Refurbing the Dash Light Rheostat (Caution: For The Purists Out There)
 

My dash and console lights were not working on the '83 240D. After doing some searching I realized that it had to be the rheostat located on the back of the instrument panel. If you own an older MB and none of your dash or console light are working, there's a good chance your rheostat is corroded. What is a rheostat you say? A rheostat is simply a device that allows you to control the resistance in a circuit. In this case, it allows you to control the brightness of the dash lights. Unfortunately, the internal contacts tend to corrode over time and eventually enough corrosion will halt the current flow through the rheostat with the end result being no more lights.

The most common "fix" for a bum rheostat is to simply solder a jumper wire across the two metal strips on the back of the rheostat. This effectively bypasses the internal components, which is good because it gets your lights back. However, in doing so you sacrifice the ability to dim your lights if you want to. The argument for bypassing the rheostat is that, even on "high", the dash lights in these older MB cars are not all that bright. After fixing mine, I agree, but nevertheless, I still wanted to open up the rheostat to see what it looked like inside and, once I did, I realized it wouldn't be a hard fix - so I cleaned up the contacts, put it back together and now it works great. What follows is a DIY on refurbing you dash light rheostat.

Step 1: Remove the instrument panel.
 

Probably the easiest way to remove the panel is to reach up from behind and push it out. You can make the little tools to pull it out, but it really is quite easy to remove the panel below the steering column (3 screws) and reach up there to push it out. Just keep in mind there are some things you will need to disconnect to get it out. Unscrew the speedo cable. Unbolt the oil pressure line (in my '83 240D the nut is 10 mm). Finally, undo the electrical connections. Be sure to keep track of where everything goes.

Note: No starting of the car while the oil pressure line is disconnected . . . ask me how I know that . . .:o.

See the little white box with the two metal strips on it (near the bottom of the panel) - that's the rheostat.

http://hphotos-snc1.fbcdn.net/hs122...._7860108_n.jpg

Step 2: Remove the rheostat from the panel.
 

To remove the rheostat from the panel, simply unscrew the one screw that holds it on. It will not come right off, it is still on the two pin electrical contacts. You will need to pull it straight out from the panel - no twisting or you might break off a pin.

http://hphotos-snc1.fbcdn.net/hs122....1_102678_n.jpg

I used a small flat-head screw driver to pry it just a little to get is started. Go slow, it will come off pretty easily.


http://hphotos-snc1.fbcdn.net/hs122....2_445797_n.jpg

Here are the two pins that the rheostat connects to, be careful not to break them.

http://hphotos-snc1.fbcdn.net/hs122...._6191306_n.jpg

Step 3: Disassemble the rheostat - but watch out for the little springs.
 

To get into the business parts of the rheostat you will need to remove the metal clips that holds the the dial and slider components against the coil and coil housing. Just pull outward on the clips and it will come off easily. You might deform the metal clips a little, but you can bend them back in a little to make sure they do their job once reassembled.

Here is what the rheostat looks like on the inside. I am holding on to the dial part that turns when you spin the little knob on the front of the dash panel to adjust the light brightness (same knob that resets the tripometer, by the way).

CAUTION: Do not attempt to take the rheostat apart in your car or in your driveway or anywhere but over a table top where you can watch for the two little springs that are going to fall out. Once you get the dash panel out of your car, I would take it to where you plan to do the work before you even remove the rheostat from the panel. It is not a pain to deal with these springs, but if you lose just one of these its all over for your rheostat (although you can always do the jumper wire trick).

