000-542-16-25 Dash Light Potentiometer

[wwheeler]

Apparently these are now available from Mercedes as seen in the "Star and Laurel" January 2018 newsletter. There have been several threads over the years from people with bad units. This new one looks different than what has been offered in the past.

Now is your chance.....

[Tyler S]

About time!
German MB link:
http://partssearch.mercedes-benz-classic.com/parts/A0005421625

[doitwright]

I was able to repair mine. Not too hard to take apart. Once disassembled, soaked in CLR,  cemented potentiometer spring into bottom of trough and used a tiny smear of silicone grease on the spring. The technical manual shows how to fix.

[66andBlue]

Tyler,
two years ago you questioned whether it would fit into the center cluster of a W113:
https://www.sl113.org/forums/index.php?topic=24235.0
Still concerned?

[mdsalemi]

If this performed an essential function, it might be useful. But my own experience is, and this is echoed by many is that at full brightness the instrument lights are not very bright to begin with. Not certain why, in the face of an easy bypass, why one would be varying the brightness from nothing to not very bright, and pay a lot of money to do so...

I don't know if the terms are 100% interchangeable, but from my early days working with electrical systems and electronics, a variable resistor that was wirewound was called a rheostat, and typically used for higher wattage applications. A semiconductor version using carbon or cermet or similar was called a potentiometer.

[66andBlue]

Wikipedia (https://en.m.wikipedia.org/wiki/Potentiometer):
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
The measuring instrument called a potentiometer is essentially a voltage divider used for measuring electric potential (voltage); the component is an implementation of the same principle, hence its name.

[doitwright]

That is what I meant to say. Rheostat. What you will find inside is a spring that is supposed to lay in a ceramic trough. There is an electrical connection at one end of the spring. Mine had corrosion between the spring loaded (2 tiny springs) contact plate (attached to shaft) and the rheostat spring. In my case, the other end of the spring was working it's way out of the trough which is why an adhesive has to be applied to hold it in. After cleaning these up with the CLR which dissolves the corrosion, I added a little silicone grease to the top of the spring to help prevent future corrosion and provide a little lubrication. As long as your spring has not broken loose from the electrical contact, there is probably just as much work to clean and fix as bridging the terminals (bypassing the potent rheostat). With many people upgrading to LED and brighter lamps, making the rheostat functional can be useful.

[wwheeler]

I don't know if the terms are 100% interchangeable, but from my early days working with electrical systems and electronics, a variable resistor that was wirewound was called a rheostat, and typically used for higher wattage applications. A semiconductor version using carbon or cermet or similar was called a potentiometer.

I certainly didn't call it that. Blame it on Mother Benz.....    

 From the January 2018 Newsletter
    Great news for owners of the classic W100/111/112/113-series. The dash light potentiometer is now available after a long absence. Take advantage and order your potentiometer today for $230.00. This potentiometer fits all W100 models W111/112 Coupes and Cabriolets, and W113 “Pagoda” series Roadsters. For more information and to place your order, please call our parts specialists directly at 1 866 622 5277 (toll-free from within the U.S.) or send us an email to classicparts@mbusa.com.

Order your classic dash light potentiometer today   

[rgafitanu@gmail.com]

There are 5 or 6 3W bulbs that are controlled by the rheostat (potentiometer). At 0.25A each they add up to 1.5A which means you would need a 20W potentiometer (oops, rheostat) which is usually made from coiled wire. Modern film or other chemical resistors cannot carry that much wattage (definitely not in the sixties). The instrument lights are indeed very dim. I replaced them with 6W bulbs but the rheostat started smoking so I had to shunt it.
There are members here that have successfully replaced the bulbs with LED's while retaining the dimming function of the rheostat so that's the way to go. I will do it too next time I have to get under the dash (what a PITA job).

[Tyler S]

Tyler,
two years ago you questioned whether it would fit into the center cluster of a W113:
https://www.sl113.org/forums/index.php?topic=24235.0
Still concerned?

Alfred, Yes I am still concerned. Below are 2 photos. One from the German Classic site and the second from the Star and Laurel newsletter. Both are the same part #0005421625
"One of these things is not like the other"

[wwheeler]

I noticed that the one in the newsletter was considerablly different than others shown. I have never seen mine. What does the original look like?

The second picture in Tyler's post looks more like the one in the Tech manual than the first. Thoughts?

[doitwright]

The one featured in the newsletter looks like the original that I just serviced in my car which is the lower photo referenced above. Mercedes calls it “Dash Light Potentiometer “. Must be an oversight.

[Tyler S]

Yes the second photo is the correct setup. But it still doesn't explain why the german site had it listed 2 years ago as being available and showing the wrong part. Bit with correct part number. This is why I questioned it and never ordered one. I could not repair my original as the stem was broken off. I ended up using a salvage sedan cluster for parts and repairing it.

[doitwright]

That’s why at $230 US it is better to try and fix yours.

[wwheeler]

Yes the second photo is the correct setup. But it still doesn't explain why the german site had it listed 2 years ago as being available and showing the wrong part. Bit with correct part number. This is why I questioned it and never ordered one. I could not repair my original as the stem was broken off. I ended up using a salvage sedan cluster for parts and repairing it.

Thanks for the confirmation. With the wild availability of "factory" Mercedes parts on the internet, I have a hard telling what is really a factory part. I saw where the combination turn signal switch for my W111 is available and is "factory" and went so far as to list the part number. Pushing the vendor and actually trying to order, turns out it isn't available after all. I know the Classic Center doesn't have them. So Tyler, you were wise not taking the bait earlier on the wrong part. More than ever, buyer beware.