Rheostat

[bpossel]

Has anyone rebuilt their rheostat?

I spent yesterday removing and cleaning my guages and thought that I had fixed my rheostat.... but no luck.

With the speedo out of the dash, I removed the rheostat with a solder gun.  On the bench, I carefully took the rheostat apart and cleaned the brush and entire unit.  I noticed that the "spring" within the unit was broken.  What I did was simply to stretch the spring over to the post and I soldered this in.  Put all back together and the unit looked good!  Put unit back in the speedo, and put all guages back in dash (after cleaning all, including glass) and unfortunately, I cant dim the guage lights.  The lights are on full strength.

Any advice?

If anyone has any questions on removing their guages and cleaning them, please let me know.  The "key" is to take your time!  Do not rush this job and you will be happy with the results!

Bob

bpossel
Memphis, TN.
1971 280SL
1997 E320

[TheEngineer]

You are so lucky! The lights could also be off. How lucky can you get? This proves an old design concept: KISS

1969 280SL,Signal Red, A/T, P/S, A/C, Both tops.    Ret. engr. 1967 230SL - Sold

[Cees Klumper]

One suggestion might be to just get another make of dimmer for 12 volts and the appropriate wattage, and rig it up to the knob. Or bench-testing the rheostat before putting the dash all back together.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[jeffc280sl]

Hi Bob,

See if this article helps.  Let me know please.  I need to fix mine also.


http://instrument.articles.mbz.org/dimmer/

Jeff C.
1970 280SL 4-speed

[waqas]

Bob,

The "spring" is actually the resistor used to vary the impedance and hence the amount of current flowing through the lights.  The resistance is varied by shorting the spring with the rotating stator (max-intensity is a complete electrical short of the resistor; min-intensity is when the resistor is partially shorted).  By stretching the spring over, you essentially shorted-out most of the resistance!  There is probably some variation of the light intensity, but it may be too slight to notice.

If you want it to work like the original, you need a new spring/resistor, which you might be able to make yourself from the appropriate length of "fuse-wire".  Last time I took the rheostat out, I measured 17 or 18 ohms of peak resistance through it.

waqas
--
1965 230 SL Euro, 4sp. man, dark green

[bpossel]

Thanks all for your replies and the information!

It helps me to understand how the rheostat works.  The article also helps a lot!

Sounds like I am lucky that the lights are operating at full brightness.  The next time I take the guages out, I will fix my rheostat.

Thanks again!
Bob

bpossel
Memphis, TN.
1971 280SL
1997 E320

[71 pagoda]

When I bought my pagoda 5 years ago I called the Mercedes classic center about replacing my rheostat which was not working. They said  that this part was not made anymore but if they get enough people looking for this part that they will put it back in production.  In the meantime I shorted the rheostat with a piece of wire and the lights work  and look fine.  If I think about how many times I  dim my dash lights in my daily driver, I cant remember ever using it.
Larry
71 280sl

[mdsalemi]

Palo Alto Speedometer rebuilt my gauges including the non-working rheostat; they may have taken one from another unit.  However, the gauges in our 113's are quite dim even at full brightness from modern IP's, (instrument panels) so Palo Alto's and most people recommend simply bypassing the 'stat and wiring to full brightness.  I don't often drive my car in the dark, but when I do I wish some of the gauges were brighter, not dimmer!


Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored