Electrical Maladies - PeachParts Mercedes-Benz Forum

sailor15015 · #1 12-22-2005, 08:30 PM

Ok. I got my '84 a couple of days ago and its got some odd electrical issues, mainly with the dash. The illumination lights on the dash come on and stay on when I turn on the key, as do the high beam and seatbelt warning lights. The fuel gauge pegs to full and the temp gauge jumps like crazy. Before about ten minutes ago it was just annoying. Now it won't let me start the car and I'm facing another day without a car of my own. Can anyone point me in the right direction?

86560SEL · #2 12-22-2005, 08:35 PM

From what I have heard from others on the boards here, it sounds like an electrical short somewhere, or perhaps a loose ground. Has it been doing it very long?

What about your cluster illumination lamps? Are they illuminating as well, or just your warning lamps? If warning lamps only, it could be your alternator or battery.

sailor15015 · #3 12-22-2005, 08:49 PM

Yes, cluster illumination lamps as well. The glow plug light is the only light not coming on and I don't think the plugs are ennergizing because the cabin light isn't dimming like it should.

sailor15015 · #4 12-22-2005, 09:05 PM

Ok. I went out just now and did some more tests. I swapped dashes from my wrecked '85 and the same thing happened. What I did discover, however, is that all the lights only come on when the speedo cable touches the cluster?

? To recap, no starting and no plugs kicking on.

SD Blue · #5 12-22-2005, 09:46 PM

First off, I would check both the batt.-chassis and chassis-engine grounds. In fact, remove and clean if you live in a corrosive area. These grounds, esp. the chassis-engine, will give you very strange symptoms if marginal. The reason is the electrical systems will use each other to "find" a ground.

sailor15015 · #6 12-22-2005, 09:56 PM

I found out on my '85 that the chasis ground is where the bell housing meets the engine so its bolted on really tightly. I suspect it might be the neutral safety switch because it had been letting me start it in any gear. Here's a hypothesis, do you think it could be shorting to the tranny and giving power to the instrument cluster via the speedo cable?

sailor15015 · #7 12-22-2005, 10:00 PM

Oh yeah and I also have no reverse lights.

SD Blue · #8 12-22-2005, 10:10 PM

Doubtful on the speedo cable. Another area that gives strange intermittent charging problems and no-start issues is the small terminal block on the passenger fenderwell. Check for corrosion or loose wires. Is there the possibility that a PO did some "modifications" in this area? (You can bypass the neutral safety switch at those terminals.)

sailor15015 · #9 12-22-2005, 10:14 PM

I don't know what's causing it but when I touch the speedo cable to the cluster, that's when the fuel gauge pegs and all the lights come on. How do I short the junction box on the fender well? I know what you're talking about and it looks norman but it is a bit corroded.

SD Blue · #10 12-22-2005, 10:37 PM

I still believe there must be a grounding issue somewhere. Corrosion at this terminal block will give strange symptoms also.

If I recall, since I don't have any information at hand, the neutral safety switch wires are the smaller ones at that terminal block. You should be able to trace it by eye and a little reasoning. The connections are to the battery, alternator, starter and neutral safety switch.

#11 12-22-2005, 10:43 PM

Just a guess, but have you tried running a ground wire from the cluster to the ground point under the dash?

sailor15015 · #12 12-22-2005, 10:47 PM

Sam, I just went outside and cleaned the junction box. There was a bit of corrosion and I cleaned it up very thoroughly and it didn't make any difference. Where do I jump to the batter to bypass the switch?

Craig, where is this grounding spot?

Why would the glow plugs not be engaging?

#13 12-22-2005, 10:54 PM

Quote:

Originally Posted by sailor15015

Craig, where is this grounding spot?

Behind the dash there are a bunch of brown wires connected to a bolt on a steel support. If you pop out the dash cluster, you can see the grounds through the opening. However, it really doesn't much matter where you ground it. I ran a ground wire from one of my cluster screws to ground (on the 240D) and it cured my "jumpy" fuel and temperature gauges.

sailor15015 · #14 12-22-2005, 11:53 PM

Do the additional symptoms of no reverse light and being able to start it in any shifter position point in the direction of the neutral safety switch?

TonyFromWestOz · #15 12-23-2005, 08:51 AM

Sailor, you do have a faulty neutral safety switch, amongst other problems.
On the termonal block in the engine compartment, with the car in "P" and the handbrake on, short between the heavy wire and the light wire. This should cause the starter motor to crank the engine. If so, try turning the key to the start position while you apply the brakes and move the shift lever. You may find a position of the shifter, where the starter will engage.

Good luck