Instrument lights still dead; rheostat bypassed - PeachParts Mercedes-Benz Forum

The Warden · #1 12-20-2004, 05:26 PM

Hello again...

I apologize if I sound mad or anytihing like that; I'm very frustrated at the moment.

This is on my '85 300D...as I've mentioned in one or two other threads, my instrument lights are dead. This isn't just the lights on the instrument panel; the ACC lights, the window switch lights, the hazard switch backlighting, and the "upper switch panel" (with the antenna, rear window, sunroof, and rear defogger switches) lights...basically every interior light that's supposed to come on with the headlights, except for the gear indicator light, is dead.

I've tried bypassing the rheostat to no avail. From the rheostat terminals, I've got 12 volts AND 0 ohms to ground, implying that the circuit's complete. But, no lights, and I don't have power at the light sockets at the ACC panel or at the window switches.

The fuse checks out okay, and everything else on that fuse works properly.

Does anyone have any suggestions? I'm getting desperate here...

This started when I jury-rigged the center-console window switches in order to raise one window when its switch failed. I've since re-wired the switches the way they're supposed to be (with a replacement passenger's side window switch assembly; thanks again mattdave!), thinking that I had messed the lights up somehow and cut a daisy-chain ground when I did the jury-rigging, but re-wiring things so they were "right" didn't do any good (at least all my window switches work now

)

Thank you for any thoughts on the matter...I'm starting to wonder where my matchbook is...

Fishslayer · #2 12-20-2004, 09:17 PM

Electrical problems are very difficult to diagnose over the net, that said the first thing I would check are the traces on the circuit board. After nearly pulling out what hair I have left, I found a bad trace on my '85, after soldering a wire from point to point got all the lights back and a functioning dimmer. Just check continuity on each trace, if you have trouble seeing them hold the board up to a light. If need be I can pull my cluster and post a picture of which trace and repair-- Good Luck, James

The Warden · #3 12-20-2004, 11:01 PM

James, would that also explain the ACC lights and the window-switch lights not working?

I'll have to check on that...although it'll have to wait for tomorrow. Thanks for the idea

Fishslayer · #4 12-20-2004, 11:19 PM

Yes, all the instrument lights go through the same circuit, its been a while but I think the trace may have gone to a Hella relay soldered to the board.
If I get home early enough tomorrow I will pull the cluster and have a look, need to paint the light pockets and freshen the bulbs anyway.
James

dmorrison · #5 12-21-2004, 12:42 AM

Lets look at this carefully, Nah lets just fix it.

This is the list of items coming from Fuse #1 in the fuse box.

Center consol illimination
Lights Park/Tail RH
Lights Marker/license
Warning Indicators/instument cluster illumination
Warning system.

Sound like you

So check to see if all these items are inop.

If so its the #1 fuse, replace it, If still a no-go then its wire harness time.

Wire harness.

The diagram shows from fuse #1 A GY/RD wire goes to the light switch. check voltage at the back of the light switch at the GY/RD wire.

From the light switch GY/VI goes to the warning system relay and the LH/RH license light. Check votage there

The GY/VI wire also goes to the instument cluster for illumination and to the center console upper section. Also the wire goes to the radio and cigar lighter then go ground G102 (Behind center of instrument cluster)

It is this GY/VI wire that goes to the rheostat. So if no power there, then we work back to the fuse. (You have checked for voltage on both sides of the fuse, correct??)

The only thing left besides the wire is the light switch . Use the ohm meter and make sure it is working correctly. You may have burnt out a contact with the switch rigging.

Switch contacts

From the fuse to the switch GY/RD wire to contact N30 (or M30 hard to read) The output ( the GY/GY/VI wire, as the manual shows it) is contact K. so check continuity with the switch ON from N30 to K.

http://skinnerbox.steaky.org/Service/W123/w123CD2/Program/ETM/123%20turbodiesel_2.pdf

page 121/4/5 and is the complete diagram for your problem.