Okay, it's not going to look like this when you pull it apart, the sprigs and slider will fall out. This is just a staged shot to show you how the slider sits on the dial mechanism (the two little springs are currently underneath it - their job is to hold the slider against the coil when everything is assembled. You can see the coil in the housing to the right.


http://hphotos-snc1.fbcdn.net/hs142....4_689117_n.jpg


Another staged shot to show you how the springs sit in the dial mechanism. The slider is removed and laying in the background. The metal braket that holds it all together doesn't actually go there - i was just using it to hold the dial upright so I could take the picture.

http://hphotos-snc1.fbcdn.net/hs142....6_904596_n.jpg


Here are all the pieces. Do not attempt to remove the coil from the housing as it is attached on one end. Break the coil off and, again, you will be resorting to the jumper method. It's not fragile, just don't pull it out.

http://hphotos-snc1.fbcdn.net/hs142...._4347444_n.jpg


The coil will pull up from the side that it is not attached. Many others have noted the white powder material that seems to build up inside (I cleaned it out of the coil area - see it below the housing.) Not entirely sure what that is. May be that the housing is made out of bakelite and it degrades a little over time. Regardless, if your rheostat isn't working you probably have some of the white stuff along with corroded components.

http://hphotos-snc1.fbcdn.net/hs122...._2203171_n.jpg

Although the finger projection of the slider runs along the coil (which is fairly clean), there are other contact points of the slider that make contact with the little disc in the center. See how corroded it is? We need to clean that . . . and the slider, too.

http://hphotos-snc1.fbcdn.net/hs122...._5458718_n.jpg

Step 4: Clean off the corrosion.
 

I started out with some battery cleaner to help loosen things up a little. Let it sit for a couple of minutes.

http://hphotos-snc1.fbcdn.net/hs122...._1225481_n.jpg


Then I took a small screw driver and scraped off the corrosive coating. See how it wants to be shiny and new again?

http://hphotos-snc1.fbcdn.net/hs122....1_398344_n.jpg

Some other materials I used, probably not all required, but I wanted to do it right the first time. I started with the battery cleaner. Then I scraped off as much corrosive coating as I could with the little screw driver. Then I used some fine grit paper to take off even more corrosive coating. Afterwords I sprayed it down with electrical contact cleaner and let it dry. You'll see what I used the di-electric grease for in a minute.

http://hphotos-snc1.fbcdn.net/hs122....402_5751_n.jpg

Now we're looking good . . .

http://hphotos-snc1.fbcdn.net/hs122...._1804321_n.jpg

Step 5: Reassemble the rheostat.
 

Now it's time to put the rheostat back together. Put a little bit of di-electric grease on one end of each spring and place them back in their holes. The grease will keep them in place as you reassemble.

http://hphotos-snc1.fbcdn.net/hs142...._1742449_n.jpg


From there you just need to hold the slider onto the spring and place it back into the housing. The metal clip is what holds the dial/spring/slider assembly onto the coil housing, so you will need to hold it in place as you slide the metal clip back on.

Once it is back together, you should try rotating the dial a few times to see if the slider runs good along the coil. When I tried it I found that the slider was now pushing the coil out of its track - see below (it shouldn't be sticking out like that).

http://hphotos-snc1.fbcdn.net/hs142...._1028821_n.jpg


So, how do you fix that? More di-electric grease, of course. I lifted up the coil, placed a few globs of grease in the track and then pressed the coil into the grease. Worked like a charm, the grease held the coil in the track and the slider moved smoothly across the surface of the coil. Note that you can rub a little dielectic grease on some of the metal components to keep them from corroding, but avoid getting it on the contact surfaces.

That's it! As they say, assembly is the reverse, right? Mount the rheostat back on the panel and reconnect the panel.

Step 5: Enjoy those dash lights and your functioning rheostat!
 

Great write up with excellent pic's. You are correct in that the surgery should not be attempted in a stone driveway, with the possiblity (near 100%) of the springs taking flight.

One little thing from a historical point, you might wish to amend, On my '83 300D there is a oil pressure sensing line, not a brake pressure.
Otherwise you are right on.

Great Job.

BTW is a clock repair in your future ? Or just happened to be midnight.

Fine pictorial and explanation. Thanks

LOL - you know, it just didn't sound right when I typed it, but it was late. You are, of course, correct. I will make that correction.

Also, I should point out that it is not a good idea to do trouble-shooting on your vacuum system while that oil pressure line is disconnected. Meaning, DO NOT, start the vehicle with the line detached. Ask me how I know!!!:o It's not like I didn't know oil would start spraying out of that line - my mind just really wanted to test the vac line at the ignition (since mine isn't curently shutting off with the key). Lucky for me the mess was minimal and since I had hooked up the Mity-Vac to the vac line I was able to shut it down fast.

