Rev Limits

Post your E12 technical questions and comments here. Please, no off-topic posts.

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RandyM
Posts: 90
Joined: Fri Jul 16, 2010 4:01 pm
Location: Ojai, Ca

Rev Limits

Post by RandyM » Sun Jan 03, 2016 12:44 pm

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
1981 Sapphire blue automatic
Hit 200k mi. on Fathers Day 2016!
Family owned since new
Route 150 between Ojai and Santa Barbara

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John in Simi Valley
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Posts: 274
Joined: Thu Dec 20, 2007 10:24 pm
Location: Formally Simi Valley, Ca.

Ignition Control Module

Post by John in Simi Valley » Mon Jan 04, 2016 3:12 am

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

Image

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.
John Savage
1980 528i 5-Speed
FirstFives COTM Editor

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Mike W.
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Location: Sonoma County

Post by Mike W. » Mon Jan 04, 2016 6:16 pm

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.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

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RandyM
Posts: 90
Joined: Fri Jul 16, 2010 4:01 pm
Location: Ojai, Ca

Post by RandyM » Mon Jan 04, 2016 6:57 pm

You guys are awesome! Thanks. I'll check the tach first, then hunt down an ICM if necessary.
1981 Sapphire blue automatic
Hit 200k mi. on Fathers Day 2016!
Family owned since new
Route 150 between Ojai and Santa Barbara

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RonDwyer
Posts: 462
Joined: Sun Mar 01, 2009 7:01 pm
Location: San Francisco Bay Area

Post by RonDwyer » Mon Jan 04, 2016 10:20 pm

I have an ICM from an 80 528i
Ron Dwyer
SF Bay Area

97 528i
03 530i
59 220S

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RonDwyer
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Location: San Francisco Bay Area

Post by RonDwyer » Tue Jan 05, 2016 12:23 am

The price on them on Real OEM is incredible. I wonder if Programa rebuilds them?
Ron Dwyer
SF Bay Area

97 528i
03 530i
59 220S

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Mike W.
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Joined: Wed Aug 30, 2006 12:12 am
Location: Sonoma County

Post by Mike W. » Tue Jan 05, 2016 1:48 am

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.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

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528i-1981
Posts: 252
Joined: Wed Feb 07, 2018 5:50 am
Location: Boston North Shore, MA

Re: Rev Limits

Post by 528i-1981 » Sat Dec 28, 2019 9:24 am

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.
(oo=00=oo) Eric
1981 528i Manual

billy bee
Posts: 18
Joined: Fri Aug 16, 2019 6:34 pm

Re: Rev Limits

Post by billy bee » Fri Jan 17, 2020 10:03 am

What's the failure rate on these ICM's? Should I get a spare for our race car?

--bb

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