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Rev Limits

Posted: Sun Jan 03, 2016 12:44 pm
by RandyM
Consistent problem at higher speeds under hard acceleration, in 2nd or 3rd gear(auto trans): My tach tops out at about 4750 rpm, past that it drops way low swings wildly up and down, engine rpms seem to stay at upper range not sagging with tach, but definely don't go higher. Am I experiencing an ECU based rev limit? Saw mention of a rotor based rev limiter in my forum search but have no clue about that. Could it be a combo of ECU rev limit and funky tach/cable? Are the tachs generally accurate? If it's all working properly, does anyone have experience futzing with changing the rev limit resistor in the ECU? Thinking I'd buy a spare ECU for monkeying with.

You see there's this mountain road with limited passing space and I need to feel more secure and not have the engine give up when I need it most.

TIA and Happy 2016!

1981 528i auto trans, California car

Ignition Control Module

Posted: Mon Jan 04, 2016 3:12 am
by John in Simi Valley
You are describing symptoms of a defective Ignition Control Module.
It's the small aluminum electronic box located in the engine compartment on the right side. Two screws fasten it in front of the right wheel well.

BMW P/N 12141267459
Rmfd control unit transistoriz.ignition


My parts bin is fresh out of these, otherwise I'd mail you one for testing.
Early BMWs from 1975 through 1983 (I think) had these, so 3,5,6 & 7 series L-Jet ignition used them. It's part of the distributor ignition circuit.

Posted: Mon Jan 04, 2016 6:16 pm
by Mike W.
There is a built in rev limiter in the ECU, it limits fuel over 6400-6600 range, clearly not what you're getting if your tach is anywhere near correct. I did recently have a problem where the tach was way off, almost double, and at higher revs would bounce up and down, but the car ran fine and didn't cut out.

The module John referenced was used on E12 528is and 633/733s thru 81, but not 3 series or 3.0 engines. I think I have a couple if that seems to be the problem. In the recent past I've seen them for sale at not bad at all prices and just stupid expensive prices. The latter seems to be the current pricing.

Posted: Mon Jan 04, 2016 6:57 pm
by RandyM
You guys are awesome! Thanks. I'll check the tach first, then hunt down an ICM if necessary.

Posted: Mon Jan 04, 2016 10:20 pm
by RonDwyer
I have an ICM from an 80 528i

Posted: Tue Jan 05, 2016 12:23 am
by RonDwyer
The price on them on Real OEM is incredible. I wonder if Programa rebuilds them?

Posted: Tue Jan 05, 2016 1:48 am
by Mike W.
RonDwyer wrote:The price on them on Real OEM is incredible. I wonder if Programa rebuilds them?
They do, but the price is still just slightly less incredible.

Re: Rev Limits

Posted: Sat Dec 28, 2019 9:24 am
by 528i-1981
Programa still repairs these as of today for $95 plus shipping. The rebuilt OEM units are being sold by them for $655. Yikes. Mine evidently died but had a spare (hat tip Lock). I'm going to repair my original while I still can. Used ones on eBay are north of $100.

Re: Rev Limits

Posted: Fri Jan 17, 2020 10:03 am
by billy bee
What's the failure rate on these ICM's? Should I get a spare for our race car?