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Old 02-11-2023, 09:09 PM
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Join Date: May 2006
Location: Boston
Posts: 1,410
W126 380SE Odometer does not work after successful gear repair.

I changed the gear set (3 gears - Z12, Z48/16 and Z48/12).
The gears work again. The Odometer moved when I manually turned the driving puls wheel (Z12).
I build everything back but when driving the car the odometer did not work.

I double checked that all the sockets on the back side were connected correctly.
Bust still the odometer does not move!

I remember that I had to bend back a small el. board that was in the way in order to remove the transparent plastic cover (holding the gear sets in place).

Can it be that I damaged the electronics?
And if yes.... how can I figure out where the problem is?

For instance: Is there a way to mimic the puls with a 12V power source ...
- finding the ground contact of the speedometer, put it on minus
- trying to find out which pin of the multi pin socket is the pulse input from the transmission.
- mimic the puls by short contacts with the input pin and watch the small gear moving with each puls

Does someone know which pin is the input from the transmission and what the el. signal character might be?

Thanks for any help, Martin

ps pictures show the odometer before gear repair.

Attached Thumbnails
W126 380SE  Odometer does not work after successful gear repair.-screenshot-2023-02-11-8.46.43-pm.jpg   W126 380SE  Odometer does not work after successful gear repair.-screenshot-2023-02-11-9.07.21-pm.jpg   W126 380SE  Odometer does not work after successful gear repair.-screenshot-2023-02-11-8.47.09-pm.jpg  
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Old 02-12-2023, 08:14 PM
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Join Date: May 2006
Location: Boston
Posts: 1,410
I was lucky to figure it out by chance.

The mechanical plastic gear replacement worked well.
The problem was the step motor not working.

I tried to gather information about the electronic part of the odometer and found some on the German web.

https://www.w126-wiki.de/index.php?title=Tachometer_reparieren

The most useful piece was the description and the circuit diagrams.

I could understand how the tachometer/odometer is working:
the signal from the transmission is rectified and cleared by the electronics and driving the 3 coils... 2 for the step motor and 1 for the tach.
Then I found that the board was repaired before as there were rougher solder points and I figured out that someone had disconnected the board (from the hardware). Then I found that the small vertical circuit board (blocking the access to the gear cover) that described yesterday was removed and soldered back as well.
Then I found that the one of the 4 pins was not soldered correctly and the pin was lose. So I resolved that pin and put the instrument back together and
voila.... it worked again.
I'd say I was clearly lucky having found this information on the web.
I am showing some pictures from the board and the pin connection and the plug

I hope that this information can give some people support at a later time when they are in need.

Martin
Attached Thumbnails
W126 380SE  Odometer does not work after successful gear repair.-screenshot-2023-02-12-8.10.23-pm.jpg   W126 380SE  Odometer does not work after successful gear repair.-screenshot-2023-02-12-4.44.31-pm.png   W126 380SE  Odometer does not work after successful gear repair.-screenshot-2023-02-12-11.02.36-am.jpg   W126 380SE  Odometer does not work after successful gear repair.-screenshot-2023-02-12-9.13.57-am.png  

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