Alternator Grounding

I was about 60 miles from home on Sunday when the alternator light came on. The ammeter showed a 30 amp discharge. The light went off and came on several times, then stayed on. I shut off all the electrical drains I could. Still a 15 amp discharge.
The EFI didn't like it too well, but the car did make it home.
Checking the alernator showed the field to be grounded. So, remove the seats and firewall panel, then removed the alternator. I checked it again out of the car and the field resistance looked fine. Huh?
Turns out that the 2 feet of field wiring from the regulator to the alternator was grounded. So, I cut the wiring harness apart and found the ground wire from the regulator to the alternator had all the insulation melted off it and had melted into the field wire grounding it. Ouch!

Here's what happened. The wire from the alternator to the battery is maybe a 6 Ga. wire. The ground for the alternator (which has to carry the same current load) is normally where it is bolted to the engine brackets.
We powdercoated the alternator and all brackets. Sinced the powder acted as an insulator, the only ground path was the 2 foot wire from the regulator ground to the alternator. It is only 16 or 18 Ga wire, not quite big enough to carry 30+ amps. So, it melted.
I've repaired the wiring and added a 6 Ga gound wire from the ground stud on the alternator to the engine block.

It lasted for 2K+ miles the way it was, but almost made me walk!
Moral of the story.....make sure your alternator has a good ground or you too may be nursing a sick GT40

Dave,

Thanks! I know my alternator bracket is coated too and I wasn't planning on using an external ground until now!!! I'll scrape it all off and use a ground of good quality.

Ron

Hi,
It might be better to use a heavy braided cable fron the alternator to the chassis, after all, thats where the battery is connected !
Tony.

I've got a braided ground cable from the battery to the chassis and another from the chassis to the engine block. You can bet that I'll ad another from the block to the alternator.

On a related note, a local NASCAR builder related a story to me where an engine in one of their cars (351 based Ford) kept fouling out one bank of spark plugs. They made MSD trade them out for another brand new ignition box. But, you guessed it, the same thing happened again. After going round-n-round with the MSD sales guy, they got hooked up with tech support. They tried this and that, but invariably, after a short time the engine would be down on power and the plugs would be fouled out on that same side of the engine. Then one of the tech support guys who had been around a while asked them to run a heavy gauge wire from each of the heads to the block and to put a braided strap from the block to the chassis. Guess what? No more plug fouling and problem solved. Seems that with sealer on the head bolts and the head gaskets, etc. The heads were not well grounded and the spark across the plugs was weak as a result. I now have a 10 gauge solid wire connecting the heads and block.

Regards,
Lynn

Good idea Lynn - having been caught by bad earths on water & oil temp sensors (courtesy of various types of thread lock / sealing compounds), I can relate to that problem. A few big earth leads to the alt & heads would be a lot cheaper than new plugs every month !

Kind Regards,

Peter D.