Pulling my hair out on this, did a lot of searching and cant find anything that's worked so far. So I am looking for any ideas.
I have a 75 Ramcharger with a 440, MSD ignition, Mopar per VR. Most of the engine bay wiring has been replaced, interior wiring is mostly stock but in good shape. I recently installed electric fuel pump with relay and tuff stuff 100 amp alt. Now I have a voltage fluctuation that you can hear in new pump and see in all lights and volt meter, Its is worse at idle but there at all rpm's.
Things I've tried:
Checked all grounds and connections.
Tried Three diff VR, using mopar per now.
Disconnected fuel pump, still fluctuates.
Moved the field sensing wire from 12v ignition to battery distribution junction and the voltage is steady as a rock at 14 volts, of course this will drain the battery if left there.
Anybody have any idea what to look at next? Alt? Ign switch?
Thanks for any help.

(1) pull/check/clean/replace the brushes that are in there now (2) as you noted sub in another alt.

put this on a key on relay.

your ignition switch shouldn't matter now.
you have bypassed the reason it could be an issue, by moving the sensing line to a constant 12 volt source instead of passing through the bulkhead, ignition switch and bulkhead again.

If you have added a ground directly to the VR case from say the engine or a line from the battery itself then you have little other possible issues other than the alt.
If you have not tried a ground strap to one of the bolt holes behind the VR, then try that next and see if you still get it.
you might also try reving to say 2k rpm to see if it goes away. still an alt problem then, if you have the extra ground.

I hope you modified your charging circuit to handle that alternator , if something tells the alternator to go full current and the wiring is not the right size for 100 amps things are going to get ugly

You found your problem as the 12 volt ign feed must have a bad connection somewhere before it gets to the volt regulator. I assume you spliced into the blue wire right at the voltage reg with the full battery 12 volts at all times to get the steady voltage reading ? If so I agree with "Andrewh" as you could run a 12 volt wire on a fuse right from the battery to a relay and right to the voltage reg sense wire and of course just turn the relay on with key on volts. As long as the unstable volts at the voltage reg sense wire is not effecting any other circuits then running a hot all the time wire through a key on controlled relay is the best repair as it best to have true battery volts to the reg sense wire and the relay controlled by the key is the best way to run the shortest circuit to the reg of true battery volts so it can control the battery voltage best this way by being able to control the field ground and charging rate. Ron

You have already isolated the issue. think about the following statement

Quote:

Moved the field sensing wire from 12v ignition to battery distribution junction and the voltage is steady as a rock at 14 volts

The VR sense line is what tells the VR to turn the field windings on and to what degree. This in turn controls the alternators output which correlates to Voltage.
By doing the above you bypassed the existing voltage sense wiring and the problem went away. I would say your voltage sense line is fluctuating thereby creating the alternator to do the same.

Now the question is what is causing the fluctuating signal. It can be anywhere between the battery, and voltage regulator. the two notorious locations are the infamous bulkhead connector and the connections to the ammeter itself.
The following may help

sorry, I guess I misread.
I read your sensing line was rock steady, not alt output voltage.
if so, then I agree with above, your sensing line was the issue, but it will be hard to tell why.
as I stated before, it passes through the bulkhead twice and then to the ignition.
each one of those can cause the voltage loss or change to cause your vr to read differently causing the fluctuations.

putting it on a relay, or rewiring the bulkhead then changing out the ignition if that isn't enough are your only fixes.

if I read correctly the first time and it is still bouncing even after changing where the sensing line gets voltage, then what I said above is still true, add a ground.

AGRRED!!! GROUNDS will wreak havoc with the charging system. one can run a good large size jumper form the battery, to the engine / alternator / regulator and see if the problem improves. If not one is back to the positive side and it's multiple locations to induce LABOR, LOL

Actually if he would put the sense circuit on a relay that is battery volts and the winding is controlled by key on volts then since the sense circuit is the J2 circuit it will be feeding battery volts right to the sense wire and the alt field and the ballast feed wire. Its a good idea to do it as the voltage reg should have the best most stable battery voltage reading it can get so it can control the charging rate by an accurate battery voltage reading.

But I do agree with these guys as I would want to find the reason for the fluxuating voltage and fix it first. Then you could still put the circuit on a key on relay if you wanted to. If it were me I would go ahead and bypass the ammeter and also check and fix the bulkhead while you are at it as that may fix the voltage problem anyway. Then install a voltmeter gauge to moniter the charging system. Good luck , Ron

Thanks for the replies.
There are new grounds from VR to eng, from eng to frame and frame to body. The amp meter was long ago bypassed and replaced with voltmeter. The dash power feeds through a bulk head stud in firewall and removed from wire harness bulkhead connector. I went ahead and changed the alt out for know good one and same thing. So I think I will install a relay like suggested for now and open up the dash to try and figure it out. Any Ideas were to start? A little more description of problem: Start her up and the voltage meter will slowly rise to about 14 volts and then start to fluctuate between 13.5 to 14.5 volts in a pulsating 1 second intervals. If motor is reved it will kind of diminish until rpms are steady then go back to fluctuating. I hate electrical problems.

so sorry, can you please clear this up for me.

When you wire the vr sensing line directly to the battery feed, you don't get the pulsing?

Yes, dead steady. No pulsating at all.

ok, then you just have to trace back the sensing line to see where you get drops. each connection that it makes could be causing it.
the bulkhead first, then back to the ignition, then back to where it feeds in from.
get voltage readings before and after each connection and that will tell you what needs to be fixed.

or let it go and just wire up the relay instead.

AGREED!! The proposed band aid will work, but, to a point. It fixes the fluctuating voltage BUT, the root cause of the issue which is a bad connection somewhere on the feed side of the "Sense" line. This circuit also feeds everything else in the vehicle and may lead to an unexpected MELTDOWN somewhere down the road due to the root cause not being addressed. The Band-Aid is not a fix I would recommend or let out the door of my shop.

yes definitely find out if it is an alt or if it is a wiring prob (keep us updated!)

I think based on what op said before, that the internal power feed is coming from
The dash power feeds through a bulk head stud in firewall and removed from wire harness bulkhead connector.

so it sounds like it is really only power coming out that is causing this. so I wouldn't expect a melt down if he leaves this fix in place.

The fact that it just started after adding a new pump makes me wonder though, where did you wire in power for the pump?

Ever wonder why modern cars have so many relays? To avoid having to run the main power for ignition and all accessories through the ignition switch.

The OP has a bad ign. switch or a faulty at the ign. switch connector...the fact that jumpering the field feed to the battery solves the problem eliminates the alternator.

,
The alternator, regulator is not the problem> The root cause is somewhere in the feed to, or out of the ignition switch to the VR sense line.

After spending some time tearing into the interior wiring harness, I guess my statement that the dash wiring was in "good shape" was inaccurate. The first thing I found was the run circuit wire from the Ing connector to bulkhead was toast. I made a quick temp replacement wire and while installing it found two more wires in the Ing connector melted together and possibly shorted. Looks like you guys were right on with your diagnosis.
Now the question is, is this the cause or the result? Any way to test ING SW?
I'll try to find the spare ING switch I have, and try and replace the damaged wires and see what happens.
Thanks again guys.

It is hard to say which came first, or why. normally when there is enough resistance in a wire to start melting the cause of that resistance is nearby. IE: melted wires at the dimmer switch would indicate the excessive resistance is in the terminals or the switch itself. hopes that helps a bit.

BTW typically the RUN wire also doubles as the sense line for the VR. That would also mean your voltage to the ignition system has been fluctuating as well,