Hello everyone!

Norm, my 1980 300TD (just for you dial up users) has been throwing me for a loop as far as his electrical problems are concerned. Now they aren't anything too terribly major, but it would be nice to have fixed!

Backstory - From what Tyler tells me, the clock, glow light and seatbelt light worked up until he pulled the cluster for the first time, then they all stopped working sans the clock which only works when the car isn't running. We are both guessing ground fault issues.

Fast forward to today. I had the cluster out once prior, but to no avail. Today I took it out as I was going to repair my cruise control amplifier (I did the "soldering trick" and it worked!!) and thought "what the heck, I'll try it again". Once again, same result, no seatbelt, no glow, no clock. Anyways, I put everything back together and take Norm out for a drive to see if the cruise works. Well I go to glow him and notice the cluster is out just a tad so I push it in and the seatbelt light comes on but is VERY dim. Then I realise that my dome lights are not working, so I start to fiddle with those. When I move the switches on the dome lights they don't come on, but they turn on or brighten the seatbelt light! I'm guessing I shook something up down there when I was messing with the amp! It's all very interesting, and I really wish I could know exactly what is causing this to all happen!

I'm going to pull the kick panel again tomorrow and shake the wires a bit more to see if it does anything! But all kidding aside, what would be the cause of this? Ground fault issues? Something else?

Thanks guys!

One of those one step forward 3 steps back instances with me. I fixed the odometer, but killed those 3 things in the process.

Murphy's law kicks me in the butt quite a bit. Today I replaced the Signal, wiper, high beam multi switch on Ol' Turbo. On final assembly, I dropped a screw which magically flew under the seat. I couldn't find it until I pulled the seat, which took an extra 30 minutes to unbolt and reconnect... I spent more time on the seat to find that screw than I did replacing the multiswitch.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

Haha well at least what you knocked out isn't TOO terribly important! I think there is some little electrical gremlin in there that is causing this that both you and I have now run in to! I'm just happy I got the cruise control fixed. Man that is a PITA to remove and replace. There are lots of wires under there, it made it hard to attempt to trace the glow, seatbelt and clock wires back to the source!! Never found where they all tie in...

This stuff is just crazy though!

Funny we were messing with the same components today, different branches on the tree, though.

I was so relieved to find everything worked, the signals canceled properly, and I didn't disable my functional cruise control in the process. No longer do I have to jam an empty water bottle between the switch and the dash to keep Ol' Turbo from spontaneously flashing its brights.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

It sounds like a bad ground situation, or it's intermittently making contact with ground. Think of all the car's electrical circuits being a "U" or loop, with the powered devices somewhere along the "U". One leg of the "U" gets + battery juice (usually through a switch for the device) and the other end of the "U" has to be tied to chassis ground, which is negative, in order to complete the circuit.

I would look behind the dash for any ground wires, which are generally brown. Test with a multimeter to see that they are indeed ground. (although if the ground is bad the meter won't show continuity to ground, in which case you just have to be sure through studying the wiring diagram that the wire is indeed supposed to be a ground wire.)

Anyway take any ground wires you find, patch a new wire into them and connect that wire to a known good chassis ground, for instance a bolt going into the steering column or any other metal part with a good connection to the unibody structure. They make crimp-on connectors for patching 1 wire into another without causing a break in the original wire.

If you're unsure if a certain wire is supposed to be a ground, in your patch wire to chassis ground put a 10-amp inline fuse. That will prevent you from burning up part of the harness if the wire isn't designed to be a ground.

I've eliminated lots of flaky electrical problems this way. You could hunt all over with your meter for hours to try to find which wire has the weak ground, or just patch in all new grounds wherever you find a ground wire and be done with it.

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'81 300CD - 180K, cannot be killed by any conventional means
'99 Ford Escort - good MPG

^I understand what your saying, and it's a good idea, but I think there is one problem.

There is only one wire coming from each of the glow, clock, and seat belt parts on the cluster! I think that now the problem is that they aren't getting power at all now, but are grounded properly.

I think there is something loose where all the wires come together under the dash. If I'm feeling risky, I'll go through one at a time and test to make sure they all actually work. That might be a days job though. I mean it all worked until a certain point, so I'm guessing there are no broken wires or anything, just bad contacts.

That will work for my dome lights though, but that's assuming they are getting power in the first place which is what I think the problem is.

Haha that is funny! I went to the yard to find a climate control amp, but instead found the cruise control amp in the CD I was telling you about instead. I was assuming it was broken, as it's a part that goes out a lot apparently, so I yanked it. I didn't want to try the solder trick if I didn't have to, so I figured I try it out on this instead. Went to pay and it was $50!! No thanks, I'll try the solder trick on my own amp for that price. I did it, resoldered all the capacitors and viola it worked like a charm! I was skeptical at first but it actually worked!

Colin: Right there's only 1 wire going to each of those devices because that's the + (positive). The - (negative) is common to all those lights and is connected via the metal outer casing of the light bulb to the socket -, which in turn has to be grounded via a common ground throughout the cluster circuit board.

Use a 12V test light to see if you have + juice at all those devices. (at the one wire you're describing.) If not then you're missing the + back at some point that's common to all of them. (a fuse for instance) If you have + juice then ground must not be connected at some point common to them.

An easy way to check all this is to get a test wire with alligator clips at each end. (Radio Shack used to sell them, but I don't know if they still do.) Clip one end to a good ground then touch the other end to the various ground points and see if stuff starts to work. That will be proof of a bad ground.

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'81 300CD - 180K, cannot be killed by any conventional means
'99 Ford Escort - good MPG

Check the multiple prong plug on the back of the cluster. Mine open up and I had to figure out the correct location of all the wires. Maybe yours only loosened and is causing the loss of power.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html

^Thanks I'll look in to that also. Hopefully it's something simple like that, but I don't see how it would affect the dome lights as well. Who know with German wiring though!

I'm about to go pop the cluster out again to see what I find. I'll test it with my multimeter and see what happens.

I'll take my camera too!

Thanks for the help guys!

Here's the funny thing. The clock used to work when the ignition was off. Does it still do that, Colin?

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

^Yep it sure does!! Go figure right?

The clock issue sure is strange. Does the cluster receive switched power?

A friend of mine had troubles with the cluster in his 300d, he ended up adding a new ground wire (from the board in the back I think?). It fixed a random seatbelt warning light and random fuel and temp gauges when the high beams were turned on.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Well after being at it again for a few minutes, I discovered that all of these things tie in together on the #2 fuse on the fusebox. The fuse looks okay, but I'm going to see if replacing it with another one might do anything. I'm desperate! If that fails then I will pull out the wires from that fuse, see if they are loose or anything, then re connect them. If that fails I'll try the same with the grounds behind the cluster.

Oh and I am now able to turn the clock on and off with the dome light switch while the ignition is off!!!

I love German wiring

So if you turn on the dome light the clock stops? That means that the clock is grounding through the dome light... which I would think isn't possible because the door switches are what complete the ground for the dome light.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

^I see what you're saying, but the dome lights, clock, glow, seat belt, and power antenna all go through the number #2 fuse on the fuse box. I think the ground is okay, but there is something preventing the cluster components from getting power.

Funny, my power antenna also does not work!

I got Norm back to square one. Glow and seatbelt out, but dome lights work again. I'm not going to mess with it much further then that. I guess this gives me an excuse to try out a new mechanic!!