W123 potentiometer question...

[Argospete33]

I just jumped the two ends on the potentiometer and finally have interior lights. I removed fhe whole intrument cluster. I went and reintalled everyhting and now its showing my temp guage at 180c and fuel tank full????

[Argospete33]

Pic from gauges

[Sugar Bear]

There is an open ground connection in the circuit for those two gauges. Could be in the circuit board or a wire/pin to the circuit board.

[Argospete33]

Will remove cluster and check all the connections and try to reinstall. Thanks!!

[Sugar Bear]

Grounds/earth are usually brown, see if you can find a brown wire/pin coming into the circuit board and test it for continuity to ground. If good go the other direction following that line in the board looking for a break or bad connection.

[ykobayashi]

Agreed. See diagram below. My guess is that C152 ground has been disconnected or is dirty. That will throw the bridge circuits in both the fuel and temp gauges out of whack simultaneously. On a w126 it would also throw off the electrical oil pressure gauge.

Let’s see, so if you break C152 you’ll get 100% of the bridge current going through the sensor leg of the Wheatstone bridge and it’ll pin the gauge at max. That’s my theory at least.

https://www.benzworld.org/attachments/screen-shot-2019-08-08-at-7-17-36-am-png.2533166/

[Argospete33]

I opened the cluster and looked for any burn marks on the board. I could not find any issues by just looking at it. Did not see any loose lines? The temp gauge worked fine before I removed it to jump the dimmer switch. May be knocked something putting cluster back in??

[Argospete33]

I reseached the board here and I might have found the problem. The ground here in the pic is disconnected on mine. I will try to resolder it and see what happens

[Argospete33]

I soldered the ground and installed the cluster.
I got the same thing...temp stuck at 120 and full tank of diesel. Everything was working fine before I removed
Clustet to jump dimmer switch

[Argospete33]

Grounded my own wire from behind cluster itself to main ground bundle behind the cluster. It works now!!!!!!!!!!

[Benzadream]

Hey now, great news! Congrats! Always so satisfying to figure it out, fix it yourself and get everything back to mormal and functioning properly. Cheers!

[Sugar Bear]

Good news.

[Argospete33]

Thanks guys..but i spoke to soon ..lol...holy cow
Was driving this morning and everything on the cluster went dead except the oil pressure...and windows also
Oh boy!!!!!...fuses look all good....maybe the ground i connected it too ..will look again

[Argospete33]

All I did was add a ground wire...now nothing works on gauges and the windows also??? What gives ?? I am lost I am no expert on electrical but I gave is a try based on the posts in the forum??

[Sugar Bear]

Suggest removing and cleaning every ground connection you can find, particularly the ones near the battery and adding a braided ground strap from the engine to the chassis. IMO, the factory braided ground appears to degrade significantly.