Crossed battery cables. What did i do ? - PeachParts Mercedes-Benz Forum

biopete · #1 07-15-2006, 12:39 AM

The Facts: My battery was dead so I put a different battery with opposite pole orientation in 1981 300D NA and hooked up cables, thus hooking the cables to the wrong poles for tlike 10 seconds or so. I dont think i tried to start car before we looked at the identifier marks on the battery (from a 1980 300SD btw).

The Story: (skip it if you want)
Hi all. I'm probably going to get the idiot of the month award for this one. I searched and found no details on this subject. Anyway, I was trying to jump my 1981 300D that has a lousy starter and low compression (still cruises 80 on the highway though unless there is any kind of incline). So it needs more than the average bear to start. After a half hour , said to my friend jumping me with his 1980 300SD "Lets switch batteries.". He said, "It should not matter. Jumper cables should accomplish the same thing.". These are AZ cables and someone told me recently that these diesels need really huge cables to provide the amps or something needed to start and regular cables wont cut it. So i'm like mad and ready to try anything at this point and convince him to switch.

So I put his battery in my holder and hook up the cables. Very soon after, 5 or 10 seconds i think, i dont think i tried to start the car, my friend looking at the markings on the battery said "Are the cables backwards?" Sure enough they were. His battery poles are completely reversed. So i put my battery on my charger and resolve to stay at home.

The Result:
I had a great evening until a few minutes ago when i go to take my car's battery off the charger to the car to start it. I put it in, turn the key, bright lights, glow it nice and long, radio playnig, and turn key some more. CLICK CLICK, lights dim, click click. ****. I go out, wiggle the cables, make em tight and triy again. Dimmer lights and nothing. While wiggling I noticed my negative battery cable was really hot. Just after a minute or so being hooked up. I've never noticed that being hot before. It was really dark doing all this. I took battery out and to porch hoping it was a bad battery. I checked the voltage with a meter -- 12.65.

The Question:
Now i want to know -- what did i do to my electrical system and :Lwhat do I have to look forward to in the way of fixing it.? This is a 1981 300D NA with almost no electrical control modules that i can think of.

The Related Question
Also, another question, my wife says the battery could have blown up on me when i crossed the cables. I dont understand this at all and she just heard this on Car Talk she says. But i got a good tongue lashing for endagering the kids while "playing" with the car.

How could a battery blow up by crossing the cables?

As always ,

thanks.

Peter

dieseldiehard · #2 07-15-2006, 12:47 AM

you probably fried all the silicon devices - radio, glow plug time, tach, alternator regulator, etc.
Batteries are not made to operate into a high current like a short circuit because the acid boils and the gasses can cause it to blow the case apart. Very dangerous and if the hydrogen bubbles ignite than you have an explosion.

Jeremy5848 · #3 07-15-2006, 02:20 AM

If I understand correctly, you didn't try to start the car with your friend's battery in the car, right? And when you put your charged battery back in, it was hooked up correctly because the lights and radio worked.

OK, looking at the schematic, the regulator is connected directly to the battery; reversing connections would fry the diodes. If the diodes fried into a short-circuit instead of an open circuit, they could conduct enough current to cause the battery cables to get hot.

There are also direct connections (don't go through the "ignition" switch) to the glow plug timer and I don't know what else. I'm afraid that dieseldiehard is right.

Didn't you get beau coup sparks when you hooked up your friend's battery?

biopete · #4 07-15-2006, 02:47 AM

Quote:

Originally Posted by Jeremy5848

If I understand correctly, you didn't try to start the car with your friend's battery in the car, right? And when you put your charged battery back in, it was hooked up correctly because the lights and radio worked.

Yep. And that with my 3 batteries from my 3 MB diesels that i swap out from car to car way to much the only way the cables reach the posts is if its hooked up correctly. With his battery it was the opposite

Quote:

OK, looking at the schematic, the regulator is connected directly to the battery; reversing connections would fry the diodes. If the diodes fried into a short-circuit instead of an open circuit, they could conduct enough current to cause the battery cables to get hot.

There are also direct connections (don't go through the "ignition" switch) to the glow plug timer and I don't know what else. I'm afraid that dieseldiehard is right.

I'm hoping for the best. The cables did not get hot until i turned the key. And I think not even until i tried to start it. I waited for the glow plug relay to kick off and heard the "clunk" . The radio and interior lights work fine until i try to start it. Thus maybe the short circuit is limited to a small area. Are you saying the Voltage regulator may be fried? I I will have to investigate tomorrow.

Quote:

Didn't you get beau coup sparks when you hooked up your friend's battery?

More than usual but nothing that made me jump. or lasted very long. It was in direct sunlight in the afternoon . I hate those damn 1980 SD's with all their differentness. They are nothing but french 300Ds .

thanks

babyjames · #5 07-15-2006, 10:18 AM

Yeah, I did this once. Not a big hairy one, really. Just fried the alternator.

Jay.

Brian Carlton · #6 07-15-2006, 05:42 PM

Charge the battery.

Disconnect the alternator........completely.

Then try to start it.

The short circuit could definitely be through the alternator when you turned the key to position II.

If you have a short circuit through the alternator, you won't have enough cranking amps to start the engine.

catmandoo62 · #7 07-15-2006, 06:04 PM

i'll agree you fried the regulator.did you have the cables good and tight,with the power these need if the cables are loose they will get hot.

biopete · #8 07-19-2006, 01:43 PM

Finally got some time to look at this. I sort of feel more stupid now.

I tightened down the negative battery cable real good. The post of the other battery must have been bigger and strected the clamp. I usually just push them on real good until i know it is going to start. Anyway, that is where the draw and heat was coming from just as someone said. I could hear a whistle around the post when the key was on.

The regulator and everything appears to be fine. I must have just put them on for a few seconds . I guess this is a lesson well learned.

thanks all.

rrgrassi · #9 07-19-2006, 01:51 PM

You really do not want to here a "whistle" around the post. That can be the sounds of hydrogen gas escaping and a defective post. While the hydrogen escapes escapes, a little sulfuric acid comes with it, and of course "the science experiment" on top of the battery follows.

Have the alterantor checked, and replace the battery.

biopete · #10 07-24-2006, 01:20 PM

Quote:

Originally Posted by rrgrassi

You really do not want to here a "whistle" around the post. That can be the sounds of hydrogen gas escaping and a defective post. While the hydrogen escapes escapes, a little sulfuric acid comes with it, and of course "the science experiment" on top of the battery follows.

Have the alterantor checked, and replace the battery.

Thanks. I appreciate it. Very good info. The whistling you are talking about would happen all the time , no? The whistle I was talking about whistle was just from the loose connection. When i tightened it down, the whistling stopped. Just electricity flowing , trying to flow to ground and having a hard time, i guess. .

thanks

Jmana · #11 07-24-2006, 04:53 PM

I did this once to a really nice Probe GT that I owned. It literally fried everything. It cost around $2,000 to replace everything. Luckily I had bought a warantee and my dad owned the car dealership, so we were able to get the warantee company to pay for it