This is likely your problem. I had the same think happen to me and tried everything but couldn't locate the problem. I then changed the signal switch and problem solved. The short was inside the flasher block which could not be seen without cracking the block open. It was shorted enough to cause the needed to swing off the scale for each flask but not enough to throw a fuse.

Starting around 73 they went to a shunt ammeter. The advantage to that is not all the current in the system has to pass through it, but as you say they seem to be less accurate.

I colorized my FSM scan just the other day!
1971 M300/400 chassis (no bulkhead connector)

I was tracking down an over-voltage regulator issue.

Check the FSM diagram I posted, my issue was a less than perfect ground from the reg case to the alt case (it is a single continuous wire) and the electronic reg is very sensitive apparently.
New reg and rebuilt alt, but put out 16V at speed!
I cleaned all and put the brown wire directly on the reg case as shown and peak V dropped to 14.9 at 4,000

Dunno if C bodies went on shunted for that year, but B bodies changed to shunted on 75 and A bodies on 76

14.9 is still too high IMO, 13.8 to 14.2 is considered ideal for a lead acid battery. One can fudge a bit but anything over 14.5 indicates a problem and can lead to shortening the life of components

Nice Diagram by the way

Note for everyone here. If the problem was the 3rd bulb added with the hight intensity filament, problem with ammeter will become the same or worst applying brakes. So is not that.

At iddle with stock alt is pretty much normal having a discharge reading with turnings and brakes. But -20 is still way too much, it should be on a 5 to 10 amps flickering. Then giving throttle things change depending on batt load conditions.

Throwing out an idea... let's try to bypass the Turning switch and check at the same time fronts and rears independiently. Disconect the plug down the column and jump out the red wire ( that's the TS flasher ) with dark green then brown wire and check the ammeter reading. It will make blink just rears, green one side, brown the other side

Then proceed the same with tan ( one side ) and light green ( the other side ) with red and check ammeter. That will make blink just fronts and cluster. It could blink at diff speed due the less bulbs handled in front.

Post results

You could make the same later, with both greens together with red for one side ( front and rear ) and brown and tan with red ( front and rear )... that will emulate the TS discarding it and check ammeter again.

P.S.: dunno how did you spliced the painless wiring into the factory TS, but I'm talking about stock wiring colors coiming from underdash harness which also matches the TS harness, if still factory ( not aftermarket replacement )

On a side note... having a bulb added on rear you should use the hazzard flasher for turnings. Hazzard flashers are rated diff and labeled heavy duty from TS flashers to handle more load.

Wouldn't application of brake lights result in a constant amp meter reading vice oscillation? Were as a turn signal or flasher has the constant spiking of the load as the relay makes and breaks contact.

You could test this with car off. Just on the battery and see what type of current load you get on the brake lights, and the turn signals. If it is high chances are a possible short to ground that is amplifying the current draw. If things are normal on the battery alone, then I would look at alternator and VR. It is possible alternator is not keeping up and your seeing a heavy swing as it flips between alternator sourced and battery sourced. Alternator can be tested at local auto parts.

Yea but there is no reason to get -20 giving brakes and -40->+40 with turning when brakes are feeding 6 bulbs while turning "just" 4... on this case with tail lights modified

I think your under estimating the fluctuation caused by flashing contact, but my point is your statement of six bulb on constant is worst than flashing Turn signal might not be true. I do agree the level of swing seems high.

Thanks!
15V is in spec for low temps, but it isn't THAT cold.


I just found some excellent drawings and descriptions of the Mopar charge system relating to all of this thread.

https://www.forabodiesonly.com/mopar/threads/new-voltage-regulator-at-15v.439417/
https://www.forabodiesonly.com/mopar/threads/voltage-regulator-overcharging.418808/

Great for helping trace Voltage issues at various conditions.

The brake pedal switch is also a fluctuating contact as soon you give brakes. Then is true while you keep the pedal pressed it becomes constant and gives time to the regulator and alternator to react to that and recover a bit, but the contact moment is the same for both.

Anyway there is no reason to get a positive reading like that with turnings at iddle or even less with engine off.

Thanks for all the input folks, I'm going to my son's tomorrow, (Tue.)
I'll try a few thing then, I still have a hunch I may have a dead short to ground somewhere.

We'll see if we can get to the bottom of this....

Thanks again, Joe

I normally check the output at the Alt. Battery stud and then compare to battery voltage while running. if you find a significant difference, the VR "sense" voltage may be low which in turn will crank up the alternator to compensate resulting in an overcharge condition at the battery.

I think that's what happening. I cleaned the bunch of connections at the ballast/ reg/ relay and it dropped one Volt, so more is likely in order for a 50 year old system.

Today I cleaned the ignition switch connectors with rust reformer and a toothbrush, then rinsed with elec cleaner. Shiny as new. The toothbrush gets into the female side on the harness.
The switch is in the sense circuit via the 10ga black BATT and 16ga red #1, and I had seen a 0.3 Ohm drop between them. The V TO the cig lighter went up 0.1V, with key off!
The only connect I couldn't find was the 10ga black feeding the switch from the alternator.
Today I saw 15.3V at WOT, then back to 14 or so.