New alternator keeps frying a wire

Is that the fat output wire that's bolted to the large terminal on the back of the alternator or one of the two smaller ones that slides onto one brush terminal?

Are you purposely trying to make this confusing? I asked which wire is red and your answer is "yes"?

The lag in replies is because I had a major house fire a few years ago, and I don't have internet service. I have to trudge outside where it's 9 degrees below zero, hope my van will start, drive 21 miles round-trip to find someplace to connect to a wireless service, and hope I don't have to do that from inside my van with an inoperative heater, all so I can try to help people solve their car problems. I can't stand the cold for more than a few hours, so I head back home where I can squat in front of a little space heater. If you don't reply back within those few hours, I won't see it until the next day. This is why I'm not skipping any days. I'd rather stay holed-up in my garage where I'm warm.

There is no one who knows more about this charging system than me. That isn't hard because there isn't much to this system, and very little that can go wrong with it. If I'm standing next to the vehicle, with a voltmeter in hand, I can have it diagnosed within 30 seconds. Since I'm not by your truck, you have to be my eyes and hands. I'm driving to town every day for your benefit in spite of asthma attacks triggered by cold air. The only variable that is making this diagnosis so difficult is your less-than-complete answers.

This charging system, in my opinion, is by far the world's best system ever developed, and is extremely easy to understand and diagnose. It is the system I always used first when teaching charging systems to my students because once they understood basic electrical theory, it only took ten minutes for them to understand this system and become proficient in repairing it.

This entire system can be diagnosed by taking voltage readings from the three wires on the alternator. The only thing that might be different, since this is on a truck, is the wire colors. Three wiring harnesses were used. One has two red wires on the alternator. One has only one red wire, and the cars had no red wires. If you won't tell me the exact colors of the fat output wire and the two small plugged-in wires, I will most likely unintentionally add even more to the confusion. This conversation has gone on for over a week. We should have had it solved in one night.

The person who started this conversation said the voltage went to 19 volts, then to 8 volts, and he drove home with no trouble. Those do not agree. I’d call 8 volts a problem. 13.75 to 14.75 volts is “no trouble”, but he didn’t say the voltage went back to normal.

The alternator does not have any control over its output voltage, yet he removed it for testing. That is the least-effective way to test it as you’re not including the rest of the system in the tests. A failed alternator would never put out 19 volts, so we know it was okay.

Chrysler’s electronic voltage regulator was the first one used by any manufacturer, and is extremely reliable. I’ve owned dozens of these cars since 1972, and have only had one failure. Replacing one because it’s old doesn’t make sense. A new one has a much higher chance of failing than an old one that has proven itself over time.

The resistor he mentioned is part of the ignition system, not the charging system. That should be a dual ballast resistor. Among all the other innovations Chrysler developed, including the alternator, (1960), and electronic regulator, (1970), they were the first with fully-electronic ignition, (1972 on Dodges and 1973 on Chryslers and Plymouths). He mentioned breaker points, but that truck did not come with that out-dated system. If he has a breaker-point distributor, it is because Chrysler was famous for great parts interchangeability, and someone popped in the older distributor because they couldn’t figure out how to diagnose the simple electronic system.

This all started with a grounded wire that was found. If it went to the ballast resistor, it was the 12-volt feed wire coming from the ignition switch. That same wire feeds the electric automatic choke heater and one of the small wires plugged into the alternator. That wire is always dark blue on cars and on SOME trucks. If I knew which color that wire is on your truck, it becomes real easy to describe how to diagnose the system. If I call it “blue”, and you don’t have a blue wire, what are you going to do?

The next point of interest that most people don’t know is an alternator is physically incapable of developing more current than it is built and rated for. It can not overheat and burn a wire unless someone replaced the output wire with one that is too small in diameter. We know he wasn’t running alongside the truck to replace that wire, and since the system had been working fine for years, that wire is not the wrong gauge. Knowing those things, we know something else besides the alternator is the problem.

The biggest clue that I overlooked is the “Volts” gauge dropping to 8 volts. All Chrysler products through the early � 80s, and some models through the late � 80s used a more-informative “Amps” gauge. I suspect he has an after-market “Volts” gauge. If so, that would be tied in to that same wire that feeds the ignition resistor / alternator field. If the charging system quit due to a defect within that system, the voltage would not drop to 8 volts. It would instantly drop to 12.6 volts, then slowly continue to drop as the battery ran down. That could take as much as an hour. The only way system voltage could drop to 8 volts instantly is if the battery had two shorted cells AND the charging system failed, or if something after the ignition switch was shorted to ground. That is what was found; something was shorted to ground.

