fried harness again. Walk me through this.

1971 Plymouth Roadrunner, 383 with auto. Electronic ignition.
The fried wire harness continues. For my original story, the original post is here.

https://board.moparts.org/ubbthreads/ubbthreads.php/topics/2373445/1.html

The car was running great, parked it with the battery disconnected for 3 weeks. Went to move it and go for an afternoon drive. Hooked up the battery and it starts crackling and smoke from under the hood. Wires cooked but not as bad as before. I will probably need to remove the instrument cluster again which I am not looking forward to.

Anyway, today I was poking around with my 12 volt test light just to see if anything looked weird.

Here is my question. I need someone to walk me through this. I am hooking up the battery NEGATIVE like normal, but I am leaving the positive cable unhooked to avoid any fireworks.
My 12 volt test light is hooked to the positive battery side, so I am testing to see where my continuity or short may be. When I ground it, the light comes on.

My ignition key is OFF.

What circuits should I expect to be “HOT” all the time? Or, when my test light lights up, what is ok and what is suspicious? I know any ground or ground wire will light up.

The starter relay wire and terminal on the starter? The Ammeter? What else?

Thanks in advance for any input.

Eightlitermopar

Battery power (at least to the switch that operates the items):
There is a bank of 3-4 fuses on the fuse box that are battery hot.
Headlights to switch.
Tail lights to headlight switch
Dome light to switch (includes curtacy lights, glove box, and cigar lighter)
4 way flashers to turn switch
Horn to relay
Radio memory
Alt and the big post on the starter. Gene

Radio memory in 71??

Sorry this recurred.

I still think it will really help tracking this down by noting every wire and connection showing damage.
Two examples here
http://www.heritech.org/cuda/Charge2.html#Fusible_Link_In_Action

Modify the diagram to match your '71. If you want help, let me know. I think I downloaded '71 FSM since last year.
We can make it a little more like tha factory like I did here.
http://www.heritech.org/cuda/Charge.html#Charging_Wiring_Diagram
I assume someone added the electronic ignition. So you'll have to help on how its been wired in.


Correct me if I'm wrong.
It did not start to smoke until after you closed the hood.
It started to smoke when you turned the key to run or start. If so which one?


Not sure what you are trying to do with the test light here.


When the battery positive is connected to the stud on the starter relay, every wire downstream is hot until up until it is interupted by a switch, or at the alternator, the internal diodes.
So the alternator feed is hot, the ignition switch feed is hot, the fuse box lead is hot, the headlight switch lead is hot.

that would be mechanical, the pushbuttons, lol.

Hot all the time: large cable to starter. brake lights/headlights/horn/dome light (you jump in the car & these will operate WO the key). I didn't read the prior thread but I will. power flows in past the bulkhead fusible link then on to/thru the ammeter then to the "welded splice" where it T's & goes to headlight switch/ign switch/horn relay then on back out past the bulkhead out to the alt. Any idea what part/area/system is dead shorting?

Graphics showing the system power supply wires.
All wires connected to the battery feed should be at battery voltage.
For purposes of tracking a short that is frying the supply wires, the wires out of the fuse box should be protected by the fuses (assuming the fuses are the right rating).

Voltage in the supply wires is like pressure in the water pipes of our houses.
There is pressure in all of the supply pipes even when all of the taps are shut.

So if a test light is placed in line at the battery positive, it should not light up.
It can't light up because the all of the switches that would complete the circuit are open.
The system is still at 12.5 Volts. Even though the light has resistance to flow, there is no flow. Voltage on both sides of the lamp is the same.

If the lamp lights up there must be a connection to ground.
If the lamp lights up and the ammeter moves, the connection must be on the other side of the ammeter.
The ammeter shows current flowing.
Using the anolgy of water pipes in the house, the ammeter is like a city water meter. If there is a leak after the meter, it will show the gallons of water flowing even if no one has opened a faucet.

If you test with a lamp in line, it would be safest to attach a small cycling circuit breaker in line with it. IIRC Chrysler suggested their techs make one using flasher module.

