Weird Electrical Problem - PeachParts Mercedes-Benz Forum

desertfox · #1 12-08-2008, 11:03 AM

I had a strange electrical problem last night with my '79 240D. My wife and I went to a holiday open house, the car started right up and we had a nice drive up PCH. We were at the party for 2 hours, we got back in the car and it started up fine again, we drove to a christmas tree lot, parked and wandered off to get a tree. Having found one, I ran back to the car to pull it up to where the tree was, it started, but it didn't feel right, like the battery was all of a sudden having a hard time giving the starter enough juice. We got the tree on the roof and when I went to start it up, nothing happened, like the battery was completely dead. I went out and asked for a jump, but when we got the cable set up, the car wouldn't turn over and we actually saw smoke coming up from the battery/cable connection - the positive grips on the cables were hot when we took them off. We ended up popping the clutch to get the car started (thank goodness for an incline!). As we drove, the lights dimmed to the point that we couldn't see the headlamps and the indicators weren't working. The inside lights were also completely off. This made me think that the alternator was dying. So we kept driving home, I turned the lights off since they weren't shining anyways. So we drove some more in the dark until I got cut off and honked the horn, which worked loud and clear. So I turned the lights back on and they all came back on full force. We drove home, turned the car off and then turned it right back on again, the starter had no trouble.

I figure there must be a short somewhere, but I don't have enough electrical knowledge to find it or even hope to diagnose it. Has anyone experienced anything like this or have any thoughts? I'd really appreciate any suggestions.

Josh

bustedbenz · #2 12-08-2008, 11:25 AM

Quote:

Originally Posted by desertfox

I had a strange electrical problem last night with my '79 240D. My wife and I went to a holiday open house, the car started right up and we had a nice drive up PCH. We were at the party for 2 hours, we got back in the car and it started up fine again, we drove to a christmas tree lot, parked and wandered off to get a tree. Having found one, I ran back to the car to pull it up to where the tree was, it started, but it didn't feel right, like the battery was all of a sudden having a hard time giving the starter enough juice. We got the tree on the roof and when I went to start it up, nothing happened, like the battery was completely dead. I went out and asked for a jump, but when we got the cable set up, the car wouldn't turn over and we actually saw smoke coming up from the battery/cable connection - the positive grips on the cables were hot when we took them off. We ended up popping the clutch to get the car started (thank goodness for an incline!). As we drove, the lights dimmed to the point that we couldn't see the headlamps and the indicators weren't working. The inside lights were also completely off. This made me think that the alternator was dying. So we kept driving home, I turned the lights off since they weren't shining anyways. So we drove some more in the dark until I got cut off and honked the horn, which worked loud and clear. So I turned the lights back on and they all came back on full force. We drove home, turned the car off and then turned it right back on again, the starter had no trouble.

I figure there must be a short somewhere, but I don't have enough electrical knowledge to find it or even hope to diagnose it. Has anyone experienced anything like this or have any thoughts? I'd really appreciate any suggestions.

Josh

First guess: you have a severely sub-par battery All those restarts in fairly close succession drained what little capacity it had left, until it would no longer start the car. Your use of the headlights took out the last remaining little bit... an alternator can't recharge a completely dead battery AND help maintain the load on the electrical system. I don't know what vehicle was used for the jump start, but these diesels are heavy and hard to turn over, and require a lot of current. I'm speculating that the jumper cable wires could have been fairly lightweight and/or the battery providing the jump was a lower cranking amperage than what was needed. Last time I had to jump-start the SDL, we had to run the helper car for fifteen minutes before the large engine would turn over.

When you turned off the lights and everything, the loads essentially disappeared, and the alternator was able to get a quick shot of juice into your failing battery, enough to run the lights the rest of the way home.

Possible? Maybe. Just a guess.

desertfox · #3 12-08-2008, 11:36 AM

I'd thought about that, but the batter is only 4 months old, I bought it at pep boys, so I assume it's not really shoddy...
Josh

81300sd · #4 12-08-2008, 11:51 AM

Sounds to me like just a poor/dirty connection at the battery.

bustedbenz · #5 12-08-2008, 12:37 PM

Quote:

Originally Posted by desertfox

I'd thought about that, but the batter is only 4 months old, I bought it at pep boys, so I assume it's not really shoddy...
Josh

You are probably right. What is the CCA rating?

Also, results from a volt meter on your system, while off, while cranking, while idling, and while running at 2000 rpm would prove interesting. This would verify the alternator functionality, at least, to rule it out. Although the fact that the system dropped out and then made a comeback makes the alternator unlikely anyway.

Jeremy5848 · #6 12-08-2008, 12:40 PM

I would recommend going over the car looking at all of the high-load connections . . . battery terminals, cable to terminal block, to starter, the ground strap under the car from the block to the chassis, etc. Something may be loose or dirty or both.

lowriderdog37 · #7 12-08-2008, 02:16 PM

Also make sure you have the right size battery...it should be freaking huge!

The smoke and sound from the connection would suggest a loose connection. Grab it and yank it back and forth a few times it should not move.

Rafi · #8 12-08-2008, 03:48 PM

sounds to me your starter is bad, battery OK

barry123400 · #9 12-08-2008, 05:13 PM

Because of the hot battery cable or cables a heavy intermittent short is present at times. The starter is the best suspect but hard to prove sometimes when intermittent.

I do not think a shorted alternator would normally clear itself. Also the next time it happens I would take a meter to make sure the glow plug circuit has not remained on. This glow plug current draw though substantial would not heat the battery cable I suspect though.

The described senario is very hard on a new battery. After you locate and eliminate the problem you might want to take your fairly new battery in for a test. Todays batteries have little tollerance for this kind of treatment.

lowriderdog37 · #10 12-08-2008, 11:24 PM

I had a problem with my starter last summer. When it got hot (usually from highway driving in the south in July), the starter wouldn't work. A jump from a large truck or another diesel would work, but otherwise I had to push start.

Not saying that it is your problem as it obviously doesn't get that hot on PCH this time of year. The starter is notorious for faulting intermittently. If there was smoke coming from the battery, I would keep an eye out for a spark (sight or sound) and check those connections. Not saying that is your problems either, just things to look at.

If you do get a new starter, don't get it from Autozone.

patbob · #11 12-09-2008, 06:00 PM

Just a guess since I haven't had it occur to me personally, but I'll bet the glow plug relay was stuck on. That would render a jump pointless unless they had super heavy duty jumper cables (like what you might use for jumping a diesel locomotive

) and be consistent with all your other observations too.