I angered the interior lighting Gods...

[Bengoshi2000]

I was driving my oldest child to a Christmas party out in the sticks this past weekend and one of the two main cluster bulbs goes out. So today, I removed the cluster and replaced the two bulbs with led's.

With the bulbs replaced and the cluster cleaned up, I reinstalled it. Now the two main bulbs work fine (as does the row of warning lights at the bottom). But I have no other interior illumination. The window switches, the ccu, transmission indicator, vent lights, hazard illumination... are all out. Dome lights works (front and rear).

Obviously I disturbed something.

While the cluster was out I removed the rheostat and cleaned it with some deoxit d5 and lubed it with some f5. I don't think that was my error as the rheostat dims the two main cluster illumination bulbs just fine. The only other task I undertook was to remove the bulb for the "bulb out" indicator (it doesn't like that I have an led in the high mount brake light and I'm tired of looking at it).

So, did I blow the cluster fuse (the one that attaches to the rear of the cluster like a bulb and is in a red holder)... or something worse?

[BWhitmore]

Is it safe to assume we are talking about your 1991 300D?

[Bengoshi2000]

Yep.

[jay_bob]

Yes the red fuse on the back of the cluster is the protection fuse for the instrument lighting circuit that extends out of the cluster.

The 124 instrument lights also have a repeater module (device N40) located behind the cluster. It is a transistor device that mimics the light bulb voltage (called circuit 58d on the diagram) and re-creates a new voltage source on circuit 58D.

Cluster connector legend:
X1 = single wire for clock (be careful, this wire is always live!)
X2 = 4 wire square connector on back of fuel-oil-temperature instrument
X3 = 15 wire circular connector on back of F-O-T instrument
X4 = 3 wire inline connector for tachometer
X5 = 5 wire inline connector for outside temperature gauge (pre 1990 models did not have VSS to the gauge, so this is a 4 wire square).

X3:13 (gray/violet) denotes a gray wire with violet stripes on pin 13 of connector X3

The cluster light circuit starts with fuse 3 which is only live when the light switch is in parking or headlight positions. This wire also feeds the external lamp warning unit, and the warning module (for the lights left on buzzer) and the ash tray light before it goes into the cluster on X3:13 (gray/violet) and hits the rheostat. The other side of the rheostat goes directly to the two main bulbs in the cluster and then to the red fuse. The other side of the red fuse goes to X2:2 (gray/blue) and on to the following devices:
X6 distribution block (all wires on this block are gray/blue) I believe this is behind the radio/CCU area. This feeds:
Climate control illumination
Radio illumination
Recirculation switch illumination
Hazard switch "dim" illumination
Center vent wheel illumination
N40 pin 4 (the sensing input)

N40 takes power from fuse 9 (live at all times) and receives that power on Pin 1 (red). Pin 3 is ground (brown). Pin 2 is the output (Gray/red). This wire goes to the X6/1 distribution block (all wires on this block are gray/blue except the feed wire from N40 that is gray/red).

N40 pin 2 via X6/1 feeds everything else in the interior that gets lit at night.
The temperature gauge illumination (X5:2 gray/blue)
Dome light switch illumination
Shifter illumination
Stereo fader
Mirror switch
Rear defrost switch
Head restraint switch
Power window switches front and back
Seat heater switches if you have those

So by looking at the list of devices that are lit and those that are not it is possible to deduce where the failure is.

If you have cluster lights and absolutely nothing else then it's probably the red fuse. It is entirely possible that you knocked it loose while messing with the rheostat.

If you have any of these lights - climate, recirc switch, hazard switch, center vent light, radio - but not any others, then you probably lost the N40 or fuse 9 feeding it.

[Bengoshi2000]

I did some additional research overnight and this morning and that all points to either a blown cluster fuse (or I bumped it just enough to break the circuit... as you suggested). So I'm hoping that's the case as I don't really feel like running down gremlins... I've got enough of that with an old am tube radio project. I'll have to wait until Saturday to get into the cluster again... joy.

[jay_bob]

You fix old radios? Got access to a tube tester by any chance?? I have a half dozen radios that I need to get working around the house.

[Bengoshi2000]

Originally Posted by jay_bob

You fix old radios? Got access to a tube tester by any chance?? I have a half dozen radios that I need to get working around the house.

I'd give my left n*t for a calibrated tube tester...

...and a functional O scope. I've got a zenith super sapphire that has an annoying buzz despite having been recapped.

[Rick76]

I almost picked up a mint B&K 747 for peanuts. I was the first to see the add early in the morning. I hastily responded saying I would take it and then left for a early e-test appointment for one of my cars.
When I got home I had no response and then to my shock discovered that the ad software had auto inserted an obsolete email address. Of course the tester was gone by that time.
I still grit my teeth whenever I recall that loss (and curse e-tests).

[Bengoshi2000]

Turns out the cluster fuse was fine. The problem seems to have been operator error. When I reinstalled the cluster, I evidently did not plug in the square 4 pin connector that sits between the rheostat and the large round connector. Amazing how well stuff works when you re-assemble it correctly.