Ammeter swings wildly, weird alternator wiring

-
W

Woodsman341

Well-Known Member
Joined
Oct 28, 2022
Messages
356
Reaction score
115
Location
Columbia, SC
Hi guys,

(Car in question is a 1975 Dart 360 Sport)

I FINALLY got five minutes to work on our new-to-us project car, nothing crazy today, just charging the battery and other minor things I could squeeze in. I noticed this problem when we bought the car and put it out of my mind until I could actually dive in. When the car is running, the ammeter swings very quickly between charging and discharging. I know the ammeter works, or appears to, because if I switch it to "on" without starting, the ammeter shows a slight discharge when the electric fuel pump energizes. What would cause that?

Before you answer, I took a look at the alternator, and found this (see picture). I realize this is a higher output alternator than '75 stock, but I always thought the second field wire would be grounded. Any ideas here?

Thanks for any input, links, etc. If I had known what term to search for I would have, but "vibrating ammeter" gets some interesting search results. :)

Pic attached.

IMG_20231025_170952361.jpg
 
Looks like you need to unwrap that mess and see what you're dealing with.
A swinging ammeter could be many things, and many of those things may reside in that rats nest.

I don't know what year specifically the alternator changed to dual fields but 75 is definitely (should be) a dual, assuming it got the right parts in the past. Look up some wiring diagrams for the charging system and see if you can make your wiring match it. Don't assume anything is good either and check part numbers and pin counts on things like the regulator before firing it up again.

Once you're confident it's right, use an ohmeter to check for shorts and a volt meter or test light to check for proper voltages at the right places. Highly recommended to do the ammeter bypass or alternator wire around while you're at it. Search the forum a bit for details, it's pretty simple.

Others will likely be along shortly with much more detail too.
 
That's been hacked. It looks like it may have been hacked to so as to run the field from the main circuit rather than from the run circuit. That would cause a battery drain. Just guessing based on the photo.

The connection at the output stud looks like it may be damaged.
1698274683302.png


A poor connection there could cause the battery to take over running the car.

Electric pumps and fans are almost always problems. if its wired to the battery side of the ammeter then the ammeter needle pointing to charge is indicating eith or both charging and fuel pump draw.
 
1975 Standard main power and key switch feed circuit is a little different than other years.

1698275066057.png


Alternator output power has two main paths into the passenger compartment (instead of one).
Otherwise its pretty similar to preceeding years. It may or may not have originally been equiped with the seat belt interlock depending on when it was built. Don't think that effects your situation, but worth knowing it about.

Ammeter indicates current flowing through it to or from the battery.

The scale is 40 amps discharge to 40 amps charging. Anything close to 40 amps is danger.
The battery should discharge about 5 amps to the ignition while starting.
It should recharge around 10-15 amps for a few sends and then work its way toward zero once hte car is running.
 
Last edited:
D
Mattax said:
That's been hacked. It looks like it may have been hacked to so as to run the field from the main circuit rather than from the run circuit. That would cause a battery drain. Just guessing based on the photo.

The connection at the output stud looks like it may be damaged.
View attachment 1716158277

A poor connection there could cause the battery to take over running the car.

Electric pumps and fans are almost always problems. if its wired to the battery side of the ammeter then the ammeter needle pointing to charge is indicating eith or both charging and fuel pump draw.
Click to expand...
Definitely some damage, and in the long run I'll be going through all the wiring, just trying to sort out one little thing at a time to make it less overwhelming.
 
Mattax said:
1975 Standard main power and key switch feed circuit is a little different than other years.

View attachment 1716158279

Alternator output power has two main paths into the passenger compartment (instead of one).
Otherwise its pretty similar to preceeding years. It may or may not have originally been equiped with the seat belt interlock depending on when it was built. Don't think that effects your situation, but worth knowing it about.

Ammeter indicates current flowing through it to or from the battery.

The scale is 40 amps discharge to 40 amps charging. Anything close to 40 amps is danger.
The battery should discharge about 5 amps to the ignition while starting.
It should recharge around 10-15 amps for a few sends and then work its way toward zero once hte car is running.
Click to expand...
It swings between negative and positive while running, but only about halfway between the first mark to either side. I'll read through your post again and check that diagram. Thanks!
 
