instrument lights blowing fuse

[piddler]

I have a '67 230SL and am blowing #7 fuse. I think I have chased the problem to the instrument lights. When I turn the rheostat that controls the brightness of the lights, the fuse blows. My problem is that I don't know how to get to the rheostat or lights to check them.

I don't have a maintenance manual. Mine was lost during some of my moves.

Thanks for any help, Pete

[piddler]

I had more time to do some checking. I have the handshake connector disconnected and when I checked the rheostat according to the instructions in the technical section there was no continuity from then pins indicated. That tells me that the rheostat is bad.
I disconnected the right front and rear lighting connections and with the handshake connector disconnected the fuse would still blow when I turned the rheostat.

Out of time to work on it anymore today. Will get back to it next week. Any advice or suggestion are appreciated.

Thanks, Pete

[Pawel66]

with the handshake connector disconnected the fuse would still blow when I turned the rheostat.

I thought this connector connects everything in instrument cluster...

Are you sure you disconnected the correct handshake connector? Sorry for a silly question...

[114015]

Hello Pete!

Do your instruments lights work at all? Do they work if you disconnect and jump the rheostat?

To me it appears as if you have a shortcut because either the rheostat itself or one of the terminals of the grey-blue wires behind the instruments touch ground - which they shoudn't.
Unfortunately the instruments bulbs are held with brass bulb holders to which the grey-blue wire is connected with a little screw in each case - and there are four of them (2 each for the speedo and the rev counter). Those "ends" are exposed o the elements (i.e. without further insulation) und thus just prone to come into contact with ground if you do not assemble your instruments with care. - Just a bad design.

Please check this and also whether your rheosotat touches ground - which it shoudn't.

Good luck !

Achim

[piddler]

Thanks for the responses.

I am pretty sure I have the right connector disconnected. It is the one directly below the steering wheel. I,too, thought that it would disconnect all of the insturment cluster.

I will get back to it next week.

Thanks, Pete

[66andBlue]

There are two handshake connectors, one for the center cluster and the other for the column multifunction switch. When in doubt disconnect both. 🤔
https://www.sl113.org/wiki/Electrical/Rheostat

Also check whether a PO already tried to shorten the rheostat and some solder remnants cause the short.

[piddler]

I disconnected the other connector and sure enough, the first one was the wrong one.

 I now have the rheostat to where I can work on it. Since I don't know if the problem is with the rheostat or the lighting I am going to melt the solder on the rheostat  and remove the wires and short them. If the short is still there then I will look further.

Thanks, Pete

[66andBlue]

Those connectors can fool you - that is why I labeled the in and out cables on one red and on the other blue as you can see in the tech manual.

[piddler]

Good idea.

[piddler]

I think I found the problem. The insulator on the bulb holder was missing on the tach.  I hope that was the problem. I had forgotten what a pain it is to get to the instrument lights.