Alternator toasted - Advice needed please

The car is a 1971 Swinger with a 340 now instead of the original 225. I recently finished a complete rebuild of the entire car, pretty much everything except exterior paint. The original wiring harnesses were butchered by the PO so I replaced everything I could buy with new M&H harnesses, including the dash, forward light, engine, heater/AC and rear lamp harnesses. Both battery cables are new repops as is the VR. I also replaced the alternator with an OEM style 3-wire rebuilt unit.

With some amount of anxiety because of all the new parts I hooked up the battery cables yesterday. Initially I was encouraged as all the lights and interior accessories worked, including the key buzzer, etc. In other words, the circuits "hot" in the key-off position were ok.

When I turned the ignition key to the run position the alternator started smoking and before I could get back inside and turn off the key the alternator looked pretty cooked inside. I checked the field wires and the 12V blue wire had some bubbled insulation within the first inch of the terminal but the rest of the length was ok. I checked continuity of this blue wire back to the bulkhead connector and it was still intact and showed no signs of overheating anywhere but that first inch. As I said, the wiring inside the alternator looked blackened.

The alternator did have the nylon insulators under the two field wires installed and they looked ok.

I have another alternator and am about to try again. I am hoping, maybe naively, that the alternator itself may have been the trouble. I read through some earlier forum posts on this topic and have since made sure my VR is grounded by scrapping off the paint under the mounting bolts and reinstalling the bolts. I will make sure the field posts on the new alternator are not grounded before I hook up again.

I would welcome advice on what else I can do to avoid a repeat of the problem before I hook up the battery again. I need the "for dummies" version as electrical system debugging is not my strong point.

KindredSpirit said:

I replaced everything I could buy with new M&H harnesses, including the dash, forward light, engine, heater/AC and rear lamp harnesses. Both battery cables are new repops as is the VR. I also replaced the alternator with an OEM style 3-wire rebuilt unit.

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New does not guarentee good, especially with electrical items like alternators.

KindredSpirit said:

With some amount of anxiety because of all the new parts I hooked up the battery cables yesterday. Initially I was encouraged as all the lights and interior accessories worked, including the key buzzer, etc. In other words, the circuits "hot" in the key-off position were ok.

Click to expand...

Good.

KindredSpirit said:

When I turned the ignition key to the run position the alternator started smoking and before I could get back inside and turn off the key the alternator looked pretty cooked inside. I checked the field wires and the 12V blue wire had some bubbled insulation within the first inch of the terminal but the rest of the length was ok. I checked continuity of this blue wire back to the bulkhead connector and it was still intact and showed no signs of overheating anywhere but that first inch. As I said, the wiring inside the alternator looked blackened.

The alternator did have the nylon insulators under the two field wires installed and they looked ok.

Click to expand...

So far there seems to be two likely causes.
1. An internal short to ground in the alternator field.
2. A poor connection at or near the field terminal where the blue wire connected.

We know almost for certain that the positive field wire had too much current because the insulation is melted back.
We know something got hot enough that it looks to you like the alternator internally was very hot.

So now narrow it down.
On the alternator.
a. Check for continuity or resistance through field. (Using an ohmmeter place a probe on each spade terminal).
b. Then check for continuity from each brush connection to ground. (place one probe on the alternator casting, and the other probe on a terminal. Then check the other terminal)
c. If you want to look inside, they are pretty easy to take apart.
see this post: Ammeter pegged...
and this thread: Alternator repair, a little show and tell. (ignore the pulley stuff)https://www.forabodiesonly.com/mopar/threads/alternator-repair-a-little-show-and-tell.431108/
tips: Remove the brushes first. Then the casing bolts - one may be a different length than the other two. When you split it, make sure the middle of the sandwich stays with the casing half that has the terminals.

On the terminal.
Slip a connector removal tool in from the front to flatten the spring tab. A flattened cotter pin will work.
Remove the plastic connector housing from the wire and terminal.
Examine the connector's crimps.

Here's some ref. photos
Original 1967 Chrysler female terminal with single wire and its connector housing.
On the left are a two similar open barrel terminals that will also work. However they may be too loose or too tight in the original Chrysler housing.
Most important, notice how it was crimped. First set compresses the wire and there should be no gaps or spaces. Second crimp hold the insulation.

Flip them over and you see the locking tab. This must be depressed to remove the housing.
When reassembling, make sure it locks in. Otherwise when the connector is installed, the terminal will back out and make a poor connection.

Rebuilts have a poor history. MAKE SURE you check continuity to ground of both field wires of the new unit. They should be OPEN (infinity)

The blue field wire is "ignition run" there is only one switched wire into the engine bay, and that is it. IT IS NOT FUSED. You "hope" you did not damage to the harness. I'd inspect the harness backtrack to the bulkhead connector, looking for burned/ melted parts of the harness

When you get it hooked up and running you should check voltage drop issues, but worry about that .............later...........

Exactly the two electrical experts I was hoping would comment. I did open the alternator and it was burned on every brush contact point. Pretty badly. I checked the terminals on both field wires and they were loose, especially the blue. I found that strange. It was a brand new harness. I know it got hot so that could have loosened the crimp on the insulation but the inner crimp on the wire itself was loose. So much that the terminal fell off while I was inspecting the wire. I have the correct style of crimping pliers so I cut and reattached both the blue and green wire terminals. Then I installed the second rebuilt alternator.

Tried the run position again. This time no problems. No heat in the blue wire at all. So I don’t know if it was the alternator or terminals. I did check through the bulkhead and to the ignition switch wiring and did not see (or smell) any signs of further damage. I will check for voltage drop once it’s running.

I must say this fits a theme of an earlier post about the inconsistent quality of replacement parts making restoration of these cars miserable. I’ve come to expect at every step that new parts will fail.

Thanks for your help as I would not have gotten past this hurdle without it.

KindredSpirit said:

Exactly the two electrical experts I was hoping would comment. I did open the alternator and it was burned on every brush contact point. Pretty badly. I checked the terminals on both field wires and they were loose, especially the blue. I found that strange. It was a brand new harness. I know it got hot so that could have loosened the crimp on the insulation but the inner crimp on the wire itself was loose. So much that the terminal fell off while I was inspecting the wire. I have the correct style of crimping pliers so I cut and reattached both the blue and green wire terminals. Then I installed the second rebuilt alternator.

Tried the run position again. This time no problems. No heat in the blue wire at all. So I don’t know if it was the alternator or terminals. I did check through the bulkhead and to the ignition switch wiring and did not see (or smell) any signs of further damage. I will check for voltage drop once it’s running.

I must say this fits a theme of an earlier post about the inconsistent quality of replacement parts making restoration of these cars miserable. I’ve come to expect at every step that new parts will fail.

Thanks for your help as I would not have gotten past this hurdle without it.

Click to expand...

I had similar incident with my blue wire. It pretty much smoked the whole wire under the hood. Opened up the harness to find bad news. It only took a couple of seconds and it was smoking good before I could shut it off. Somehow stupid me got that blue wire to make contact with the alternator case. Poof!