Instrument Lights out - 1987 300SDL - PeachParts Mercedes-Benz Forum

jia300d · #1 01-02-2015, 12:08 PM

First, Happy New year to everyone.

Now to my problem. My instrument cluster lights were working normally before I removed the cluster to replace light bulbs and clean the inside of the clear plastic front. I re-installed it and all of a sudden there is no illumination for the instruments. The warning, turning and center console lights get power and work normally. The front courtesy light next to the sunroof switch also works. I mention this because Fuse #2 controls the cluster illumination and courtesy lamp. Regardless, I replaced Fuse #2 and nothing. Although Fuse #6 controls the warning lights (and they are all working, as all the instruments) I still replaced it and nothing. My first thought is that power comes from the round knob connector behind the cluster, on the left side and feeds the rheostat which controls the intensity of the light. I wanted to bypass the rheostat but I really cannot see how to do that. So, I guess I am asking if anyone can provide some guiding thoughts to get the lights back on and if someone knows how to bypass the rheostat because I really do not have a use for it. I always like the brightest console possible. Thanks a lot for any help.

Assault · #2 01-03-2015, 12:37 AM

Perhaps this: http://www.peachparts.com/shopforum/diesel-discussion/308524-intermittent-no-dash-backlight-problem-solved.html

jia300d · #3 01-03-2015, 01:20 PM

Thanks Assault ! the link is a great starting point for my troubleshooting. My car does not have the new solid state rheostat but the old one so it should be simpler and of course I will check the soldered points.

Graham · #4 01-03-2015, 06:57 PM

Make sure you haven't blown a fuse. If you don't disconnect battery while removing cluster, this often happens. Don't ask how I know!

Worse situation, is fuse does not blow,, but you blow one of the traces on the cluster circuit board that feed the cluster bulbs.

Regarding bypassing dimmer, there are threads here on how to do that. Just a short jumper wire across dimmer terminals.

jia300d · #5 01-13-2015, 03:50 PM

UPDATE- After 10 days of silence I have to report success. The problem was something very unexpected that drove me crazy until a friend who knows more about electrics than I do explained what my problem was. I must also mention that I short changed the readers by not mentioning that I was trying also to upgrade to LED lights (I will document the upgrade in a different post) and come back to include the link here.

Ok, here we go:
Fuses were all gook and I did not have to replace any.
From looking at the electrical diagram, I could see that the Warning Lights are in a different circuit than the illumination for the cluster and that is why they were working fine.
I checked the voltage coming from the lights ON-OFF switch to the Pin #5 of the round knob that connects to the cluster and I was reading 12.5V.
I checked the voltage that went from Pin#5 thru the Rheostat (lights dimmer) and out thru Pin#8 of the cluster a couple of times and it read once 11.9v and the second time 12.0v. This voltage feeds the large light bulbs under the light tunnels part of the speedometer cluster by connecting to the Pin#8 on the speedometer side.
So far, my thought was: I have voltage coming from the battery and continuity thru the rheostat, so everything should be good. Nope, the lights lights were not turning ON. Since they are LEDs I turn them around to make sure that the + matched the capacitor in the led, otherwise it would work. Well, here it did not work at all.
I read a post that recommends to install a am additional ground strap to not depend on the Pin#3 alone for ground.
This did not help.
For the heck of it I connected the speedometer Pin#8 (that feeds the lights) DIRECTLY to the 12 v coming from the battery and voila! the lights turned on. Here is where I began scratching my head because I had previously check voltage thru the rheostat and I was basically reading 12 v, so in theory it should be enough to make the lights work.
I could not figure our what it was and here is when my friend explained why. Incandescent lights (original lights) illuminate more of less based of the amount of voltage they receive and that is why the rheostat cam be used. But LEDs do not work like that. They require a certain amount of voltage for them to work and in this case, a very minor drop in voltage thru the rheostat (remember that I measured 11.9v and 12v out of it) was not providing the voltage needed for the large LEDs to turn ON.
Solution: I am bypassing the rheostat (did not want it anyway) and connecting the lights directly to the 12.5 v coming from the Lights ON-OFF switch and getting very bright lights. BTW, I left in place the incandescent bulbs but I inserted a strip of LEDs ($0.99 cents in ebay, including shipping) inside the instruments cluster and I cannot ask for more light.
That's all folks. Thanks for your initial input.