Instrument Cluster Repair

[BillGrissom]

Notes on repairing my 1985 300D instrument cluster "printed wiring board". I hope it simplifies the task for others. Problems I noted were no gage lighting and both turn signal bulbs continuously lit, though they would still flash.

I removed the cluster by taking off the under-dash panel and pushing from behind. My speedometer cable doesn't have slack to unscrew from the front. My oil pressure tube was dry, though I had a napkin ready. On the bench, view the back side. The middle board has a finger that hooks under the instrument board, making it tricky to re-assemble (springs pop out), so insure you are commited before taking the next step.

After removing the gages, I found a burnt ground trace, as reported in another post. I soldered 2 wires to bypass those traces, as shown. I then managed to burn another trace in testing some new LED bulbs (see repair photo). They wouldn't light when applying voltage between the rheostat output pin and pin 15 (gnd) with a 120 mA max DC supply, which did slightly light the original incandescent bulbs. I foolishly pushed on, using a more powerful source. I then realized that the LED leads were bent in a way that shorted both sides of the bulb holder. I rebent them in a U, so each lead stayed on one flat side. The fuzzy photo shows this. The LED's should be installed so the thicker lead contacts the positive terminal.

Below are the connector pin terminals. I also wrote them on the board traces. The back side is all you will see when installed, which I numbered (fuzzy photo). 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 board, which you can see well if you back-light it. You can verify the pins in the factory wiring diagram, but tricky to read.

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)

Details
It appears that the warning buzzer was originally on the printed circuit board. In my 1985, it comes from another box, perhaps the security system, thus the missing pin.
The "alternator not charging" warning bulb appears to light from the difference between alternator output and battery voltage. There are a few diodes and resistors on the board, which may be part of this circuit.

[TnBob]

You have great patience and lots of spare time to provide that pin out reading. THANKS.

As far as LED's go, they come in a variety of styles. I used flat tip ones on my turn signals and they are bright looking in from the rear bumper.

[BillGrissom]

Thanks for the comment TnBob. Today, I ran across a hand-written sheet in my manual that lists the same pinouts. I must have written it years ago. Not sure if from the web or I figured it out. For the current post, I used info from another website (lost link) and the schematic.

BTW, does anyone know what the switch at the top of the photo is for? It appears designed to sense if the instrument cluster is removed. I don't find it in the schematic, but my Haynes manual has wiring only to 1984, so perhaps a 1985 feature. My car has the factory security system, so perhaps part of that. I pulled the plug on the security box long ago because it had a mind of its own and didn't want to disturb the neighbors.

[yvairguy]

that's funny i looked at the pic before i read the post and was wondering what the heck that switch at the top was......

[yvairguy]

thinking about it i bet the switch kills the ignition so you don't pump oil out everywhere from the oil line being disconnected.....

[BillGrissom]

Originally Posted by yvairguy

thinking about it i bet the switch kills the ignition so you don't pump oil out everywhere from the oil line being disconnected.....

Great idea. I tried it (oil line connected) and the starter still actuates with the cluster pulled out. Edit 12-1: I was under the dash today and found that the "cluster in" switch cable routes to the alarm box, so it is for the security system. I guess M-B figured that clever thieves could bypass the security by popping out the cluster. Anyway, my alarm box is disconnected since it caused problems.

[BillGrissom]

Update. The round LED bulbs above didn't put enough light on the dash. I bought some different LED's that project light forward better, which works great in the light tunnels. My dash is now so bright I must turn down the rheostat slightly. I bought qty 20 them on ebay for $0.60 ea. Search for "T10" and "LED".

Be careful to not get any bulbs that loop the wires across the flat base (like mini-circuit board ones), since that would cause a short circuit (see above). The M-B bulb holders are a bit unique in the way they electrically connect to the bulb. I expect the wire on even regular incandescent bulbs could bridge the two fingers if the wire gets bent slightly. That would cause a short circuit, and may be why my ground trace was fried (happened to others). If not, perhaps someone tried installing brighter bulbs (more current). LED's draw much less current and generate less heat, so less risk to your dash, plus they should last the life of the car.

[Graham]

For anyone who finds those burned out traces, it may be worthwhile finding the reason.

It would appear that the traces blow before the fuse for the dash lighting. That would be a design problem.

Two of us here have found that the actual short that caused the fuse or traces to blow, was in the pillar between the front and the rear seats. Part of the same dash lighting circuit goes to the window switches on the back doors. We found that if lights were on and the back door was opened, the fuse blew every time! I removed the harness in the pillar and found the wires were shredded (documented elsewhere on PP) There appeared to be some sharp edges of metal inside the pillar and they had sawed through the wiring.

My solution was to rewire because I had everything apart. Another solution, is to remove the trim and cut the blue wire that runs up the pillar. It is the one going to the switch. Having that switch backlit doesn't seem super important.

You can also blow the fuse/traces if you don't disconnect the battery before removing the cluster. There is a live wire that can ground out.

[jay_bob]

Originally Posted by Graham

I removed the harness in the pillar and found the wires were shredded (documented elsewhere on PP) There appeared to be some sharp edges of metal inside the pillar and they had sawed through the wiring.

It's not a sharp edge but the fact that M-B made a poor choice in the stranding count for the door wires. The wires should have many fine strands instead of a few coarse strands.

We have to observe this fact in our control panel designs at work. The UL standards for controls require any wire going over a hinge point to be high strand count. #12 AWG needs 65 strands and #14 needs 41 strands. To be safe we wire everything with that kind of wire.

The constant flexing of the wires causes metal fatigue which causes the wire strands to break. The broken ends of the wire are quite sharp and can punch through the insulation makiing it appear to have been cut.

[Graham]

Originally Posted by jay_bob

It's not a sharp edge but the fact that M-B made a poor choice in the stranding count for the door wires. The wires should have many fine strands instead of a few coarse strands.