In the previous thread I think you mentioned the alternator feed connection was damaged.
I'd look there again. if its damaged then you know its down that line. Is something is spliced into that line? or is insulation damaged? If not, then it could be the output stud or rectifier inside the alternator is grounding.

Might do just as well with a cheap analog ohm meter rather than the test light and breaker.
Disconnect the battery, then start checking for continuity to ground. Disconnect the alt, and then continue diconnecting and checking until you narrow the location,
Thats how I'd do it.

Correct me if I'm wrong.
It did not start to smoke until after you closed the hood.
It started to smoke when you turned the key to run or start. If so which one? [quote]

It started crackling and smoking as soon as I connected the positive terminal. I never even had a chance to get into the car and turn the key.

And you are right, I need to get the multimeter out and start checking.

These sorts of things always start happening when there is no time or money to mess around, doesn't it?

I will keep you posted.

eight

I did get a new alternator last time things went crazy. I will start looking around tomorrow.

I know it's going to end up being something simple or stupid.

eightlitermopar

I had a "new" (rebuilt) alternator on my 70 Chrysler 300Hurst . . . the insulator on the output post "failed" and shorted to ground, immediately frying the fusible link . . . full stop ! Changed the alternator, replaced the link, and everything was fine . . . maybe check the Alternator & associated wiring ??? . . . good luck !

If I was a betting man I'd say you lost your alternator. It's the only failure that accounts for everything you see. What I am puzzled by is why the fusible link didn't fail and protect the rest of the harness.

Thinking about it this morning, if there is a lamp in series as illustrated, current will be limited to whatever the lamp needs. So no need for the circuit breaker as long as the lamp doesn't draw much current.
I'd put your test lamp onto the battery positive post and touch the battery clamp on the positive cable to see if it lights up. If so, the short is still present and you can try to trace it with an ohm meter or the lamp.

This happened last time too. It fried the insulation on the link, but did not sacrifice the copper wire itself.

This time was not near as bad as the last time it fried the wires.

I will take the alternator off and test it to see if there is a short. I do have a multimeter, so how exactly should I test it to see if there is a problem with the alternator itself?

eightlitermopar

Also, I have a pretty busy schedule which makes this "hobby" more frustrating at the current time.

However, as I work on the car I will give updates and ask questions as I go along. One of my pet peeves is when people don't update after asking questions, so I don't want to be that guy.

If you don't hear back from me, it doesn't mean I am ignoring anyone. I will give updates on my slow progress.

Thanks again!

eightlitermopar

Mattax hit the nail on the head. "If not, then it could be the output stud or rectifier inside the alternator is grounding." If it is the stud, a careful disassembly of the connector on the back of the alternator should show a problem. As to the diodes (rectifier) a resistance check to the alternator case from the output stud should show an open circuit. I am curious though, does the output wire from the alternator show any evidence of overheating at the alternator? If so, this would probably be the last nail in the coffin.

As said: take off the black cable from the alt threaded stud. clamp test light clip on batt positive post. touch test light pointer to the alt stud then touch it to the cable end. NEITHER should make it light up.

Thanks. I will try this next on Saturday and report back. I will even take pictures if needed.

Ok, I had a quick minute to do some a test.

As stated, I hooked up the test light clamp to the positive post.

I touched the alternator case just to make sure I had a good circuit. The light came on, as expected.

I touched the alternator threaded stud (with black wire removed) and the light DID NOT come on.

I then touched the black wire (still disconnected) and the light DID NOT come on.

It looks like the alternator is ok at this point.

My next day off I will poke around and report back to see what else I could find.

*On a possible unrelated note, I had my 1992 dodge truck not start the other day. Long story short, a varmint had chewed wire on the ignition pickup on the distributor. I replaced it and got it going. I set up a trap and caught a good size rat.
While poking around on the roadrunner, I found a fresh nest between the driver door and inner front fender. It had rags and acorns stockpiled. This may not be chewed wire related, and I have not seen any wires chewed yet, but I will continue to search.

Thanks again for all your help.

eightlitermopar