1983 300D Electrical Issue - PeachParts Mercedes-Benz Forum

rmmagow · #1 05-28-2007, 11:49 AM

Hi All,
My 83 has developed some kind of electrical problem. It won't start and something is draining the battery. Battery's a new Autozone. Car has had it for about 3 months with no issues. When I turn on the ignition, sometimes the lights light up, the glow plug lights, all the regular dash lights. Go to turn the key, the tach pointer sort of goes crazy. No click or even an attempt to turm the engine. After this, there's no dash lights, no electrical activity at all. Number 12 fuse was fried. It controls the tach and in my car had a power-feed wire sort of sloppily put in. Think it's for the radio. I disco'd that for now. Battery's taking a 4 amp charge now via a 6 amp charger. Are there some kind of main-relay fues in the car that I should check? Could the battery just have gone bad in a short time? The battery terminals and the ground connection are clean and just cleaned this morning. They weren't dirty at any rate. Flaky ignition switch??
Any and all troubleshooting advice appreciated.

Knightrider966 · #2 05-28-2007, 12:02 PM

I would remove the instrument cluster and start looking there! It sounds like this is the most likely are of trouble since that is where you get your goblins from just by turning on the key. My ignition switch began to wear and the wires fitted into the plastic body started moving around after being turned back and forth after awhile. I had to replace mine and problem solved! Not many think to remove the lock cylinder and add contact lube back there!

rmmagow · #3 05-28-2007, 10:27 PM

I pulled the gauge and looked around. Nothing obviously wrong, loose wires, burning smell or anything. Is there a write up on removing/replacing the ignition switch? I actually think I have a new one stashed away someplace. Took the battery to AutoZone, they charged it and essentially declared it good. I have it sitting in the car disconnected. Voltage seems to be slowly dropping. From around 12.98 volts when I got it home, it's now at 12.1. I want to see if it drops under 11 overnight. They said they'd replace it anyway. Before doing this I did hook it up and started the car. At idle I was reading 13.9 volts at the battery, alternator seems OK. Worst case, I've got another gauge cluster I can throw in too. Funny how one accumulates spare bits for these old beasts.

rmmagow · #4 05-29-2007, 07:21 PM

OK, I traced the short to something fused on fuse #2. It covers a lot of circuits. Funny observation, you can see the clock tick on the meter when measuing the current. It is pulling a lot of current, enough to pop a 250ma fuse in my gauge. I suspect the power antenna since it operates erractically and seems to be stuck part way down. How can I disconnect the power antenna? Are the connection in the trunk (sedan) or do I have to pull the radio. The radio is not stock, it's some kind of Alpine unit that you can pop off the face to discourage theft I guess. The antenna up and down switch would ocasionally work too. The inside lights are also on this fuse. My front light hasn't been working. I took it out altogether but still have a heavy current drain. I'd like to try disco'ing the antenna before pulling the radio.

Thanks for any guidance.

#5 05-29-2007, 07:48 PM

You can disconnect the power antenna in the trunk by unplugging the connector to the unit, behind the plastic trunk liner.

rmmagow · #6 05-29-2007, 07:59 PM

Found it, The brown connector. Now when I connect my meter between the negative battery post and the neg cable set to 30ma it still pegs then backs off to about half of the needle movement and pulsates. It will peg the meter at 300ma too but then quickly returns to a low reading. The pulsation must be the clock but I don't know why the thing pegs for a few seconds. Could a faulty antenna up and down switch cause this even with the antenna unpluged? And yes, with fuse #2 pulled the meter barely moves. Can the clock cause this? It always kept good time and never acted up on me but...

#7 05-29-2007, 08:16 PM

It's pretty easy to unplug the power to the clock, a single connector on the back of the clock/tach.

You can unplug the antenna switch if you pull the switch out (be careful of the wood trim).

rmmagow · #8 05-31-2007, 08:10 AM

This is getting messy. I disconnected the power antenna but still have a high drain via fuse number 2. I disconnected the antenna up and down switch and that didn't improve anything. What might be the next most logical thing to disconnect?, the radio, the instrumment panel? I noticed that when I did the checking for current draw and forgot to close the door, I had a high steady draw from the interior lights I guess. The light in the center roof front doesn't go on although the bulb tested good. I've removed this and tested the draw but nothing changed so I put it back in. Should I just pull it and leave it out while I try to narrow down my problem.
I am not terribly familar with the multi-meter I'm using. It is an ancient Micronta unit from Radio Shack. With the DC Amps set to 300 MA the gauge decks itself for a couple of seconds then moves back into a lower range and pulsates with the clock tick I think. When set to 30 MA it basically does the same although the needle stays higher in the gauge and the pulses are more pronounced. Another multi meter I have, a somewhat newer but cheap digital/needle type gauge can't handle the draw and it blows an internal fuse, a 200 MA/250V rated small thing. The alternator is able to overcome this problem meaning it still charges the battery, but I have to disconnect it each night so the car will start in the morning. It is a sedan so the seat lock thing shouldn't come into play. Are there any known areas that I should be checking that I haven't mentioned here?
Thanks for any and all info.

Lycoming-8 · #9 06-01-2007, 01:29 AM

Not sure if you have the full list of items on fuse #2, but the card in fuse box shows the following:

Clock, trunk lite, dome lites frt & rear, Hazard system, warning monitor term SF, power antenna, standing lites frt & rear.

It is not clear what is meant by "warning monitor", but this is what is listed on the card. You might want to unplug all of these with the meter still sitting in place of the fuse and see what results. Could be a wire going to one of the above items has been crimped or rubbed to point of shorting.

barry123400 · #10 06-01-2007, 11:00 AM

An aftermarket radio is always a suspect. In too many cases I have found them drawing when turned off. Make sure the trunk bulb is removed temporarily as you canot verify if really off when trunk lid closed. Unplug your power antenna as they have been a common source as well. Just a few of the common offenders that are quick to eliminate usually. I think the glove box light is on the key circuit. May depend on the year.

Brian Carlton · #11 06-01-2007, 11:17 AM

Did you disconnect that clock yet........and repeat the test?

Mustang_man298 · #12 06-01-2007, 11:34 AM

Quote:

Originally Posted by barry123400

An aftermarket radio is always a suspect. In too many cases I have found them drawing when turned off. Make sure the trunk bulb is removed temporarily as you canot verify if really off when trunk lid closed. Unplug your power antenna as they have been a common source as well. Just a few of the common offenders that are quick to eliminate usually. I think the glove box light is on the key circuit. May depend on the year.

Just to clarify, I think he means to make sure the "on" circuit of the radio is not hooked to unswitched power (you would easily know if the radio does not turn off with the key). A newer radio, which you say you have, will also have a "memory" circuit that retains your presets, that wire should be hooked to unswitched power and should be a very very tiny drain that normally won't hurt a battery for weeks. (I have let mine sit for 5 weeks waiting on parts before and it still started right up, I also have an aftermarket radio). You should verify the connections though, too often they are scabbed with butt splices or the like and have a stray strand of wire poking out or stripped too long in the crimp and some bare wire exposed, that would be all it takes....

barry123400 · #13 06-01-2007, 02:18 PM

Mustang man, thanks for the clarification. I have also seen these aftermarket radios draw even when turned off. I wondered if a diode failed or whatever causing the amp to still draw. Something was strange with them.
It was substamtially heavier than a memory draw. Never dug into the radios to establish the exact issue with them.
Auto radios almost passed the point of being serviceable a long time ago. You had to get too much stuff out of the way before addressing the actual problem.