Fire Alternator - 1977 911S why..?

[submerge]

Hi.
I just replaced my alternator with a new Bosch unit. I took pics when removing the old alternator so I could re-wire correctly. It did appear as though the short ground wire from top of the case to the alternator was not actually connected but, I"m not sure that is possible. When I hooked up the new unit I put the wires back as they were. I did connect the braided ground wire from the top of the case to the D- post with the other ground wires - oops? That is the only change as far as I can tell.

I connected the battery and walked to the back of the car and smoke was billowing out of the fan. I disconnected as fast as I could and blew out a tiny flame coming off the positive post I could see through the fan.

Question 1 - Would attaching that ground wire do this?

Question 2 - Considering the state of my wires, what do I do?

Thanks all!




and after .....

[Rawknees'Turbo]

Unfortunately, you are going to have to remove much more of the covering for that section of the engine bay wiring harness and see where the burnt insulation stops, and replace what is damaged. The two, large gauge red wires go all the way through the tub and to the positive battery clamp, so you are looking at a potentially big job.

No way would attaching a ground wire to the ground terminal on the alternator cause that, and based on your pics, it looks like either the new alternator shorted internally or that you accidentally reversed polarity on your battery . . .

[timmy2]

Ouch, that hurts to look at.
According to your photos it looks connected correctly.

I'd send that alternator out to be assessed as to if it was the cause of the failure. (Internal diode or winding short)
If so, it should be replaced for free. And ask for restitution for damages to your expensive engine harness.

If it wasn't the fault of the alternator, You may have had wires that were already compromised in the old harness. (40 years old now)

Pull the entire engine harness, send what remains to me, and I'll build you a new one to replace it. PM or Email me for details on shipping it and replacement cost.

Here is a photo of a new '77 engine harness I built for someone else recently.

[Rawknees'Turbo]

Originally Posted by timmy2

If it wasn't the fault of the alternator, You may have had wires that were already compromised in the old harness. (40 years old now)

This is a really good point, Dennis; just moving the very old wires around might have flaked off insulation enough to cause a short.

[submerge]

I thought I might have reversed the polarity on the battery and almost ran back into the garage once I finished disassembly and taking pics. But, no I didn't.

[timmy2]

Engine harnesses see the most abuse in our cars due to repeated extreme heat cycles.
Just lying across that hot engine tends to cook them quite well over time.
Insulation becomes brittle and when wires are moved, the insulation cracks and flakes off leaving bare wires just looking for a path to ground.

Building SC and Mid Year Engine Harnesses

[submerge]

[QUOTE=timmy2;9405458]Ouch, that hurts to look at.
According to your photos it looks connected correctly.

So the ground wire from the top of the case to the alternator was correct?

[Steve@Rennsport]

Actually, there ARE new alternators available at the dealer, however minor adaptation is needed.

Certainly not cheap, but far more reliable than the vast majority of remans & rebuilt ones you find. Due to poor MTBF and QC issues with rebuilt ones, we ONLY use new alternators (and starters) anymore.

[timmy2]

Yes, case ground to the same point as the brown harness grounds is correct to terminal marked D-.
Same point electrically as all the studs/nuts on the outside frame of the alternator if it was built correctly.
(Some people attach the ground strap to the outer case)

[mysocal911]

The two 6mm B+ studs should have round insulators on them to insulate them from
the alt housing. Remove the hex nuts and check for the round insulators. There should
also be insulator washers under the hex nuts with metal washers between each of them.

A closer look indicates the current was conducted thru the diodes, i.e. missing diode lead
on middle diode, which indicates current flowed thru the diodes and not directly to the
alt housing. The middle diode actually got so hot that the solder melted and it moved.
Mostly likely the diodes were the wrong polarity, if the battery was installed correctly
or not jumped incorrectly.

[Walter_Middie]

Submerge,

Here's the notes I took of my 1975 alternator. When I removed it, the red wires were draped across the top of the engine, and the insulation was cracked and pealing off. I ended up replacing the entire engine wiring harness. I'm surprised mine didn't end up like yours - I'm sure it was close.

B+ Battery positive big red wires
D- Ground, brown wires AND a braided ground strap to the top of the engine;
D+/61 small blue wire, the warning lamp circuit; and
DF Dynamo field, which furnishes current to the brushes from the external VR.

[Avanti]

I may not be looking at it right so someone confirm what I see but it looks like its connected wrong.

Photos show The B+ wire is connected to the first terminal (located at 9 O'clock) which is to the left of the DF terminal. It should be connected to the B+ terminal which is the second terminal to the left of the DF and located at 7 O'clock.

Paul

[Rawknees'Turbo]

^^^

Good eye (I think) - it looks like the B+ terminals are in different locations from the alternator that the OP removed and the new one he installed . . .

[mysocal911]

Originally Posted by Avanti

I may not be looking at it right so someone confirm what I see but it looks like its connected wrong.

Photos show The B+ wire is connected to the first terminal (located at 9 O'clock) which is to the left of the DF terminal. It should be connected to the B+ terminal which is the second terminal to the left of the DF and located at 7 O'clock.

Paul

The two 6mm studs connect to the same point on the diode plate. The red wire can be connected to either one.

[mysocal911]

Originally Posted by Joe Bob

So your opinion is that the rebuilder boogered it up? I would agree....that isn't a new alternator.

That's very possible, but I'm sure any problem would have been discovered at final test.
I had to say it, but it appears like a reversed battery or it was jumped backwards.

[submerge]

Originally Posted by mysocal911

The two 6mm B+ studs should have round insulators on them to insulate them from
the alt housing. Remove the hex nuts and check for the round insulators. There should
also be insulator washers under the hex nuts with metal washers between each of them.

A closer look indicates the current was conducted thru the diodes, i.e. missing diode lead
on middle diode, which indicates current flowed thru the diodes and not directly to the
alt housing. The middle diode actually got so hot that the solder melted and it moved.
Mostly likely the diodes were the wrong polarity, if the battery was installed correctly
or not jumped incorrectly.

I thought missing insulator washer and leads was a good variable. I went and removed the nuts, but it is quite difficult to tell after the fact.

[submerge]

At this point would the best course of action be having a independant take a look before I send this sucker back to Bosch? I just bought it from Pelican last week. Is there any actual likelihood they might cover the wiring harness replacement?

Thanks for all the help as usual!!

Wyatt

[submerge]

Originally Posted by mysocal911

That's very possible, but I'm sure any problem would have been discovered at final test.
I had to say it, but it appears like a reversed battery or it was jumped backwards.

Completely agree this looks like reversed battery. But, that is not possible in this scenario. Its pretty darn easy to look and eliminate the possibility.

Wyatt

[submerge]

[QUOTE]

Originally Posted by Walter_Middie

I ended up replacing the entire engine wiring harness.

I can tell you I'm on the fence about undertaking the wiring harness replacement myself. It seems as though I need to have this custom made, and then I'm looking at a healthy number of hours in the garage which I just don't have to give righ now. Man I thought I would have been driving today! Anybody know a fantastic wiring guy in Socal? I'm in Long Beach....haha.. but seriously.


Wyatt

[fanaudical]

Yikes! I'm glad you were able to disconnect things before it got out of hand.

My opinion: Patching the harness is a temporary fix at best (and another potential ignition source).

Replacing the harness is not that difficult or time consuming. Making the harness with all the correct parts and wire sizes/colors/lengths is the hard/time-consuming part.

I think that the fastest path to repair is to replace the engine harness (it disconnects at the rear panel). I've seen Dennis' (Timmy2's) work in person and it's very good work.