In this case, the wires did not break where they hinge. They broke half way up the B-Pillar in a straight run where they would not be bending. I could not see the exact place, but in measuring and then reaching up inside, I cut my finger on something sharp. It appeared to be approximately at the welds that hold the door strap hinge in place. No doubt the wires move up and down a bit as the door is opened, and this must have eventually sawed through both the sheath and the insulation.

Next to useless expandable braid sheath removed and not in picture. Rewired with flexible sheathed triple conductor cable plus a single ground wire - all things I happened to have.

Problem and Solution (with help from my123ca!) are covered in this thread.

[greazzer]

Originally Posted by BillGrissom

Update. The round LED bulbs above didn't put enough light on the dash. I bought some different LED's that project light forward better, which works great in the light tunnels. My dash is now so bright I must turn down the rheostat slightly. I bought qty 20 them on ebay for $0.60 ea. Search for "T10" and "LED".

Be careful to not get any bulbs that loop the wires across the flat base (like mini-circuit board ones), since that would cause a short circuit (see above). The M-B bulb holders are a bit unique in the way they electrically connect to the bulb. I expect the wire on even regular incandescent bulbs could bridge the two fingers if the wire gets bent slightly. That would cause a short circuit, and may be why my ground trace was fried (happened to others). If not, perhaps someone tried installing brighter bulbs (more current). LED's draw much less current and generate less heat, so less risk to your dash, plus they should last the life of the car.

Any way you could post some pictures / more pictures for dummies like me. Please ...

[BillGrissom]

Someone also asked in a PM who I bought from. I answer here because info should be for everybody. The ebay seller of the 8-LED bulbs was "cocogiftcity". I don't know their bulbs are any better. They were nearby and I needed this done before my son takes the car away. To answer another question, not selling my extras because shipping would cost as much, plus I can use the bulbs in my other cars.

To repeat, for any bulbs you use, insure their wiring doesn't bridge the 2 contacts of the M-B bulb holders, or you risk frying your board from a short circuit (like I did). Measure the resistance between the 2 contacts before installing. The M-B holders are a bit unique. Indeed, one ebay seller mentions that.

I'll look into the possibility that bulbs elsewhere in the car might be wired thru the instrument circuit board and thus risk frying traces. I don't recall that from the wiring diagram or from working on the circuit board, plus it sounds unusual. I would expect that body wiring would branch off from the lamp switch before the circuit board. I wonder if there is a missing body ground that caused back-feeding thru the instrument cluster, maybe from ground current flowing thru the speedometer cable.

[Graham]

Originally Posted by BillGrissom

I'll look into the possibility that bulbs elsewhere in the car might be wired thru the instrument circuit board and thus risk frying traces. I don't recall that from the wiring diagram or from working on the circuit board, plus it sounds unusual. I would expect that body wiring would branch off from the lamp switch before the circuit board. I wonder if there is a missing body ground that caused back-feeding thru the instrument cluster, maybe from ground current flowing thru the speedometer cable.

By all means check the circuit diagrams for yourself. You WILL find that there are a number of other bulbs and leds on that same cluster lighting circuit.

I have exactly the same car.

[jay_bob]

The 123 lighting system carries the entire load of all interior illumination bulbs through the cluster. The rheostat dims everything that is not part of the cluster. This circuit is called 58d.

The following devices are on circuit 58d:
HVAC control lighting (2 bulbs, the A/C wheel, and 3 rotary switch connectors for the 240D, or into the PBU controller connector on the 300s for the internal bulbs).
FO Spider light (for the upper panel switches - sunroof, antenna, rear defrost, rear dome)
Console Power Window switches (LED)
Rear Power Window switches (LED)

I believe there were 2 radio options on the 123s.
The 599 (analog tuner with 2 big knobs) has no separate illumination wire. The bulb glows all the time at full brightness whenever the radio is on. There is just one switched hot wire (circuit 15) that is shared with the lighter socket on a quick connect and a ground wire.
The 612 (digital tuner) has a separate 2 pin plug that is left hanging if you use the 599. This plug has the dimmed light circuit (circuit 58d) and a constant live (circuit 30) for the memory backup.

The most likely contributors to blown traces are:
- Replacing the bulbs in the cluster and getting the bulb socket incorrectly seated, shorting out the two halves of the bulb socket circuit
- Shorts in the B pillar harness due to fatigue or damage shorting out the 58d line going to the window switches in the back doors.

Note: anyone reading this that has a 1986 or newer 300D, this does not apply to you, as you have a 124 chassis. Different animal altogether. Best to start reading here:
W124 electrical fun: Two problems

[funola]

See this post (#18) http://www.peachparts.com/shopforum/diesel-discussion/189069-stranded-immediate-help-needed-2.html for some interesting issues I encountered with the instrument cluster not long after buying my first Mercedes (83 300D).

Originally Posted by Graham

For anyone who finds those burned out traces, it may be worthwhile finding the reason.

It would appear that the traces blow before the fuse for the dash lighting. That would be a design problem.

Two of us here have found that the actual short that caused the fuse or traces to blow, was in the pillar between the front and the rear seats. Part of the same dash lighting circuit goes to the window switches on the back doors. We found that if lights were on and the back door was opened, the fuse blew every time! I removed the harness in the pillar and found the wires were shredded (documented elsewhere on PP) There appeared to be some sharp edges of metal inside the pillar and they had sawed through the wiring.

My solution was to rewire because I had everything apart. Another solution, is to remove the trim and cut the blue wire that runs up the pillar. It is the one going to the switch. Having that switch backlit doesn't seem super important.

You can also blow the fuse/traces if you don't disconnect the battery before removing the cluster. There is a live wire that can ground out.