Clock repair??? How did you know my clock wasn't working? Unfortunately it wasn't midnight . . . more like 3 AM . . . dumb me. That looks a tad more difficult, but . . . who knows.

Thanks for catching my mistake.

Hey I got lucky with my dash lights,just turned the knob about fifty times,on and off, work now but very dim. 83 300d

Yeah, others have reported success by doing exactly what you did. I tried that . . . but to no avail. I think the reason these get this way in the first place is because they don't get much use at all. I mean, honestly, how often do you adjust the brightness of the dash lights . . . especially when they are already dim on high. Unfotunately I just wasn't getting any response from mine . . . and after cleaning it I can see why.

...
now wire in some LCD's and get good brightness on the dash!@

LEDs?

That is what I love about these cars. It is possable to fix most everything instead of replacing.

This is a great write up and pictorial. This needs to get placed in the DIY Articles.

A new Rheostat is about $90.00 by the way.

Charlie

I should point out that in 4/5 cases of a failed rheostat I have dealt with, the resistance coil itself had completely disintegrated. Thus, a repair would be impossible unless a new coil could be sourced.

An excellent write up though, that process would work well for units that are only suffering from corrosion.

Thanks Charlie, I had no idea what a new one would cost. It's nice to have the rheostat working again, but the working rheostat reveals that I have some console bulbs to replace.

Well that would certainly put a damper on the fix. Thank you, I didn't realize it was that severe in that many cases. Luckily, you can see part of the coil without disassembling the unit, so at least you could get a sense of how good or bad the coil might be to decide if it is worth cleaning, or if jumping the rheostat is better.

I must admit this is one of the best write ups I have read on this site. I'm sure there are other good ones to as this site is amazing to say the least.

Very complete and extremely well written.

From reading it I feel like I had just accomplished the task!

In fact time for nappie! Great Job!!

My car had a semi working rheostat when I bought it. It eventually quit working all together. In the meantime I did a lot of twisting the knob back and forth till the bulbs lit up sufficiently. When the rheostat failed completely, I could feel that something, was hung up on something.

I took mine apart too, to see how it works and if it could be fixed. IMO it was beyond fixing, and I didn't want to lose functionality - so I bought a new one. When I opened up my Rheostat/Potentiometer, I was a little shocked to see how corroded it was inside. Two and a half years ago they weren't $90. Mine was $58.34.

I'm guessing the combination of living near the beach and a leaking windshield seal were the culprits. (28 years, is not a bad service life, though.)

As pawoSD mentions above, the coiled wire had coroded all the way through. What was left of it was stretched and bent from me twisting the knob back and forth after it broke.

I performed a few more steps while I had my cluster removed. Because the plastic tube the light passes through had become somewhat discolored, I painted the insides of the tubes with an ultra white paint. It's the flat white basecoat for my Pearl White Lexus. That lets a lot more light come through. I also repainted all my needles with signal red. I blended fluorescent orange with a little red, but it's still a little redder than the original, which to my eyes looks more like a pumpkin color. It's always bugged me how the needles fade unevenly.

Perfect timing on the well-done DIY thread since I just discovered my rheostat only worked at between 6-9 volts. I refurbed it and all is well. The console lights work now.

One other tip I used for re-installing the gauge cluster - since the carpet/felt/cloth insulation is no longer glued to the dash above the cluster, I had a hard time getting the gauge back in without jockeying it around. I used a thin piece of cardboard (like cereal box thickness) as a guide/shim between the cluster and the insulation. The cluster slid smoothly across the cardboard and went it was almost in place I slid the cardboard out - a lot like using a shoe-horn. The IC seems to fit better in the dash this way.

I also used Stabilant 22A on the rheostat and the dash bulbs. Removed and replaced the bulbs a few times to loosen or remove corrosion, then put the Stabilant on for the final re-install.

