Dash lights dont come on - PeachParts Mercedes-Benz Forum

tobybul · #1 10-18-2006, 12:16 AM

I just acquired an 84 300D with just about everything working fine except the dash lights dont come on. I'm talking about the one you dim with the rheostat. Could it be that the rheostat is bad? Can't be all the bulbs are out. Even the lights by the a/c switches dont come on.

sokoloff · #2 10-18-2006, 07:04 AM

Common problem - likely the rheostat. Do a search. Lots of into about it.

Len

winmutt · #3 10-18-2006, 08:00 AM

You can hotwire around the rheostat, its not like the MB dash will ever be tooooo bright

tobybul · #4 10-18-2006, 08:16 AM

Quote:

Originally Posted by winmutt

You can hotwire around the rheostat, its not like the MB dash will ever be tooooo bright

It may come to that. I haven't taken the cluster off yet to know exactly whats wrong but. Because none of the dash lights work I thought theres a fuse that controls all of them. But there does not appear to be one. I guess the rheo acts as the "fuse".

Jim O'Keefe · #5 10-18-2006, 11:25 AM

Don't know how much effort you intend to spend....but I tested mine by simply puting 2 alligator clips wired together on the posts and reinstalling the gauge module...turned on the lights....they worked fine....left it there for the last 4 months and have had no issues

tobybul · #6 10-18-2006, 12:50 PM

Quote:

Originally Posted by Jim O'Keefe

Don't know how much effort you intend to spend....but I tested mine by simply puting 2 alligator clips wired together on the posts and reinstalling the gauge module...turned on the lights....they worked fine....left it there for the last 4 months and have had no issues

am assuming you mean the posts of the rheo behind the cluster. If I decide to jumper the rheo, I will probably solder a wire.

winmutt · #7 10-18-2006, 01:11 PM

Quote:

Originally Posted by tobybul

am assuming you mean the posts of the rheo behind the cluster. If I decide to jumper the rheo, I will probably solder a wire.

Amazingly enough usually just some electric cleaner will do the job.

Zerohour3k · #8 10-18-2006, 02:23 PM

I tell this to everybody who asks this question...

Before you think about soldering a wire onto your rheostat, you might want to remove the clusters indivudally, first. Look at the PCB boards for any breakage in the PCB tracks, or any burn marks. If you find one, re-connect the break with a small piece of copper wire, and solder it into place.

I've seen more issues with the PCB, than I have with the rheostat. Granted, the Rheostats don't hold up too well over the years, either.. but if you're pulling the dash, may as well check both places!

tobybul · #9 10-18-2006, 04:06 PM

Quote:

Originally Posted by Zerohour3k

I tell this to everybody who asks this question...

Before you think about soldering a wire onto your rheostat, you might want to remove the clusters indivudally, first. Look at the PCB boards for any breakage in the PCB tracks, or any burn marks. If you find one, re-connect the break with a small piece of copper wire, and solder it into place.

I've seen more issues with the PCB, than I have with the rheostat. Granted, the Rheostats don't hold up too well over the years, either.. but if you're pulling the dash, may as well check both places!

sounds like a good idea

patbob · #10 10-19-2006, 04:22 PM

Quote:

Originally Posted by tobybul

I just acquired an 84 300D with just about everything working fine except the dash lights dont come on. I'm talking about the one you dim with the rheostat. Could it be that the rheostat is bad? Can't be all the bulbs are out. Even the lights by the a/c switches dont come on.

I recently fixed my '83 300D, but with a wiring diagram and observation I was able to deduce that it was very likely the rheostat before I started. On mine, the a/c lights are on the same circuit, but not controlled by the rheo, so your problem might lie elsewhere (like maybe a fuse.. or maybe all the lights really are just burned out?).

It's a week later and my gasket-sealant rheo fix is still working

. It's not much, but its the only thing that's gone right for me with this car so far.

Pulling the instrument panel was a bit of a pain.. not hard, just took time. Hardest part by far was wrestling the underdash panel back into place before I'd had my coffee. Get a multimeter and test every bulb you can before you replace it -- no sense having to do that again anytime soon.

Hodakaguy · #11 10-19-2006, 05:20 PM

I just replaced the rheostat last night, its a 5 minute job to just change out the rheostat. Mine was toast so in went the new. I took a little longer and removed the instrument cluster and took a burnishing tool and cleaned all the electrical contact posts that are sticking out from behind the cluster. I then applied a light coat of dialectric grease to them and re-installed the cluster. Now my lights work great and my fuel gauge is rock solid. I had a bouncing fuel gauge and replaced the fuel sender since it was shot, the gauge would still bounce once in a while so I reached up under the dash and wiggled the plug and the gauge would jump as I wiggled. After cleaning the contacts on the plug and plugging it back in its now working like new. Something to think about cleaning while your in there.

Hodakaguy

arkie · #12 10-19-2006, 11:52 PM

Easy fix on this one fellows - and you don't have to pull anything. The rheostat gets corrosion/patina and no longer conducts electricity through it. Twisting it on-off anywhere from 25 to 500 times (literally - had a 76 that took nearer to the 500) will usually strip the corrosion off and magically restore the instrument lights.

Hodakaguy · #13 10-20-2006, 07:52 AM

I tried turning the rheostat back and forth a bunch of times and finally got my lights to come on but once I removed it and installed a new one you could see where the old switch was busted and the inner windings of the rheostat was hanging out the bottom, would hae never worked right without removing it and installing a new one.

Hodakaguy

tobybul · #14 10-20-2006, 08:49 AM

Quote:

Originally Posted by patbob

I recently fixed my '83 300D, but with a wiring diagram and observation I was able to deduce that it was very likely the rheostat before I started. On mine, the a/c lights are on the same circuit, but not controlled by the rheo, so your problem might lie elsewhere (like maybe a fuse.. or maybe all the lights really are just burned out?).

I wondered about this too. I have not gone in there yet to check all these. My priority right is to replace the header exhaust pipe. The diesel smoke coming out of it and in the engine comaprtment is quite annoying. Next would probably be the central lock vacuum system, then the dash lights.

I ordered a book online that supposedly will help diagnose the vacuum system. Also, I called my buddy svc mgr at the mb dealership who said he'll try to get diagrams for me. Hope he comes thru.

Bill Ladd · #15 10-20-2006, 08:56 AM

Quote:

Originally Posted by arkie

Easy fix on this one fellows - and you don't have to pull anything. The rheostat gets corrosion/patina and no longer conducts electricity through it. Twisting it on-off anywhere from 25 to 500 times (literally - had a 76 that took nearer to the 500) will usually strip the corrosion off and magically restore the instrument lights.

I did this on my '82 300sd. Probably twisted that thing fifty billiondy times. I now have maybe 75% light out of an already dim setup. It will do until I pull my cluster to fix my recently busted odo and dead clock.

I will probably bypass the rheostat. I don't think I've ever needed to dim any dash lights.

I'll also check the circuit boards for broken traces -- thanks Zerohour.

Bill