Short in Instrument Lights

[Harry]

I have had an intermittent short in my dash lighting circuit for some time now and I was wondering if others have perhaps shared this malady and could offer some hints/advice?  Number 7 fuse inevitably gets blown but they may operate for a while before that happens.  I thought that I must have a ground getting in contact somewhere with a positive but i don't find that to be the case.  I have the tach, center cluster and speedometer out this evening, as well as #7 fuse.  When I check continuity between any of the instrument lamp positive wires and ground, it shows as continuous but perhaps that is because of the bulbs in the tail light and license plate light (right) that remain in the circuit?

Suggestions for a culprit?

Thanks,
Harry

[Harry]

More information.  When I clearly isolate the positive lamp wires going to the tach and speedo, put a fuse back into #7 and turn the lamp switch on, it blows the new fuse.  This tells me that the short is either in the switch or in one of the other lamps powered through the switch.  I guess I can remove the other bulbs in the circuit and begin tracing shorts that way.  Advice?

Harry

[JohnnyC]

While in process of rebuilding my 280SL, I found a few toasted instrument wires. I think they are gray/purple and gray/ blue wires. Upon further tear down, I concluded that the lamp rheostat had broken causing the short.

Cheers and God Bless,
JohnnyC

[Harry]

Thanks JohnnyC.  I don’t think the 230SL has the rheostat but I’ll confirm.

[rwmastel]

My 230SL has the rheostat for dimming the dash lights.  I always assumed all Pagodas had it.  It often doesn't work because it stays in one spot for years and corrosion builds up.

[Harry]

Oh - the tapered knob on the center instrument cluster is the rheostat.  Completely forgot that.  And mine is "crusty" to turn.  Hmmm.

[BobH]

Hello, this could be almost anything fed from fuse 7, front right lights, rear right lights, plus the cluster lights

I would pull the 12 way connector apart, the one connected to the cluster, and unplug the front and rear lights, so there's just the wiring with nothing else attached and then start metering the cables out.  I have read that these rheostats can fail, i guess they could short to ground which would rupture the fuse

There's a good diagram of the connector, i'll try and find it and post it, it will help for tracing the circuits

12 way connector schematic

https://www.sl113.org/wiki/Electrical/InstrumentCluster

[Harry]

I have disconnected all lamps, front and rear, left and right, and the instrument cluster to get those paths to ground out of the picture.  I did notice that the female connector for the right front headlamp had two sockets that showed as continuous to ground.  Same thing on the right rear socket connector.  Not the case for the left side sockets.  It seems to me that only one socket/pin in each connector should be continuous to ground.

Another thing that doesn't agree with the wiring diagram is that fuses 7 and 8 both have power on the top lug when the battery is connected.  I read the diagram to indicate that these fuses wouldn't get power until the rotary light switch has been turned to the parking or headlamp position?  Does that make sense?  Am I missing something?

Thanks again,
Harry

[Harry]

So, I removed the back of the instrument cluster an it looks as though it is not connected to the gage lights at all.  I’ll have to look in more detail tomorrow.