Well, thanks for saying so, but you should also check out the DYI articles section if you haven't. Based on the complexity of the task, I don't think it can really compare to some of the great DYI articles (i.e. dmorrison's insanely thorough 123 AC evaporator replacement or bgkast's 123 front end suspension rebuild - which I will be thankful for when my replacement parts show up).

Glad someone got some use out of this. Thanks for the tips on reinstalling and using the Stabilant 22A - I've never used it, but might give it a try.

DMORRISON`s write up`s are incredable, college level.

I do a project, take pictures, document everything I do, then when Iam done and go back and look it all over.

Iam still in kindregarden doing finger painting :o.


Charlie

Maybe he's tripp'n from dropping too much LCD. :wacko2:

Well, you never know what kind of mods people are putting in these days - maybe I have seen too many episodes of Pimp My Ride. vstech has a good point though - assuming he did mean LEDs - that would brighten things up, of course the color might seem a little off. vstech or anyone else done this?

Or you could just solder a jumper across the two pins with the rheostat in place.
The pic shows this done in front, but you can do the same thing in the back, at the bottom two points.

Two quick questions on this (great DIY BTW).

1) does the black knob have to be pulled off first from the frontside?

2) not to be too lazy or anything, but if one didnt want to break all the connections and take the risk pulling the cluster, how feasible would it be to unscrew the rheostat from behind and just pull it off with the cluster in place or just pulled out into the gap between its hole and the steering wheel? Sure it is a bit tougher to do it by feel, but there are other benefits to doing it this way.

Thanks!

Any idea if you could do this refurbishment on an 84 W126?

No, there is a metal piece that fits into the plastic knob - it pulls out from behind when you remove the rheostat.

Hmmm, not sure. May be possible, probably not very easy - but I never tried. I think would would at least have to push or pull the cluster out enough to see the rheostat, but probably still tough to get at. It doesn't take too much to disconnect the cluster, just keep track of where eveything goes - draw a simple diagram to help out as you undo things. Or use your camera to snap a couple pics first.

If it has the same style cluster with rheostat, then yes. If you want to find out, pull or push out the cluster and look to see if there is one there. I think it is the same, but not positive.

Did this today on my W126 1984 300SD. Easy and great improvement. Mine was to the point where it wouldn't adjust the lighting. It would only work on a single setting which was high. Took it apart and the resistance coil was in fine shape. Lots of white powder in and around it though. I think that was keeping it from working.

Many thanks for the pictures and explanation. I was planning on needing a new one.

many thanks
 

Thanks for the post, worked like a charm. No more driving at night with a flashlight. An added bonus, from taking the instrument panel out and re-installing the circular pin connector my temp gauge no longer jumps around. And I thought i needed a new temp sender.

Great Post thanks. I just bought an 83 240D and the instrument lights and console lights went out. Originally I thought it was a fuse but none of the fuses seemed to be bad. I actually couldn't figure out exactly which fuse controls those lights - there is one that says inst. clust. but I wasn't sure if that was for the actual gauges or for the lights. At any rate, when I pulled that fuse the gauges stopped working so presumably the fuse was good. Does anyone know the specific fuse for those lights? Since nothing else is not working, it probably is not the fuse but I'd hate to pull everything out just to find out later that it is. Also, in the event that I can't get the rheostat working after I pull it, does anybody know where I can find info on repairing using a jump wire?
Thanks

It's probably fuse 1, but you probably pulled fuse 12. http://www.peachparts.com/shopforum/general-information/242913-fuse-box-chart-what-fuse-goes-where.html Post #5.

Have you looked in the DIY wiki? W123 dash light rheostat rebuild. No it's not a jumper, but you can try the rebuild first, and if your's is too far gone, you can easily jumper it while out. PeachPartsWiki: Dash Light Rheostat Rebuild (yes, that's also this thread).

There are plenty of other threads w/ pics of various jumpers.

Or the easiest temp fix of all: gently but quickly rotate the switch a few times with the headlights on in a dark environment. Do the lights magically appear? Maybe the rheostat is dirty or corroded in that spot. Moving the switch may clean it enough for contact.

