New to repair, bear with me if I'm missing anything obvious.

Since buying my 84 300CDT, all electrical has worked except the instrument cluster lights and climate control lights. On the cluster, the indicators bulbs do work (red brake light, blue highbeam light, etc), but the two bulbs that light all the gauges don't light up.

My understanding is that these cluster lights are connected to the climate control lights. So it makes sense that they are both not working.

I followed advice to solder the dimmer switch. This fixed the climate control lights, but not the cluster lights. Which indicated that it was probably the wrong bulbs in the cluster. Problem is that at the time, I didn't know that the cluster lights are connected to the climate control lights, and so didn't realize the solder worked. I redid the solder and ever since, once again the climate control and instrument cluster both don't light up again.

Hope that makes sense, appreciative of any advice that can help me troubleshoot.

So you jumpered the potentiometer?

I think the test then is continuity. My first guess would be a bad solder joint.

The fact that some lights went on but not all is an added twist, as the potentiometer iirc only has two pins. So the next step would be the traces on the board. Asking the obvious - you’ve checked all bulbs?

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

I sent a reply to this but got a note that it's being reviewed by moderator, figured I'd try again.

Exactly - I jumpered the potentiometer. I tried three different methods, 1: on the outside of the potentiometer, 2: removing the potentiometer and soldering the two pins, 3: taking apart the cluster and soldering those two pins from the inside. All reported to work on forums, but no luck other than the very first successful attempt, which I undid because at the time I thought it didn't work.

I did clean the areas with methyl hydrate, might I not have done a thorough enough job?

As for some cluster lights working but not all, I recall reading that those indicator bulbs (brakes, highbeams, etc) are a different connection to the two bulbs that light the gauges. On this note, in the climate control area, the back window defogger switch does light up, but everything else doesn't. Similarly here, I've read that this switch is a different connection to the others in the climate control area.

As for cluster lightbulbs: I purchased new ones that appeared to be to spec, but I think they actually aren't the right ones, as evidenced by my first attempt fixing the climate control area but not the cluster. Given my uncertainty on those bulbs, my strategy is to get the climate control to light up, which should mean the cluster is working too, and then go about finding the right bulbs.

Ok. Sorry if this seems overly basic, but back to basics...

- have you tested the LED bulbs to actually work?
- have you put a multimeter in the potentiometer to test for continuity/resistance of your jumper/solder?
- have you verified continuity from the “out” pin that takes the potentiometer output, to the bulb sockets?
- have you verified continuity from the 12v socket to the “in” side pin for the potentiometer?

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Basics are more than welcome, I'm a first timer at any of this.

-The new LEDs don't work so I've taken them out, replacing them with standard bulbs that do work elsewhere, but not in the spot that lights up the cluster
- I do have continuity when checking the two solder points. However, when removing the potentiometer, the in and out pins do not have continuity with eachother. Should they?
- Confirmed, yes there's continuity from the out pin to the bulb sockets
- I'm not certain where the 12v socket is. Can you point me in the direction?

Am I right in guessing that the climate control area and instrument cluster not lighting up are from the same issue? My logic is that the potentiometer dims the lights in two places: the instrument cluster and the climate control area, and those happen to be the two places I don't have light.

You may have a problem with the instrument cluster ground. There is a spot on the back of the cluster where a separate ground wire can be screwed onto the cluster and then ground the wire at the location on the steering column support where the other brown ground wires are attached. By adding this ground wire I was able to make my instrument lights work again.

After installing leds instrument night lights stopped working. Do not know why but reinstalling the regular bulbs stilled did not work. Answer: the leds burned the printed
circuit board. Repair the board by bridging the open gap or best from pin to pin.

Also note if clock wire (always hot )touches back of cluster will pop the circuit board.

Ok great stuff.

Glad there’s continuity from the out pin to the bulb sockets. That means your circuit board traces aren’t burned up.

Should there be continuous between the two pins? No, because they have an air gap.

How to find the 12v in? I’d start looking for continuity from the potentiometer “in” pin to the main harness that connects to the board. Haven’t seen a schematic, but I’d assume that’s how it works.

Someone else brought up ground. Good point. So for that I’d check between the bulb socket and metal on the back plane of the cluster. Then with the cluster plugged in via its main harness, I’d check continuity between the cluster metal back and some known good ground point on the car.

I have a bad potentiometer, so I know that it affects cluster and hvac controls other than the temperature wheel, and the shift selector. At least on a 240. The ACC in a 300 might be different. I don’t exactly recall.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

First thing I thought when I saw instrument cluster is ground.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

If the clock hot touches the cluster before the main harness is unplugged,can overload
printed circuit board,ground side.Same goes for shorted leds,not open but shorted.
Easy to ck. Remove back plastic cover.and easy to see,burned area.

Thanks everyone. Can't seem to find a guide for setting up ground wire, is this the way?

1. Strip the ends of a 16" length standard copper wire (14 gauge?) and add ring terminal connectors to each end
2. Remove the bottom right screw connecting the middle and right sections of the cluster (when viewing cluster from behind), and attach one of ring terminals there
3. Remove one of the bolts behind the cluster that has many brown wires connected to it and attach the other ring terminal

Will have a chance to test these in the next day or two and will report back with my findings.