Woodsman341 said:
It swings between negative and positive while running, but only about halfway between the first mark to either side
Click to expand...
The entire circuit is hot (positive with no switch or disconnect).
The meter simply shows when current is flowing to or from the battery.
Black wires in the diagram are hot unless the are connected to a ground. For example the battery negative. Its the only ground wire shown in this schematic.
 
How long does it take to swing, aka a second, or 1/2 second, or is it a very fast flutter?
 
So it swingd back and forth while the engine is idling?
 
Sounds like a bad connectin, AKA ammeter, bulkhead connector, or welded splice in harness. Do you have a "gun" thermometer? Or reach up under dash and feel for heat, and or wiggle harness, see if it changes

Bad/ worn/ sticky alternator brushes. Bad diodes (intermittent) or damaged stator. Run engine under load in dark, open hood, look for serious sparking between windings in the alternator. You'll need a load to do that, EG maybe let it sit 5-10 min with headlights on and engine off
 
67Dart273 said:
Sounds like a bad connectin, AKA ammeter, bulkhead connector, or welded splice in harness. Do you have a "gun" thermometer? Or reach up under dash and feel for heat, and or wiggle harness, see if it changes

Bad/ worn/ sticky alternator brushes. Bad diodes (intermittent) or damaged stator. Run engine under load in dark, open hood, look for serious sparking between windings in the alternator. You'll need a load to do that, EG maybe let it sit 5-10 min with headlights on and engine off
Click to expand...

I do have a non-contact thermometer, and I can try your suggestions tomorrow evening probably. Thanks!
 
I'm not much help, but wiring for me is no "little thing", and by the looks of that I would suspect ALL of the wiring.
I would not leave that battery connected while unattended.
Good luck!
 
Definitely need to remove tape from harness at alternator.
Then take a photo of that area.
 
Woodsman341 said:
I'll try to get a short video of it tomorrow.
Click to expand...

I say hold on just a min.
Do you know what a normally functioning amp meter does on these cars?
The reason I ask because of the fuel pump showing a discharge on the needle question, that would be expected and normal.

The normally working meter fluctuates quite a bit depending on circumstances.


The video along with a couple of light revs, and any other you think is odd would tell us a lot.
 
TrailBeast said:
I say hold on just a min.
Do you know what a normally functioning amp meter does on these cars?
The reason I ask because of the fuel pump showing a discharge on the needle question, that would be expected and normal.

The normally working meter fluctuates quite a bit depending on circumstances.


The video along with a couple of light revs, and any other you think is odd would tell us a lot.
Click to expand...

Oh yeah, I'm familiar with the way it usually looks. I owned my original 1975 for about 10 years before (stupidly) trading it in.
 
TrailBeast said:
I say hold on just a min.
Do you know what a normally functioning amp meter does on these cars?
The reason I ask because of the fuel pump showing a discharge on the needle question, that would be expected and normal.

The normally working meter fluctuates quite a bit depending on circumstances.


The video along with a couple of light revs, and any other you think is odd would tell us a lot.
Click to expand...
That's what I was gettin at. They will fluctuate right after startup sometimes too, until the system recovers the battery from start.
 
RustyRatRod said:
That's what I was gettin at. They will fluctuate right after startup sometimes too, until the system recovers the battery from start.
Click to expand...

Or even “bounce” off the voltage regulator enough to make headlights blink dimmer and brighter.
Practically every Mopar owner has seen that.
 
TrailBeast said:
Or even “bounce” off the voltage regulator enough to make headlights blink dimmer and brighter.
Practically every Mopar owner has seen that.
Click to expand...
Mine did do that until I upgraded to the square back alternator and put @crackedback's headlight harness on it.
 
RustyRatRod said:
Mine did do that until I upgraded to the square back alternator and put @crackedback's headlight harness on it.
Click to expand...

Mine did as well, and I always had a square back.
In my case I rewired the regulator sense wire (blue wire on the regulator) through a relay direct to the positive post on the starter relay.

In the run position the relay closes allowing the regulator to see direct battery voltage instead of it running out through all the other ignition and ballast resistor wires.
The regulators voltage sense wire is basically a straight shot to battery pos.

That eliminated bouncing.
 
You must log in or register to reply here.
-

Back
Top