenshoes3

I'm really interested in this now, my 16v does act like its going to quit while idling and has fluctuating idle, my friend told me to spray carb cleaner somewhere, could someone please pm me on how you use carb cleaner to fix the fluctuating idle. thread starter, how many miles are on your car?

944bucky

Here's a little information on the ICU. Sorry for the run about but I don't know how to upload pics yet. I think I need to pay for a membership. Anyways, at my registry site http://registry.rennlist.com/registry/profiles/944bucky there is a picture from the 944S DME test Technik.
Anyways the heat conductive paste is Bosch No. 5 942 860 003 It says too keep it off of paint.

gtroth

That paste is just ordinary heat sink compound. Get it at radio shack, etc.

I beleive the ICM is just an array of power transistors. When I replaced mine, the car came off throttle more slowly to idle than it did before. But it did this for just for a few days. So it's got some more complex interaction with the DME than I can figure out.

What exaclty is she doing, there, krahmlow?

944bucky

nice black box problem, I'll check the wiring diagram.

944bucky

It looks like it functions like points in an older style ignition would, i.e. it tells the coil when to discharge. It has 5 connections, 1. output signal from DME, this should be the signal that can been seen at my registry a square wave. 2. Ground/shielding for final stage, also from the DME. 3. Ground to body, right below the icm attachment. 4. power from the ignition switch i.e. the key. 5. ignition coil.

I wish I could post this diagram instead of describe it, but the power from the ignition at 4 above branches to the coil, then becomes number 5 above to the icm. It also has an identification symble identical to the dme.

-|<

Try to imagine with no spaces and the bottom right line having an arrow at the end. Makes me think CMOS logic for some reason, but I'm not sure.

944bucky

After some though I guess I can convince myself of this: for a coil to function it needs an alternating current. The ignition switch provides a constant supply to the coil but the coil can either send power to the icm or through the trasister portion to the plugs. If the icm is low or off, then the power from the coil is sent to ground and not throught the transformer. This would explain the need for heat disapation. When the dme goes high or on, it changes the switch in the icm so that there is a high impedance or infinite resistance or open circuit, causing the power back at the coil to go to the transformer and plugs. This switching action would also provide the changing current needed to cause the transformer to work.

krahmlow

944bucky: thanks for checking this out. I'm crap with electrical diagrams/theory, but I'm following the basics.... I think. Interestingly, the replacement has an extra pin (seven altogether), whereas the original only had 6, with an empty slot for the 7th. Of course, that doesn't mean it's incompatible with my car, but....

gtroth: When I switch in the new unit, the car only turns over. Doesn't even attempt to fire. Though it turns over faster, which is interesting. When I put the old one back in, a funny thing happens. When it first starts to fire, it almost seizes, like the timing is off or there's a pre-ignition problem. Then the car seems to correct something and the engine starts up. And it runs like s---t. Rough idle (shuddering), occasional misfires, hunting idle, hesitation on acceleration.

The new Bosch ICU didn't get here as planned (back-ordered), so I won't be able to re-check that 'til next week.

I should add that I may have missed a grounding point when I was cleaning things up. The diagram shows an "Engine block/DME" ground, somewhere near the grounding point on the clutch housing, but I don't remember seeing it when I was in there. Anyone know exactly where I can look for this? My car is reassembled and I can't really see in there.