Gauge Cluster Issues
 

While troubleshooting a different problem, I had my gauge cluster out and apart. I've put it back together wrong, apparently. New issues: Gauge lights don't work. Fuel gauge pegs at full on startup, and temp gauge goes to full within a minute or two of starting. High beam indicator stays on. Other info: The rheostat has a jumper soldered across it. There is also a jumper from the rheostat to the #3 pin on the round electrical connector to work around a bit of the PCB that flaked off years ago. I originally installed it years ago, and it came off as I was pulling the cluster. I got Pin 3 based off a post I saw on this site, but I'm not sure that's correct.

What did I do wrong?

Edit: The patient is the '83 300D in my signature.

You may have put the 15 pin plug in wrong and shorted some stuff. It's a keyed plug, but it does happen. Or your ground might be wonky now after this last cluster R&R and may need a new ground cable from the cluster to the ground behind the dash.

I'd consider having the keyed connector in wrong unlikely. However, the ground thing sounds quite plausible. What are some good places to connect an auxiliary ground?

Proper Grounding
 

Use an part of the body, use a star typ lock washer under the nut and ring lead .

German cars use brown wire for grounds .

I have seen where the keyed part of the plug had broken off making it very easy to misalign the connector.

X2

The D-shaped key was still there when I had it apart last, and I didn't have to use much force to get it in. We'll see when I take it apart this weekend. I do wonder how a ground issue could result in the high beam indicator being on, albeit too dimly to see it during the day.

One way to test out the cluster ground circuit is to use a long test cable. Connect one end to the battery grnd point the other end would go to the grnd point on the wiring harness. Using a continuity meter will verify the wiring harness grnd point to the cluster. If the key "D" shaped connector is still connected to the plug then I can't see how this would cause a problem.
MB's have a funky issue with with grnd circuits. In electricity devices will seek the least resistant path to ground and can take out other circuits in the process.
Please don't ask how I know this......

multi plug
 

Any good .W123 diesel

Poor Grounds & Current Backflow
 

" I do wonder how a ground issue could result in the high beam indicator being on, albeit too dimly to see it during the day."

It glows because the indicator lamp is providing a poor ground path .

Like the millions of old 50's & 60' GM products that have a dim taillight that goes out when you step on the brake : the secondary filament provides a ground path until you energize that circuit then the light goes out .

Auto electrics isn't rocket science, you just need to learn it by doing, you'll get this fixed ere long .

Where on the cluster?

Any screw on the speedometer.

I'll try that.

Binnacle Ground Connection
 

I'd take a photo and post it if it wasn't so god damned difficult to do so, it used to be simple, now it's almost impossible to upload any attachments .

It seems to have worked. The fuel gauge is now reading a plausible value instead of pegging full. I'd take it for a test drive, but I'm going to walk over and grab a pambazo first. It will be a few hours before it's dark enough to tell if I fixed the dash lights. I may work on the Jag a little bit.

The white wire is my grounding connection. The other end goes to the ground point in the dash (where the bunch of brown wires are) with a Deutsch DT connector pin as a quick connect between.


The LM7812 voltage regulator keeps the cluster LED lights alive. Otherwise they start flickering and fail within a year, oddly out of every other light being replaced with LED its only those that failed consistently until I added the regulator.

It turns out I had an incomplete success. Everything's working right except the dash lights. Know that I know I have a good ground, it's got to be a power issue. Like I said before, the PCB is knackered, and I have some wire bypassing the fault. I guess it's time to re-do that.

Its a good point to NEVER remove cluster bulbs with the lights on, they can short across which will melt the PCB traces.

Problem solved. I soldered a new jumper wire from the back of the PCB to the far side of the rheostat. Then I realized I had the wire on the wrong pin. Moved it to the right one, and now I'm good.