W123 Instrument Cluster Lights Not Working (it isn't the rheostat) - PeachParts Mercedes-Benz Forum

will_t · #1 11-03-2019, 06:26 PM

Greetings All:

I have a 1983 300D whose instrument lights are dead. I put another instrument cluster in and same thing. It isn't the rheostat. I have not tried to figure out which of the pins on the main electrical connection is for the instrument lights - does anyone know? And if the pin is not hot on mine when the lights are on any ideas on tracking down where it might have failed? And oh yeah, I did check the fuses....

Thanks in advance,

Diesel911 · #2 11-03-2019, 08:34 PM

Maybe someone will post a free online site with the Factory Service Manual so you can get a look at the wiring diagram.

Do the warning lights come on when you first turn the key?

What fuse goes to the instrument cluster?

Long ago people used to traces some of those troubles to where there is a common ground point behind the instrument cluster.

Could be the light switch itself. If the warning lights don't come on when you first turn the Key and the fuses is good and making good contact could it be the Ignition Switch?

will_t · #3 11-04-2019, 09:08 AM

Warning lights come on. Gauges all work. It is the same fuse as the backup lamps - number 8 I think. The backup lamps work.

I could ground the cluster. I've done that before for other reasons on other 123's. Not sure it would help though.

psaboic · #4 11-04-2019, 10:04 AM

Ok, if the reverse lights work, then the fuse is good as you said. Also, it sounds as if the entire cluster is good since the warning lights come on and gauges work. You need to check for power at the plug that goes into the back of the cluster. Here is the pin callouts from the maintenance manuals I used back in the day. It is for a 1985, but it worked fine on my old 1983.

The blank pin on my board is #9. The numbers increase cw, viewed from the rear. There are tiny pin numbers on the component side of the cluster board, which you can see well if you back-light it.

Pin, wire color, Function or Source
1, wht/blu, High Beam indicator bulb, +12V = on
2, grn, from Coolant Temperature Sensor
3, blu/blk, from fuel level sender in tank to gage
4, blu/grn, from low Fuel Warning in tank to bulb, grounded = on
5, blk/blu, +12V supply from fuse #12
6, blu/red, +12V from key switch #15 (run) to charge indicator bulb
7, blu, +out from alternator terminal D+ to charge indicator bulb
8, brn/red, from brake pad warning switches to indicator bulb, grounded = on
9, brn/gry, from warning switches (doors, relay) to buzzer
10, brn/red/wht, from brake fluid low and e-brake switches to indicator bulb, grounded = on
11, gry/grn/vio, +12V from headlamp switch terminal K to dimmer rheostat and to warning buzzer
12, gry/blu, rheostat output to bulbs in console
13, blk/grn, from turn signal term 8 to right turn signal indicator bulb, +12V = on
14, blk/wht, from turn signal term 6 to left turn signal indicator bulb, +12V = on
15, brn, from M1 ground (behind cluster)

I'm thinking you have 1 or more of 4 possible scenarios.....a broken wire from the rheostat output to the cluster, a broken wire from the headlight switch terminal "K" to the rheostat input, bad rheostat itself, or a bad headlight switch......

Hope this helps, Good luck

Diseasel300 · #5 11-04-2019, 10:54 AM

Do any of the other dash lights come on? Climate control? Auto shifter? Center console switches? Window switches? If they do, there's an issue in the cluster between the rheostat and the bulbs in the cluster. If they don't, it's a bad rheostat (jumper it to find out), or as Psaboic said, connection from the headlight switch to the cluster or a bad headlight switch.

gregp1962 · #6 11-04-2019, 11:38 AM

I'm having a similar issue as the OP. My dash lights don't work. Climate control lights also don't work. But, the shifter lights DO work. And, this may or may not be related, the tach and clock don't work. 1982 300CD, US model with tach and clock on right side of cluster.

For the OP, I guess he'll want to tell us if all lights are out or just some. (climate control lights and shifter lights)

will_t · #7 11-04-2019, 02:37 PM

Thanks all, I will go now to the main electrical plug into the back of the cluster and see if it is getting power at the applicable pin, and if not, check the switch. I might even have another switch here somewhere.

Everything else works in the car - all the lights, cruise, signals, climate control lights, shifter lights, backup, brake, headlights, running lights, radio, blower fan, horn, everything. All the fuses are good. And, I have various instrument clusters laying around and the instrument lights not working is equal on whatever instrument cluster I stick in the car. That's why I pretty much rule out the rheostat.

Thanks for the help!

psaboic · #8 11-04-2019, 09:28 PM

Well, unless the clusters all have burned out instrument light bulbs, I'm betting on bad wiring to the big round plug, or a bad headlight switch.

BillGrissom · #9 11-05-2019, 09:54 PM

When you say "not the reostat", did you try removing it and jumpering the 2 pins together? In general, the dash lamps need full 12 V anyway. My dash is brighter since I changed the 2 bulbs for the light pipes to forward-shining LED's.

gregp1962 · #10 11-05-2019, 11:48 PM

If I'm reading the info in post #4, pin #5 is a good starting point to see if there is 12v to the cluster. Isn't that the power input to the cluster? Shouldn't you check power there?

psaboic · #11 11-06-2019, 09:53 AM

With all the interest in this, I hope to God the OP posts the answer to the problem once it is figured out!

will_t · #12 11-06-2019, 04:03 PM

Hi all, thank you for your kind suggestions. I have not had time to check pin 5, which I agree will likely not have power. As soon as I have a chance to get the cluster back out and stick a test light on it I will let you know what I find. I will also check to see if the headlight switch is sending power where it is supposed to send it.

Thanks for your patience,

Diesel911 · #13 11-07-2019, 12:21 PM

some of the wiring diagram attached.

The diagram shows #11 as being where the power comes from.

Looks to me like if you turn the light switch and take connector C151 and put the + probe of the volt meter on #11 and the negative probe on #15 (the ground). If you get no power try the negative probe a different ground.
If you are still getting no power then it is in the light switch or the connector to the light switch.

If when you moved to a different source of a ground you got power then it is the #15 to ground that is the problem.

will_t · #14 11-07-2019, 01:45 PM

Pin 5 was not the correct one. Thank you Diesel 911. I tested pin 11 with 15 as the ground and it tested good, which means that the main plug and the wiring within the car are fine. I had a fuel/temp/oil gauge laying around so I plugged it in by itself and found one working instrument cluster bulb and one not. So, I found another bulb that worked and put it in that gauge. I replaced the gauge that had been in the original cluster with the "new" gauge and now everything is good. As it turns out the second instrument cluster I installed had both bad instrument cluster light bulbs, and that's what threw me off first thing. I believe the original problem of no instrument cluster lights lay in the original gauge itself - and as soon as I typed that I realized that I should have tested it by itself before putting everything back together withe the "new" gauge. Anyway, now, with the new one, everything works.

Thank you all for your help. I definitely learned something here!

Best,

psaboic · #15 11-07-2019, 02:39 PM

Glad it is fixed. Guess my way out theory I posted a couple days ago about all the bulbs being burned out was not so far off after all. Glad you learned something. I learn something new from this forum on an almost daily basis.