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Tacho inop.

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ron
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Tacho inop.

Post by ron » Mon Apr 16, 2018 8:00 am

Had an email from a friend with a tacho problem as below:-(Car is '86 M635)

However, have you any thoughts on this. Tacho not working, but speedo, temp, fuel and service indicators all correct and resettable. SI lights all green now. Was working before the winter. Have removed cluster and wiggled/re-inserted the blue connector and coding plugs, pulled out tacho and checked the connectors but still dead. No jerk as you turn on the ignition ( which I recall it usually does). Detached/reinserted the DME plug. There are 4 wires going to the tacho, and each seems to be a pair reading about 300 ohms – so think the tacho is OK. Fully dismantled the board and the only thing of note is that of two new NiCd, one is 1.27V and one is 1.02V, despite being connected to a battery all winter. I believe they are in series to support the SI processor. I’ve buzzed out the tacho right angle plug back to the IC under the SI batteries and all 4 leads are OK, so what next?

Board is not the original, since many years ago, before I understood about the effect of the SI processor on the gauges, I sent it to be repaired and they sent back another one. I think they had to do that since the corrosion was pretty bad..

Any idea what the minimal reassembly is to check if it works? It would be nice to be sure the DME is putting out the signal. Any way to check this? One for Bigcoupe? Or Brucey?


Any bright ideas on what to do next?
They are ALWAYS rustier than you thought!!!!!!
'88 High line.
'85 M #228
'85 M #207
'80 735i

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Hefeweizen
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Re: Tacho inop.

Post by Hefeweizen » Mon Apr 16, 2018 3:58 pm

All indications point towards a broken gear. It is relatively easy to fix this on an electronic speedometer. If you are not confident, there are places who do that for you.
Rainer
85 635CSi (Silver, Daily Driver)
79 633CSi (Red Euro, Project)

"If you think a good European mechanic is expensive, try a bad one!"

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wattsmonkey
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Re: Tacho inop.

Post by wattsmonkey » Mon Apr 16, 2018 6:25 pm

I don't think I've seen the tacho go bad, even with otherwise knackered SI boards. If it's anyone near me I've got a known working cluster to try. I'm guessing a rev counter from a cooking 635 would work for diagnostic purposes - I don't suppose the red line position is the priority here!

Anyway, let me know if I can help with this,

Rob
"Most of it necessary; all of it enjoyable[!]" LJKS
'86 M635CSi #367 Bring the Noise.
'84 635CSi, dogleg fun

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Hefeweizen
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Re: Tacho inop.

Post by Hefeweizen » Mon Apr 16, 2018 7:18 pm

Hefeweizen wrote:
Mon Apr 16, 2018 3:58 pm
All indications point towards a broken gear. It is relatively easy to fix this on an electronic speedometer. If you are not confident, there are places who do that for you.
:oops: Sorry, my bad! It is Monday and I confused it with the odometer. Disregard my Monday statement!
Rainer
85 635CSi (Silver, Daily Driver)
79 633CSi (Red Euro, Project)

"If you think a good European mechanic is expensive, try a bad one!"

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