70 Challenger 340, points ignition system.
Car ran fine , drove it over 200 miles recently and no issues at all.

Got gas a week or so ago, and on the way back I notice the ammeter is charging positive.
The faster I go the more it charges. Get car home and at all gauge needle is in the middle, rev the engine and it will charge heavily positive.

My first thought was voltage regulator, and since my battery was old as well I changed both. No change.
Ended up changing alternator too as it had a broken ear and not aligned properly. Just got an autoparts store replacement for now.

Testing the system:

With all new components, and after cleaning all connections from the alternator through the bulkhead connections to the gauge itself all my voltages seem within range.

I am getting about 12.6V at battery at rest.
I am getting the same at the alternator positive wire.

With key in ' run ' position, I am getting a little over 12 volts at alternator field to the voltage regulator.
Ground of voltage regulator was checked with meter and good.

Voltage through bulkhead connectors and gauge itself was about 12.4V

Voltage at battery with car running is 12.8 at idle and will go up to almost 15V when car is revved high.

Can it be the ammeter itself ( can I just put an aftermarket ammeter instead)?
The ignition switch?

Any suggestions?
I know there is a bypass but after reading a myriad of posts on the subject I am confused on how to do it without potentially frying my electrical system. I need a simple step by step if this is a viable solution.

Appreciate the replies.

The factory service manual has a decent explanation of the system as well as troubleshooting tips.

The ammeter should not be an issue, it is not in the circuit that the VR uses to sense charge voltage, the ignition switch is as well as a number of connections, all of which can cause a voltage drop to the VR causing an overcharge situation.

Also, the charging voltage is temperature compensated, to a degree, so 15v isn't necessarily out of spec, if it's -20F, lol. The FSM gives expected charging voltage ranges based on temps.

I retrofitted essentially the same setup into my 51 Plymouth, it works fine and does get up to the mid/upper 14v range when initially charging the system, but it comes down as the battery charges up. That is normal.

Thanks.
I have referenced the FSM as well with no luck.

I would check your VR "sense" voltage as that is what the VR uses to control the field current or Output of the alternator.
If that reading is more than 2. - .5V lower than the battery, The VR will increase that alternators output to compensate and try to bring the V up. Ideally all the voltage readings should be the same but, every connection is a potential drop.
I might also suggest a ground strap between the engine and firewall cleaning the paint off both. one can read no resistance but when Current is required to flow across a bad connection the resistance can/will increase.

Should your reading be substantially lower at the VR start working backwards to find the V drop.

keep us posted

Just a couple of thoughts...
1. Double check to make sure the voltage regulator is grounded well
2. Check the bulkhead connectors, the connectors often become oxdized causing a resistance/weak contact. Take apart, burnish the contacts and coat with dia-electric grease


Just my $0.02..

Grounds going bad?

Sounds to me like you have a load on the battery side of the system that increases with rpm ...If the amp meter is good. Maybe the battery itself? You should have 14.7 volts at the battery when running at any rpm so that kind of tells me that the loadis there at idle to with you only getting 12v then. When you rev it can actually cover the load and you see it on the gauge/voltage. Some thing has a heavy draw on system Try popping fused circuits to see if there is any effect. Do you have any added on accessories?

As a flat out, across the board statement, this is wrong.

The FSM gives the expected voltage ranges at specified temperatures, 14.7v is rarely specified and this time of year not going to be seen. Basically, that value is the high end of the range if the temps are at -20F.

maybe it's just me, but i like to see 13.8-14.2 or so when running, with 14.5-14.8 for a few seconds or so after initial start up.

Per the FSM at 80 degress Farenheit voltage range should be between 13.8 - 14.4

The car did have accessories on it, but I have been removing them, and each time I did I tested the system and had no issues. There are still some installed though, could be one of them. Pulling fuses couldn't hurt assuming the ammeter is working properly.

I will double check engine ground , I know voltage regulator ground is good. FSM also suggests checking those , along with ignition switch circuit between battery terminal of ignition switch and voltage regulator. Not sure how to do that.

That's the issue right there. The regulator will now overcharge by that ~half volt.

You can also put the positive lead on your voltmeter on the positive terminal at the battery, and the negative lead on the sense line at the alternator/regulator. That will give you the voltage drop too. Then you can move the negative lead back 'upstream' following the sense line through the bulkhead connector to see if there's a particular connection that's bad.

To test it, if you add a jumper wire there (bat+ to sense line), the overcharge problem should go away. You'll also never be able to turn it off, lol. But it will show the problem.

I fixed it with a relay triggered from the key, which feeds power to the ignition system.

so to help eliminate some of the guess work.
run a line from the pos of the battery to the ballast resistor. one side is higher/closer to battery voltage than the other, when in run.
then start the car and try your voltage tests again and you will see your charging system is fine and it doesn't ramp up to 15 volts.

typically I am lazy and will make a jumper like that powered by a relay with the key on kicking it off.

but since you already did a lot of cleaning, it seems you missed one or it isn't good enough.

2 things when testing. the ground makes a difference as well.
so make sure you use whatever ground the feed is using.
so for the voltage to the VR sensing line, use the case of the vr as the ground, not the battery ground.
run through all the connections again till you eliminate that .5 volt drop.

The VR supplies a variable ground via the green field wire..if this wire contacts ground anywhere on its path from the alternator to the VR, the the alternator will have excessive output.

Ok give or take a few tenths. What do you think it should be at Idle?

It depends on the alternator and idle RPM

Thanks for the additional replies, didn't have a chance yet today to go through the suggestions, but will try later or tomorrow and check back in, thank you.

That answer has already been given.

As well as Alternator and crank pulley sizes. Idle voltage can be influenced by many things so I would ignore that for now.

I might also suggest a ground strap between the engine and firewall cleaning the paint off both. one can read no resistance but when Current is required to flow across a bad connection the resistance can/will increase. I remember my 383 having a ground strap, but no on any 340 car I've had, other than the negative battery cable to the block

All vehicles should have a braided ground strap between the engine and body. In fact multiple straps are not a bad idea. Without one you are relying on metal components connected to the (Engine in this case?) to carry the current required. Paint, oxidation, corrosion and other things that occur over time impede the current flow and wreak all kinds of problems. Grounds are like HP/Torque, more is better.
Your .5V drop between the VR sense and battery is at the upper limit of what I would consider acceptable. But there are variables that come into play such as the age of the harnesses, repaired connections etc.
Might be interesting to watch that V at the regulator as the RPM is increased. It should follow the output of the alternator. IF not there's a clue there as to why the output keeps climbing.
Keep us posted

EDIT: just re read the entire post. If you're topping out at 15V, that .5V drop is a contributing factor as mentioned. That alone would drop you to 14.5. Check battery, regulator and Alternator output all at the same tim, RPM, Conditions etc .