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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!"

johnn635
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Location: South Wales UK

Re: Tacho inop.

Post by johnn635 » Sun Apr 22, 2018 2:04 pm

Hi All,

It's my tacho that's not working and I know it's the cluster because Ron ( Thank you Ron ) lent me another cluster to try and the tacho works. I do not know exactly how the tacho movement is driven - there are four connections which appear to be 2 and 2. One pair, if you apply a 1.5V battery, drives the needle, and the other changes its resistance when the needle is moved, so I think the movement is fine. One SI battery was low, and it appears that it was not charging, due to a bad ( me culpa) solder joint on the battery. Fixing this means the batteries are charging but the cluster seems to be taking more than the expected few mA when the ignition is off. There is a 50ohm WW resistor which clearly supplies the memory current. There is no immediately obvious dry joint, but it was working before it idled away the winter in a nice warm air conditioned garage. MOT good, and ready for a nice summer outing, but need to 'red line' it!

Experts on the E24 cluster seem thin on the ground now.

John

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Brucey
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Re: Tacho inop.

Post by Brucey » Sun Apr 22, 2018 5:23 pm

I don't know the answer to your problem but a few comments

- there is a plug in the back for the instrument cluster which (IIRC) configures the cluster for different cars. Do make sure that the correct plug is fitted.

- there is more than one version of the cluster.

- I have not seen a detailed diagram of how the tacho works

- when the SI batteries are bad the tacho often stops working for some reason

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

johnn635
Posts: 37
Joined: Thu Jan 28, 2016 4:59 pm
Location: South Wales UK

Re: Tacho inop.

Post by johnn635 » Sun Apr 22, 2018 5:35 pm

Tks Brucey,

The only thing i am wondering is - because the SI batteries went to a ' half-charged state, is the SI processor confused. Typical of uP devices, it might need a processor reset, which I assume is not the same as an SI reset, because I want to clear the SI memory as well.

Not sure if simply desoldering the batteries and then reconnecting will work. worth a try?

John

johnn635
Posts: 37
Joined: Thu Jan 28, 2016 4:59 pm
Location: South Wales UK

Re: Tacho inop.

Post by johnn635 » Fri May 11, 2018 3:31 pm

Hi All,
Delighted to report that systematic fault finding on the cluster revealed the problem. It was a 100u/16v small can electrolytic that decided to go short circuit after 30 years.

Diagnosis was aided by powering the unit from a bench current limited power supply. Measuring voltages and ohming out the tracks suggested the short, confirmed by removing the capacitor. Normally electolytics go open, but there's always one.

In the process, I also worked on the old style SI board. Hadn't realise how much havoc a leaking SI battery causes on the PCB tracks. Sections where copper just disappeared beneath the lacquer finish. Only buzzing out the tracks revealed the extent of the problem.

Not offering my services - must have put 40+ hours into it, but satisfying to resolve a problem.

John

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