This boils down to the engine is running now that the grounded wire was repaired, but the charging system is still dead. All indications are the wire feeding the alternator’s field got overlooked where it was spliced to the wire feeding the ignition resistor. That can be verified by measuring the two voltages on the two smaller plugged-in wires on the back of the alternator. If I’m right, you’ll find 0 volts on both of them. If that is not what you find, tell me the three voltages on the three wires, first with the ignition switch off, then with the engine running, and please be sure to state the exact wire colors.

You can't fault the originator of this question for making mistakes. I wasn't here yet with my wondrous wisdom, and there are fewer and fewer people around who are so familiar with this system. Now that I AM here, there's no reason we can't figure this out, but if I have to drive there to look at something, you're taking me out for lunch!

Voltage on ground? At what points exactly are the meter probes?

If there is still a charging problem, what are the two voltages on the two small wires on the back of the alternator and what are their colors?

I think there's something going on with how you're taking the voltage readings. There's no ground wire on the back of the alternator. That is the output wire that's bolted on and should have full battery voltage of 12.6 volts with the engine not running, and normal charging voltage of between 13.75 to 14.75 volts with the engine running. That voltage should not go much above 14.75 volts to avoid over-charging the battery.

If the engine starts okay when cold, then immediately runs too rich, there's an "S"-shaped link connecting the choke pull-off to the choke blade. Squeeze that a little tighter with a needle-nose pliers to make the blade open more. Also, to check if that pull-off is okay, unplug the vacuum hose, press the link in, then hold your finger over the vacuum port. Release the link, then it should hold in that retracted position until you remove your finger. If it releases right away, the rubber diaphragm is dry-rotted and leaking.

If the engine only starts to run too rich after a minute or two, there's two things to check. The first is there's an exhaust passage that goes between the two cylinder heads and runs under the base of the carburetor to prevent icing, and it runs under the choke's thermostatic spring to cause it to pull the choke blade open all the way. That passage used to commonly become plugged with carbon. The original symptom was the engine would barely run and you'd have a huge cloud of black smoke from the exhaust. I don't know which year this started, but on the later engines, if you unbolt the thermostatic spring and remove it, it sat in a stamped stainless steel bowl that could be lifted out. That provided access to that passage and it was pretty easy to chop that carbon out to clear it. In earlier years, the intake manifold had to be removed to remove the carbon.

Related to this is the heat riser valve on the passenger-side exhaust manifold. That has a thermostatic spring on it too. That valve closes when the spring is cold, to force all the right-side exhaust to go through the right cylinder head, through the intake manifold passage under the carburetor and choke thermostatic spring, then through the left cylinder head and out the exhaust on that side. That valve spends most of its time open, so that's where it tends to rust tight. The hot exhaust gas looks for the easiest route to leave the engine, and that is not through the passage, so it doesn't heat the choke spring. The Chrysler dealer's parts departments have a REAL good product to free up those heat riser valves. It's called "Rust Penetrant", and comes in a small spray can about half the size of a spray paint can. This stuff will do in 20 minutes what it takes a weekend for WD-40 to do.

The second thing to look at is about the same time they started using that removable bowl under the choke's thermostatic spring, they added an electric heater to that spring. You'll see a wire coming out of the spring's housing, and it usually went to a thermal switch right behind the right side of the carburetor, bolted to the intake manifold. I don't recall ever seeing either of those parts fail. The easiest way to tell if that heater is working is to turn the ignition switch to the "run" position, then check for voltage in that area. A slower way is to observe if the choke plate starts to open by itself after the ignition switch has been on for five or ten minutes. I can't remember how long that should take, but as I recall, you may feel the housing getting warm. If that heater is not working, the best suspect is the 12-volt feed wire is cut, disconnected, or somehow overlooked. That feed wire is the same one that feeds the 12 volts to the brush on the alternator.

The spray I mentioned is "Rust Penetrant" from the Chrysler dealer's parts department. It goes on black, and sizzles as it eats its way through rust. It was real effective at loosening rusted heat-riser valves, which is what it was originally made for.

Rust Penetrant will burn off from hot exhaust parts, but the problem I found with this product is you need to wash it off once it's done doing its thing on other parts that don't get hot. I was the suspension and alignment specialist at a very nice family-owned Chrysler dealership through the '90s. On every alignment, I sprayed some lube on the threads joining the inner and outer tie rod ends. I saw a lot of the same cars once a year for a regular maintenance alignment, and I thought lubing the threaded connections would insure they would not be rusted tight next year. In fact, what I found is Rust Penetrant opened the way up for moisture to follow it in, then the parts got so rusty, I needed a torch next time to free them up. Instead, now I use their "Spray White Lube" on the threads. That is a lithium-based grease that goes on real juicy. The liquid sneaks into tight places and takes the grease with it. The liquid evaporates and leaves the grease behind. It's real good for door hinge pins too.