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.