I'm needing assistance with a charging system problem. Vehicle is a 1979 Magnum, factory 100 amp alternator, external voltage regulator (1970 & later electronic type/ triangle connector). The Magnum uses the shunt wire to bypass the amp gauge so charging current does not pass thru the bulkhead or the ammeter. There have been no charging system problems for years and everything was fine when I parked the car last fall for the winter season.
Over the winter I replaced the ignition switch (bad mechanically, not electrically) and the starter relay (just because it was original). This past Saturday, I replaced the battery because it was 12 years old and did not turn over the engine. Sunday, I fired up the car for the first time this season. My voltmeter (Auto Meter) spiked to 16+ volts on start up but after a few seconds came down to the 14-15V range. I consider 14-15V at idle (1000 rpm) normal. After a few minutes running, the voltmeter spiked to 16+V again and remained. I connected my digital multi meter to the alternator output post and read almost 18V. I checked the voltage regulator ground and it is clean-I also use a braided grounding strap from one of the VR mounting bolts to the engine. I had a spare VR (not new but know good) and swapped that on but it did not change things. There is a ground wire from the alternator (alt. ground post) to the left cylinder head where the negative battery cable connects-that is good. I let things sit for a day then started some checks. Battery voltage, key off-13.1V. Blue wire at VR connector, key on-12.9V. Blue field wire at alternator, key on-12.9V. I checked continuity of the blue and green field wires, each has about 0.5 ohms resistance, and no continuity between the two field wires. I have not yet pulled the bulkhead apart but my readings appear to be normal.
I next bench tested the alternator according to the FSM-"Rotor Field Coil Current Draw Test". The result was 4.2-4.3 amps. According to the FSM, "Field coil draw should be 4.75-6.0 amps at 12V. A low rotor coil draw is an indication of high resistance in field coil circuit. (Brushes, slip rings or rotor coil)." This is where I'm stumped. Is this the cause of the high voltage output of the alternator? To double check the alternator, I took it to a local parts store for testing and they said the rectifier is bad.
I don't want to throw parts at this, but rather learn. I have a handle on electrical basics but that's it. Is the problem solely the alternator or am I missing something? Sorry for the long post but wanted to provide as much info as possible.
Thank you.
Steve

My experience with rectifier failures is no or low charge. Most common problem is the voltage regulator plug. If the regulator sees an absence or low input voltage on the blue wire it will cause an overcharge. The regulator uses that signal to know what the current battery voltage is to allow the correct rate of charge. If it were me I would check it real close and most likely replace it.

You may have an issue with feedback voltage to the regulator. If the feedback voltage is low to the regulator, then the regulator will want to kick the voltage up to compensate. That is why grounds are first suspect, but it does not need to be the ground - could be the feed.

Do you have a schematic for the circuit? I would measure the voltages at the voltage regulator. It could be ignition switch related.

Looks like the voltage regulator is fed from the ballast resistor. Make sure you have full battery voltage on the ignition side of the ballast resistor.

While it may not seem like a lot, you show a .2 or .3 voltage drop to the VR for the sensing line.

What did you use for ground when you tested that? If you used the battery, check it again with the case as the ground and see if it changes.

For now I might run a line to the ballast directly to the positive on the battery and start it. See if the voltage still ramps up to 18 volts.
If so, then it probably is the alt. since you eliminated everything that could affect the vr.
Well there is one other item, the plug itself on the vr, have been known to get loose. But since you aren't driving, I am not sure if that would be an issue.

make sure you run it on the feed side of the ballast.

Mark & Andrew,
I do not have a ballast resistor in the system. I run a MSD6AL and Blaster 2 coil-these have been on the car for years. When I checked the sense line voltage at the VR, I removed the VR plug, positive side of the meter directly into the plug female connector and negative side of the meter to the VR bolt that also holds the ground strap to the engine. The VR plug connects quite tightly to the VR terminals-it requires some effort to remove the plug.

I would try to measure the ignition side of the voltage regulator when it is running. A bad connection in the ignition switch can cause a voltage drop under load.

You might need to trace the wire to the bulkhead (assume a 79 Magnum has a bulkhead?) to get access to the wire. Then measure from positive to the voltage regulator case - my suspicion s that you will see some drop and it may be inconsistent.

Had high charging volts on my son’s 90 Cummins truck after we changed the alternator and voltage regulator, it was still over charging to to 17 volts. It had blown most of the light bulbs. It ended up being the connection to the voltage regulator was rusty and corroded. Cleaned that up and fixed the problem.

might not make a difference, but don't use the bolt head for ground. use the vr case body. just in case.

then see if it is different if you use battery ground.

again, last test would be to run a pos line direct from the battery to the feed to the vr. I think you might be able to tap into the msd feed, but not familar enough with that wiring. That is to say without cutting and splicing into a line.

One of 2 possibilities. Either the alternator field is being shorted to voltage causing full charge output. Or the regulator field output is high because the regulator is being tricked into thinking alternator current output is low. Since you just replaced the battery I'd be looking at those cables first. Voltage drop test from negative battery post to reg case and alternator case. Voltage drop test from battery positive post to reg ignition feed, alternator ignition feed, and alternator output. Voltage drop regulator field control to alternator. Do all these while running under load. You'll find it.
Doug

Another thought was the battery. Seeing it started doing this after replacement. As said be sure the cables are good and if possible try another battery. Couple quick ideas that do not cost you anything.

Since it all started after a new battery was installed, try another battery. New battery could have a shorted cell.

Thanks everyone for the responses so far. I'll be checking things this weekend.