Dave

To add, the wire does continue to the window switches from the instrument panel to the switches as a GY/BU wire to the C124 distribution block then as a GY wire to the switches. So there is a connection between the switch rigging you did and the circuit.

The Warden · #6 12-21-2004, 12:42 AM

...and my car is no longer condemned!! For the moment, at least.

James, you were right...found a break in the board right after the far side of the rheostat terminal. Soldered that (only had a gun, so it's ugly...but it works and didn't compromise anything else...I hope), and the lights all work now.

Now, while the instrument cluster's in many pieces, I get to renew everything I can, clean up, etc...guess it's time to bypass the rheostat (even though it was okay).

Also, has anyone tried replacing the dash bulbs with LED's?

Thanks again

Fishslayer · #7 12-21-2004, 07:56 AM

Excelent!! glad it worked out and you could save the matches for another day

As to the led, the only ones I have seen have forward ilumination, so I dont think they would help. gonna try the white paint trick and fresh bulbs myself.
Good Luck, James

lrg · #8 12-21-2004, 01:08 PM

Warden,
I tried the white LEDs, in fact two kinds. They were a waste of money and actually worse than the stock bulbs. Just repaint the prisims bright white and put in fresh bulbs. One trick I though of but haven't tried it to line the white prisims with some kind of shiny foil. It might help but also might melt the clear plastic light "channels". I've done that too with higher wattage bulbs, a really bad idea.

The Warden · #9 12-22-2004, 01:41 AM

BTW, wound up just getting "normal" replacement bulbs (couldn't wait for Fastlane, so I had to get them from the dealer...$50 later

) and bypassed the rheostat completely...and just finished putting the instrument panel back in the car. Even without painting anything, I think the lights are a bit brighter, and certainly it's nice to see everything properly working.

I'm still in the process of putting everything back together, but I'd call this a success...the car should be ready to go to southern CA for the holidays tomorrow; also did a valve adjustment...

Thanks again

redbaronph123 · #10 12-22-2004, 03:55 AM

i'll be putting in LEDs as replacement.. i dont have the prisms anymore..

3 on each side - hyper white - diffused light
1 150 OHM resistor on each one to control the voltage...

i'll post the pix after i get around to doing it...

Fisherman · #11 04-06-2005, 09:33 PM

When you guys were refering to the circuit board here is it a board mounted tto the light switch assembly or someplace else?

I have the same problem as Warden and need to start tracing... Although the other night while driving I don't remember not having dash lights...

go figure... Maybe I never looked at the instruments? not like me at all...

whunter · #12 04-07-2005, 01:19 AM

Quote:

Originally Posted by Fisherman

When you guys were referring to the circuit board here is it a board mounted to the light switch assembly or someplace else?

Attached to back of instrument cluster is the circuit board.

Fisherman · #13 04-08-2005, 10:34 AM

After further investigation it appears that I have only lost lighting at the acc controls.
I have the radio out currently. I wonder if this could be affecting it as i have traced all the wires and checked all fuses and it seems okay.

It had to have been working because I just wouldn't not notice something like that.

Really scared me when I thought i lost it all...

billrei · #14 04-08-2005, 11:43 AM

Quote:

Originally Posted by lrg

Warden,
I tried the white LEDs, in fact two kinds. They were a waste of money and actually worse than the stock bulbs. Just repaint the prisims bright white and put in fresh bulbs. One trick I though of but haven't tried it to line the white prisims with some kind of shiny foil. It might help but also might melt the clear plastic light "channels". I've done that too with higher wattage bulbs, a really bad idea.

When I had my instrument cluster out I lined both prism pockets with shiny foil tape and cleaned the end of the prism. Made a big difference I can actually read the panel without the rheostat turned all the way up.

Anyone trying to trouble shoot electrical gremlins without a good wiring diagram is just p*ssing into the wind...