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Lazy Tachometer?

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PhoenixOp
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Posts: 146
Joined: Mon Aug 17, 2015 11:19 pm
Location: Kuna, Idaho, USA

Lazy Tachometer?

Post by PhoenixOp » Sun Jul 08, 2018 1:40 pm

Gentlemen;

My 1985 E24 (12/4/1984 production) 5 speed, ECE 635CSi, has developed a lazy tachometer. Most of the time it does not work, other times, it seems to come out of it's slumber struggling to start reading RPM's as it should. It almost acts like a bad connection somewhere. :cry:

After staring at the wiring diagrams until my eyes crossed, I can't find a logical place for said bad connection. The wiring seems to come from the motronic control unit straight to the cluster.

I'm hoping someone has had a similar problem and knows what to do with this. I believe there is an engine speed sensor on the transmission (of all places), but because my car is a manual-euro spec, I'm not sure the wiring diagrams I have necessarily apply.

Any and all ideas or suggestions are most certainly welcome.
Mike Drives: :D
BMW's...
85 635CSi (US)
85 635CSi (Euro)
90 750i (Euro)
91 850i
94 540i
94 740i
97 528i
97 740iL
98 740iL
98 750iL
00 528i

Also:
67 Buick Rivera
02 Mercedes Benz SL500
14 Dodge Dart Rallye XT
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wfwright2
Posts: 91
Joined: Sun Mar 10, 2013 4:23 pm
Location: Chesapeake, Virginia, USA

Re: Lazy Tachometer?

Post by wfwright2 » Sun Jul 08, 2018 5:35 pm

My M and previous 635CSi have (had) this feature. It appears to be a symptom of low voltage/poor state of charge in the batteries for the oil change/inspection count down module.
Sorry I have forgotten the real name of the board in the instrument cluster.
I have found with more regular (often ) driving the proper corrects itself until the Ni-cads finally fail.
Wfwright2
Chesapeake, VA 23322
88 M6 Black (5/87)
07 X5 4.8l
02 X5 4.4l I killed, ignored chain rattle, 00 528iT took out by 89 t-bird
87 635csi A. Gone to new keeper, 88 535is Gone to new keeper

User avatar
PhoenixOp
Bigcoupe supporter
Posts: 146
Joined: Mon Aug 17, 2015 11:19 pm
Location: Kuna, Idaho, USA

Re: Lazy Tachometer?

Post by PhoenixOp » Mon Jul 09, 2018 2:27 pm

Thank you for the prompt response.

I completely forgot about the SI board and all it's weird problems. I installed a ProGrama replacement board that did not require batteries in my U.S. Spec E24. Looks like I may have to do the same with this one.

Again, thank you,
Mike
Mike Drives: :D
BMW's...
85 635CSi (US)
85 635CSi (Euro)
90 750i (Euro)
91 850i
94 540i
94 740i
97 528i
97 740iL
98 740iL
98 750iL
00 528i

Also:
67 Buick Rivera
02 Mercedes Benz SL500
14 Dodge Dart Rallye XT
15 Ram 3500 Dually

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