Well, my issue in my 82 is that even with new speedometer cable, the unit is VERY sensitive to position to prevent bounce at low speed and/or high speed.

So minimal movement to the cluster would be great.

And to have to disconnect the oil pressure gauge connector is always a risk in a leak-free system.

Thanks for the tip Yak

And most of the time the fixed used part is better than a new one.

This is an interesting thread. I recently had my instrument cluster rebuilt. When he was doing that he went ahead and cleaned the rheostat for me. Now it is bright without the flickering I use to get when changing the illumination setting.

Yes, I'm talkin ta ya :)
 

Special request:
Can you re-post all of the pictures, this looses a lot of value without them.

Thanks.


.

Yeah, I'd like to do this, if the pics were attached to the thread...

I found the wiki for it though...yippy http://www.peachparts.com/Wikka/W123DashLightRheostat

I wish the pics had not been deleted. Cant see any of them. :(

Great post. Just did it on my 1982 W123. Only had to unscrew the speedometer cable and the large electrical connector on the left side. Then pull the panel up and from the underside extract the rheostat. I was able to easily unscrew the philips head holding the rheostat and easily pull it off without removing the steering wheel.

Here are some pics. I left my clusters in place because at least my 82 is very sensitive to cable routing for needle bounce. It’s easy to unscrew the unit and pull straight out the back..

Then clean well, fit some decent wire, solder, and replace.

https://i.imgur.com/muaUTpll.jpg

https://i.imgur.com/ugtE37Ml.jpg

https://i.imgur.com/4XPKsQPl.jpg

https://i.imgur.com/Q0BJa8Gl.jpg

https://i.imgur.com/SwcLV7Ul.jpg

https://i.imgur.com/pfzINZUl.jpg

Note I think in the end the resistance metered out to 0.3 ohm or something.

Note also, this was the first one I did. After that, I bought a seller soldering gun from a Lowe’s, which was much better for doing this...

When dash lights failed on my 82 300Sd, turn-turn-turn of dimmer knob got back very weak illumination. So, pulled cluster and when 30+ y.o. soldering iron failed, cut approx 1" scrap off new bicycle brake cable, pried up each of the brass strips just enough to insert an end. This solderless jumper "cable" quick fix worked fine. Could eventually corrode, but should last for years. Don

The second one I did, I considered such an approach. It’s not flowing much current, and the internal wire was tiny, but all the same, I had zero control over a high resistance contact. And I wanted the insulation on the jumper personally. I could get a very tunable friction fit from the little hole, and additional strands could be wrapped under or soldered on top. That’s what I ultimately did second time...

That and LED lights, I can see the dash at night!

I just back fed the ashtray light and they all lit up just fine.

I just repaired the rheostat in my 300D w123. Major problem was that the rotary slider was pushing the resistance coil out of its groove. Second problem was a high resistance connection between the end of the resistance coil and the rivet.
The free end of the resistance coil was fixed in its location by a small dab of muffler repair putty (silicate formulation) at its tip within the ceramic base then heated in my wife's oven at 150 degC for 30 minutes.
There was a grey powder like deposit at this point which may indicate a similar compound had been used previously -obviously it would need to be heat resistant, because the dimming function would be dependent on shedding excessive watts as heat. The high resistance connection was fixed by using a square ended awl pressed into the rivet to oscillate it backwards and forwards (about 45 deg total). The rivet was then firmed up with a punch very cautiously (to avoid cracking the ceramic) while backed up with a second punch in a vice. Resistance of the connection was then checked at 1 ohm or less.
I tied the rotary slider to its post with a few loops of cotton thread to help in replacing it. They were easily cut and pulled away when the assembly was complete. It is important to bend the retaining arms at the top plate with using needle nose pliers to obtain a firm re assembly.
I lubricated the sliding surfaces with dielectric grease after reassembly. Otherwise everything is too sticky and slippy.
While an ohmmeter indicates regulation over about 10 ohms, time will tell whether all this works.
Andrew