My dad's 69 Coronet RT has a charging problem that we can't seem to figure out so we're looking for advice and help. Here's a description of the problem:

At idle and RPM's below 2500, the needle on the volt gauge rapidly "dances" between 13.0-13.5 volts and this is confirmed accurate because we measured the voltage directly off the battery as well. Above 2500 RPM, the volt gauge will drop to roughly 12 volts. Here's the weird part, when my dad takes the RPM's up to 2500+, I'm at the battery with the volt meter measuring the battery and it spikes up to 200 volts but volt gauge inside the car is holding steady around 12 volts. The charging system doesn't seem to be working properly at higher RPM's and even at the low ones, we'd expect a higher voltage reading and the volt gauge needle has never "danced" around before.

Here's what we've tried so far:
Brand new alternator - no change
Checked alternator belt tightness - no change
Swapped voltage regulator from his 68 Coronet RT - no change
Swapped ballast resistor - no change
Swapped coil - no change
Swapped battery - no change
Inspected and reseated the following wires: battery, coil, voltage regulator and ballast resistor. - no change

Some background on the car:
The problem surfaced back toward in May when we drove the car to the race track. Prior to that, the car had been garaged and sitting for at least 6 months. No modifications or changes have been done to any part of the car. The car has a full MSD ignition.(analog 6AL) The only thing we haven't tried yet is swapping MSD boxes which we can do although it would require a little extra effort. There are no other symptoms. The car starts, runs and drives just fine. We're just afraid that the battery will get drained on prolonged drives. This is a street/strip car.

Any help or ideas would be greatly appreciated. I'm sure there are details I've left out so let me know if you have any questions about the car and the set up.

Thanks!

Two things come to my mind, one is the belt driving the alternator is starting to slip at that RPM due to the load increasing at or above that RPM
The other thing is related to the sitting allowing corrosion to occur on the grounds from the motor to the car body That is what I would look at first, cleaning and tightening them first
Let us know what you find and do to correct this

And don't forget the firewall disconnect; the reference voltage that the regulator sees to determine the output comes through connections at the firewall disconnect, if the connections are poor ........

so I would recommend running a new line directly from the battery to the vr. eliminate the wire from the harness temp.
fire it up and see if you get the same voltage issues.

if you do, add a ground wire directly from the block to the vr body.
again see if you get the same issues.

this will tell you where to start looking. what chunk of wiring is your issue.

if you do all of the above and still see fluctuations, the last part is the wire from the vr to the alt.

since it is charging and you see voltage changes at the battery that circuit would seem to be good.
voltage fluctuations and spikes generally are not affected by the alt to battery path.

are these point regs? (wire wound resistors on bottom side) & if so was the one from him OK when it was in his car?

"spikes to 200 volts". could you elaborate on this statement ? i'm confused at that high a voltage. i think that is impossible to obtain, plus if so, the battery should have exploded. i hope you are wearing protective clothing and a full face shield when checking the battery input.

it isn't impossible.
I have seen it myself too.
had an old 64 modified to use the new vr and a 150 amp alt.
this was before I knew enough to do it myself and they screwed up the connectors.
It constantly overcharged and one day spiked the voltage like that for a long time. vaporized about 3 inches of 10 gauge wire insulation and about an inch of the copper istelf.

Thanks for the replies everyone.

We were puzzled by this as well. So my dad is in the car and takes the RPM's above 2500 while I'm measuring the battery with a volt meter. The voltage reading I get on the meter jumps around and progressively gets higher as the RPM's get higher and I've seen it spike as high as 200 volts. We don't see this behavior at lower RPM's. The voltage appears normal although lower than we would expect with the car running and a properly functioning charging system.

borrow another meter

I did think about that as well and will definitely try a different meter. That still won't change what volt gauge is reading at those RPM's though. (Drops to the low 12's) I guess it's possible the volt gauge is going bad.

I also had another thought, this car has a functioning factory amp gauge. We have not done the MADD electric bypass on the amp gauge. Would that be worth a try? (Probably a good idea to do anyways)

cheaper digital meters can read wonky. first we gotta find/fix the charging issue (& we will) then (if needed) the Mad bypass can be considered (some like it some dont, I do not)

I agree.
to be clear, MAD bypass doesn't affect voltage like this. It is clearly on the VR side, you just have to find out where. As I said earlier, bypassing chunks of wiring will help you figure out where to focus your search.

i can see a spike in amp readings into possibly the 200 range with a 150 amp alternator, but i can't possibly see how a 12 volt alternator can produce 200 volts. please explain how that is possible. 20-25 volts maybe, but putting 200 volts into a 12 volt battery will end bad quickly.

https://forums.anandtech.com/threads/modifying-alternator-for-higher-voltages.155626/

on purpose and accidently it is possible.

I had no idea it was possible, I just assumed it was a cheap gauge function plus unsteady tendency of digital gauges in general

quite an interesting link. i'm beginning to understand, but need to go over it again. thanks for posting this ! old guys can learn stuff if they have the right teachers !