switch for dash lights

That's rheostat... dealer item only... boy, it is expensive. Try ebay. I may have a spare... I'll let you know if I have one.

I have a similar problem on my '77 300D: my instrument lights are very dim. My rheostat (brightness control) works as I have verified because the lights on the center console (HVAC) are bright and can be dimmed correctly. Since this rheostat also controls the instrument lights, I believe the connection is good that far, but something is causing the instrument light branch of the circuit to be dim even when the rheostat is turned to bright.

Please give me any suggestions on correcting this.

Thanks!

I battled this with my older 380SE... changed all the bulbs... changed the rheostat when it died... I hooked up the cluster direct to battery and the light are super bright.

The only thing I can think of that cause this problem is the headlight switch, but I never changed this so I don't know.

The dash dimmer rheostat is the culprit. I was able to *repair* mine (naturally *after* I got a new one).

The relationship between voltage drop and luminous flux is far far from linear. It's third order or something like that.

Bottom line is, crud on the rheostat plus old bulbs means dim dashes.

Replace or clean the rheostat, replace all the bulbs and clean the plastic on the instrument cluster. I did this and it was like night and day.

Great article... I bought a new one when the other failed... but still same thing... dim... sold the car already.

The bulbs age. The filament is not as bright being covered by oxide and the inside of the glass willl have been blackened by deposition of, again, oxide. Tis the way of all incandescent bulbs - except halogens where the halogen cycle (for the most part) solves this problem.

The bulbs were ~ 20 years old. When was the last time you got 20 years out of an incandescent bulb? :-)

Also do not underestimate how much the film of whatever it is on the inside of the cluster bezel cuts down light. Clean that up and it makes quite a difference.

I think that is what is missing... the cluster bezel that allows the light to shine into the cluster. I had new bulb... hooked up the speedo directly when out of the cluster... real bright.