We, my father and I, were in the process of changing the alternator (the old one froze) http://www.914world.com/bbs2/index.php?act...81&hl=newto914s

We got a rebuilt from autozone, but after putting it in we figured out it had a bad diode(never go to autozone). I ordered another one from a place online I get a lot of parts for my old Benz from. It was cheap. $80 for a bosch rebuild + 20 core charge, they have free shipping, and I usually receive my order the day after I place it(crazy fast).
Now I wasn’t there for this, but after my Dad reinstalled the alternator when reconnected the Battery cables the wire harness on the altenator started to fried. By the time he got the wire off the battery again it had completely melted the wire going from the Alt. to the starter, and singed the wires going to the relay board.
What might have caused this? I already ordered a new harness from Jeff, but after I change it I don’t want to melt another one.
Could it be the wrong amp Alt. It’s a 55 amp
Oh, 1.8 block with D-jet
Thanks guys
Samson

There are 2 posts on the back of the alternator. One is the voltage output, the other is ground. You probably hooked the battery charge lead to the ground.

Well, that is a possablity. I was not there at the time it happened. But when I put in the autozone alt(the first try). It looked pritty clear where to hook what(a clip and a wire), but anything possably.
Do you think that would have fried the new Alt?

If you put the charge wire on the ground post, then you didn't fry the alt. It would only be a metal case hooked to ground at that point. Use an ohmmeter to check to see which post is ground, and hook the charge wire to the other one.

Cheap alt rebuilds are just that cheap....either buy a new one or have it rebuilt locally.

Bet the rebuilder hooked the posts up ass backwards.

Once when I was in band camp and had a positive ground Austin Healy....the generator was replaced four times. I was in school and had the local English shop do the swap out.

The rebuild company sent us generators that were wound the opposite direction.....worked for about fifteen twenty minutes and gave up the ghost.

It would be near impossible to hook up the harness wrong. There is a 3-pole connector just like on the relay plate end of the alt harness and a single fat red wire with an eyelet type wire terminal.

If its wasnt a bad alternator, then my bet would be that the fat red wire insualtion chafed off and shorted to the engine tine (a direct shor to ground).

I see this all the time. Be sure the grommets are in place, to isolate the wire from the cooling tin edge.

there was an old, old thread that addressed an issue with a portion of the back of " some" alts making contact to the engine. could this cause this problem?

kevin

I recall that the stud on the alternator that the main positive wire is connected to is taller on the rebuilds than the original. This allows it to contact the air duct/cover and short out. The original wires had rubber boots on the help prevent this problem. If you look at the picture I posted you can see there are two studs. The second stud could be shorting. Apparently this is only a problem with the cheap rebuilds. Mine are not cheap, but give you a great boost in output.

http://www.914world.com/bbs2/index.php?act...=ST&f=4&t=13777

Let us know the cause of the problem. Would be interesting if it was the alternator, What result would you get by trying to get the alternator rebuilder to foot the bill for a new harness if he is indeed at fault?

Compair the stud length between the two alternators, are they different.

Also i suspected my alternator wire insulation, so i re-sleaved it with heat shrink, and as other poster noted, make sure you got god gromments to prevent chaffing of the insulation.


good luck, lets hear the autopsy result.

Again, I was not there for the Melting, but after taking to me Dad he sounded fairly sure he hooked it up right. The Rebuild came back with the resister on one pole, so hooked it to the other pole. The boot on the wire was in good shape, so I don't think it shorted that way. Mabye the insulation on the Red wire caused it to ground out, but their is no way to check now cause it all melted off. The old gasket the seperated the airguid from the ALT disintegrated, but thank could have done this.

ClayPerrine how would I use an "ohmmeter to check to see which post is ground"

My Dad is going to pull it out again on Friday so we'll see then

As much as anything else, the thing to look for is a burn spot. I would look on the inside of the duct/cover. The cover is bolted to the alternator case, and that is grounded through the fan shroud/engine case/tranny case/ground strap/body/and battery ground to the battery. The big red alternator wire goes to the starter, and via the heavy black wire to the battery positive post. Thus, if a stud on the alternator shorts to the cover you will have a good arc-welding circuit. In this case the weak link is the heavy red wire to the starter.

This has certainly happened before with rebuilds. Clearancing the stud to the cover is the only solution. I'll try to provide a stud height measurement when I get home tonight.

Okay, stud height measurement:

1/4" for the short stud, to which the capacitor is connected

1/2" for the tall stud, to which the heavy red wire is connected

The tall stud needs the rubber cap for protection.

Right, look for the burn mark, you should see the effect of the electric arc, ah la arc welding. Should be melted metal on the wire, and short location. However you will not find any melted metal is it was not a dead short to ground on your harness. An internal short in the alternator wiould melt the insulation down the wire, but the arc would be inside the alternator.

If the alternator insides (look thru the slots) are burnt, and smell burnt, it is likely the fault is with the alternator, If the case is that, have the alternator company buy you a new harness, refund your money, buy from someone else, and tell us on the list who it is so we can avoid them.

If your voltage regulator is fried, it might cause a direct path for the alt output to ground. The harness would be the only thing available to take the heat, since the path from the VR plug on the relay board is straight to the alt harness plug.

Since it was the red wire from the alternator to the starter that burnt it pretty much has to be a short from the battery positive to ground. It is very unlikely the regulator or the alternator are damaged. The stud in the alternator where the red wire connects is the first place in that circuit that comes close to shorting. Considering the current required to burn that wire, it pretty much has to be a direct battery short to ground.

since it's an Autozone part, there should be a limited lifetime warranty on this alternator....go swap for another and see if it happens again....

Here is the previous thread, found in the Classics area.
Classics, a must read for anyone working on their car.

http://www.914world.com/bbs2/index.php?act...t=ST&f=5&t=5913

Thanks Davep for a link to the Classics artical. I think that's it. Yestarday my Dad took off the Alternator and had it check and it was shot. I told him to look for a spark mark and he said it looks like it shorted through the resister, but found no marks on the air guide cover. The VR check out ok.

dmenche914 the guys I ordered the ALT from are thebenzbin.com They don't have a ton for porsches but have just about everything for old Benzs. Overall I've been really happy with them, super fast shipping and cheap Euro parts, but their return policy sucks I'm not even sure if they'll take the ALT back. One of those "no return on electrical parts if installed" clauses. Then it could take up to 4 weeks to receive a credit.

Heh... b-b-b-b-b-been there.


M

I'd like to know if the stud lengths were as long as I posted.
If it did short out through the alternator, I'd guess the alternator was bad to begin with. That is to say, it caused the wire harness to burn out and damn near take the car with it.