'83 300SD - dead short across the battery.
 

I figured I'd bounce this off of you guys first before I go any further with it.

I got a dead short across my battery last Saturday night on my way home, frying my brand new battery. It was somewhat intermittent in that a couple of times on the hour-long drive, when I hit some bumps it would clear up for a split second.

I've done some tracing and troubleshooting with the multimeter and it's in between the junction under the fuse box under the hood and where it meets the back of the tumbler. I've done a little searching but I'm a bit leery about peeling back the dash still. I've only had the car for a month and a half so I'm still learning my way around it, but I'm learning fast.

Anyone got any wise ideas? Able to point me in the right direction?

Dead short? I bet if you shorted an MB diesel battery with a wrench across the terminals you'd melt the wrench and posts.


Like, when it was happening did your lights shut off along with everything else? How bad of a short? No fuses blown?

Ya, if you had a dead short you would KNOW... there would likely be a fire, and at the very least a ton of smoke from the glowing wires and overheating battery.

-Jason

Something odd about your description. You say it is a short across the battery but describe a wiring problem under the dash? Are you sure it destroyed your battery? I can't see how a wiring short could cause a battery to fail. If it completely discharged the battery, I agree with the earlier posts that you would be surrounded by charred material.
What exactly are the symptoms?

I don't think anything could pull that much power except the positive battery cable shorting to the frame. It would instantly burn small wires and fuses.

Check That wire all the way to the starter.

Just at terminology issue; not everyone knows Auto Speak.
What I got from the description he has an intermittent short in the wiring near the fuse box.
If that is the case he will need to find the wires or wire that the Insulation has rubbed through an shield it or replace the wire.

However, also in the description concerning the "bump"; could also be a symptom of a loose/broken wire cutting off the electricity and, a resulting temp loss of power.

Thanks for the quick responses guys.

I used my meter to check all of the connections at the junction where all of the wires go from the battery to the start & alternator.

When I was doing testing, there was a leak in the battery telling me that one of the cells had ruptured, and it was refusing to take a charge. My battery charger just had no response when I hooked it up and my battery reads something like 6 volts now.

I've had intermittent problems with my charging system that I'm planning on fixing now that I finally have all the V-belts so that I can rule out the alternator or not. The car had been sitting for a year or more when I bought it so there was some stress on the old battery from that of course. After a while and a couple of times I drained the battery down real low, so it wouldn't hold a charge anymore. After I bought a new battery it turned over like a top every time for the next week+.

What happened was I was driving home at night. The car was fine all day, but all of the sudden the battery was basically dead (power accessories, etc were all real slow and unresponsive) when I drove it a couple of miles to the restaurant we were eating at. When I parked it I could tell it wasn't going to restart so I had to get a jump. It fired right up and everything was fine. On my way after this, after a few minutes all of my electrical components in the car basically just died. My headlights basically dimmed to nothing and all of the electrical in the car just more or less quit. Quite an intense 30+ mile drive home through the country, needless to say...

The only fuse that blew was the heater blower (common, from what I hear) a week or so before this. All of the wiring on the starter and alternator looked fine to me, and I disconnected all of the wires at the junction like I said and managed to trace the only dead short to ground going all the way to wires under the fuse box, and then the big three wire red hot lead that goes through the firewall and to the ignition. I jiggled my key around in the ignition a bit and a couple of times for a split second the short lifted. The previous owner also had a massive keyring and I know that causes issues as well, so that might have anything to do with it.

Work in progress! Thanks in advance.

-Sean

I don't see how a simple short in small gauge wiring can destroy a battery like that. Something else must be going on because if the power drain was enough to completely drain the battery, it would have started on fire.
Start with the basics. Make sure the alternator is charging. Check the battery for at least 13+ volts with the engine running. It sounds to me as if your alternator is not charging and the battery simply lasted a few weeks without being charged until it finally gave out.

kerry - I agree with you there. The alternator definitely wasn't charging like it should, but it was still somewhat functional, even with the tired old belts which are no doubt out of adjustment as well. I tested weeks ago and there was a positive difference between running and non.

Even at that, I don't believe a non-working alternator would cause wiring that was perfectly functional and all in order when I tested a few weeks ago would suddenly be causing a now legitimate short.

I'll post photos later tonight if anyone is confused about exactly what areas I'm talking about.

What were the voltage readings at the battery with engine off and engine running?

My first MB I purchased the owner didn't know if it was the battery or fuel pump. I picked up a battery, installed it, and it turned the starter for about 3 sec and died. I was thinking crud... Got a volt meter and the battery was only reading 9. something volts. It shorted out internally. Went back got a 2nd New Battery. Car started and off I went.

Check your voltages, not running and running.

Good luck

I tested it a couple of times a few weeks ago. The voltage on the toasted old battery after a good long charge would be around 12 before a start. Afterwards it would raise over 13 but I know it wasn't quite the charge it should have been from the alternator.

I don't think this is a quick pop in and play fix. At this point I can't even hook up a battery with the car wired up properly because as I said, there's a dead short between the two cables and I've traced it from the cables to the fuse box. If I threw another battery in it'll likely get destroyed too. With everything unhooked at the junction on the frame beside the engine I've tested all of the wires that go to the battery and alternator and none of them are shorting, all of them are linked to where they're supposed to be.

I know there's a short in the system but I haven't taken off the dash to find out where it is. I was hoping someone could help me with some possible points to look for and point me in the direction for some instructions for the dash removal proceedure as my searches came up kind of flat. Sorry for all the hassle, but hopefully we're onto something here. Thanks again.

Well you could pull all the fuses and replace them one by one. Do you have an amp meter? That would be one way of figuring it out.

Have you considered that the short might be in the battery? Problems with new batteries are not unheard of.

I realize that, but that doesn't explain why I have a short across my battery cables on the car when they're unhooked from the battery terminals and everything is wired up correctly in the junction. If that were the case, simply removing the battery would remove the short. The short remains.