W123 dash lights too dim
 

'83 240D, dash lights are so dim sometimes I'm not even sure they're on. Is there a knob to adjust the brightness? Is there a way to make them brighter?

Any suggestions for troubleshooting?

I suppose the most obvious question is have you removed your cluster to inspect?
Are both bulbs working?
Have you bridged the rheostat?

There's a rheostat? Where do I find it?

There is a black knob near the right bottom side of the instrument cluster. This rheostat knob can be turned to adjust the brightness of the dash lights. The rheostat is subject to,old age failure. An east fix is to remove the instrument cluster and remove the rheostat (held in place by two Phillips head screws). Solder a wire across the pins on the rheostat. Reinstall the rheostat. The dash lights will be at full brightness.

Found it! On mine it's the trip odometer reset, lower left. Until now, i hadn't seen that line in the manual. It doesn't do anything visible in daylight, so I'll try to jumper across it from below.

Correct - the knob also functions as the trip odometer reset. New rheostats (or good used) show up on Ebay at times for around $50.

They're all lower left, not right. Just take the cluster out and assess on the bench. Nothing to be done "from underneath"

Thanks for the correction.

There's two on fleabay right now, both in Europe, both close to $100, both used. I already have a used one...

I don't ever dim dash lights. I think I'll just jumper this, maybe with a wire, maybe a resistor.

If I may suggest, while you are already in there, look at the light "tubes" and the two prisms for the lights. Clean them real well, and replace the bulbs, possibly with some brighter bulbs of the same size......You an also use some bright white paint to paint the inside of the light tubes to reflect more light. I have done all three and never had to bridge my rheostat as the increase in light output was so much better. Just my .02

Get warm leds. I got mine from a rich uncle in Bellingham . Problem solved.

SCAMMER!
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There must have been just a speck of corrosion in there. When I twisted the knob back and forth several times, the dash lights came on. I'm good for now on that. Now I'll have time for an oil change and checking the valve lash this weekend, maybe.

You can spray "contact cleaner" on the rheostat wire and slide. You probably want it at max all the time, so solder across the pins does that or substitute a jumper wire for it. I put forward-shining LED bulbs which better direct light down the light tubes.

There is a tool to pull out the cluster, basically thin straps which hook the back. But, I remove the lower dash cover so I can push it out and also better direct the cables and unscrew the speedo cable for more access. Don't disconnect the oil pressure tube unless you must (mess, future leak), and surely never run the engine with it disconnected and open.