78 amp alternator upgrade issue

My 40 amp alternator in my dart took a dump last night. I figured since I've added injection and electronic ignition, I would upgrade to the 78 amp alternator and the newer style voltage regulator. I do have the injection system, electronic ignition, and headlight system attached to the back of the alternator. I did add in the second field wire. Now my ammeter is pegged to the charging side, but the battery is only getting 12.8 volts. What could cause this? I do have another voltage regulator I could try. Could the field wires be plugged in wrong? Electrical is not my strong suit. Thanks!

Do you know what the amp rating is on your amp meter? Is the 78 amp alternator a newer Mopar replacement factory type or a after market one?
If Mopar type try reading the voltage at the output side of the alternator on the big wire going back to the battery with the motor revved up above 2500 RPM and see if the voltage output increases Is the alternator belt real tight or not? If not tight, tighten it up so it is
Let us know what you find

1st... don't worry about the ammeter capacity anymore. Your ammeter will be safe with high output alts since won't read anymore power coming and going from batt because wont be discharged like previouslly did. The stock 78 amps mopar alt is still not the MOST efficient of all but WAY BETTER than the old stock one ( is the one I'm using ) specially with the small pulley they use to come. Ammeter reads what the batt sucks and provides, not what the alt sources,

Now, about the voltage... not everything is on alternator, but on wiring conditions. Did you JUST upgraded the alt without consider the wiring and ALSO ammeter conditions itself? After all these years of abuse over a not correct charging plattform from factory lot of stuff can be affected

Charging system upgrade

Alt fields can be connected in either way, so don't worry about that.

Reg must be good grounded.

What's the voltage at the alternator terminal stud?

If the alternator is full fielded (max output all the time) then idds are the green wire is grounded, or your regulator is bad.

Don't newer alternators have the voltage regulator built in ??

No stock Mopars that I know of
You can buy high output after market single wire and use them also
I did on my current S/P bracket car, Powermaster 100 amp single wire

I did use the factory 78 amp alternator, and in going to run the ammeter bypass tonight. I started the car this morning to check the voltage at the back of the alternator, and it was showing 15 volts there. I checked the battery, and it was showing 13.5. I didn't change anything from last night to this morning. Any thoughts on what to check?

so you converted a 68 single field charging system to the dual field charging system for the 78 amp alt.

one field wire from the new vr, the second field wire from the key on 12 volt power that went to the original VR. with a split to the new vr for sensing.


you need to verify sensing voltage first. Be sure you use the same reference ground.
Battery pos to battery neg on the meter for battery voltage.

sensing line to case of the VR for sensing line voltage. Do not use the battery ground or you may get a false reading.

Alt from the back post to the alt case.

I have had charging issues because of 3 things.
the new connector is bad for the sensing voltage line. causing intermittent over charge conditions.

The sensing voltage has dropped too much passing through the bulkhead 3 times. you can clean the bulkhead connector, or use a relay to give power directly from the battery.

The VR was not grounded properly. I always add a ground cable to the bolt holding the vr to the firewall.

15 volts at the back of the alt is a bit high, but 13.5 at the battery is fine.
amp meter might have issues, your battery could be low and is charging to compensate.
or you are actually over charging and don't know it.

If your seeing 15 volts at the alternator stud or on the output wire and only 13.0 volts at the battery you have a bad connection( high resistance, corrosion or loose) somewhere between the two Two volts drop between the two is way to much
take your volt meter and trace it back from the alternator to the bulkhead connector and then back on the other wire that feeds to the battery If your seeing 14+ volts into the bulkhead connector and less than than 14.0 volts coming out your on the way to fixing it
I have seen more than one Mopar, 1966 to 1971 model yrs. have the inner nut on the amp gauge loose causing what you may have now
Please let us know what you find so others on here can benefit from your work also
What year, make and model car are you working on?
Hopefully someone on here will have the correct year and model factory service manual (FSM) so they can look up the wiring schematic and tell you which wire color and pin location to test from

It appears the sensor wire is getting an incorrect reading. At idle the battery voltage shows 13 ish volts, the sensor wire shows 11.5. I have read about putting a relay in to correct the sensor wire issue. Does anyone have a wiring diagram for that? How difficult would that be? I have limited use of my arms, so I'd prefer not to have to tinker under the dash. Thanks!

I put mine under the hood.

the sensing line comes out from the bulkhead and you tapped it for the extra line and the VR.
Just before that split, I cut it and wired in the relay.
Used the original sensing line to trip the relay, grounded at the body, for the other leg ran a new hot from the battery to the relay for power.

That's exactly what I was thinking, but wanted to verify before I did it. Fires suck! Thanks for your help!

As said recheck how you have everything wired & I would temp wire it all up with jumpers & confirm everything is copacetic on your readings then hard wire everything. A relay is good but as you know there is an issue elsewhere that needs to be addressed first.

Have you look at a FSM wiring schematic yet ? If not look at one so you know about which wires feed the field on your alterantor
One is from the ignition switch and the other is from the ignition ballast resistor which is fed from the ignition switch
If you don't want melted wires find out what is ( loose wires, bad connection, corrosion)causing the voltage drop between the dash and engine wiring harnesses and fix it !
I've had to fix a bunch of burnt up wiring harness in many Mopar cars I've owned, the factory wiring and assembly of those cars suck
The old cars, pre OTB-1, have very simple circuits, no magic in them at all, battery and ground only except inside the alternators

Hopefully you have it fixed. If not I agree with Cab about a voltage drop problem. If it still has problems I would do a charging system output voltage drop test next. Good luck with it. Ron

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If there is nothing on the battery circuit other than the battery, then there is a big problem in the wiring between the ammeter and the battery.
Recharging of the battery should never be drawing 40 amps (gage pegged).
30 amps for a few seconds is OK right after starting,
5 amps for a few minutes.
Once recharged it should show zero.

Over 30 amps for anything more than a 1/2 minute and the weakest connections will get hot and then fail.

If the positive field wire is at 12.8 volts and the alternator output is at 14 Volts then there is a bad connection or wire as the others have already posted.

You'll have to make your own diagram, but this should give you a start.
Its a schematic for 67-9 A-body with the mentioned mods wired in as guesses about the connections.
On the 70 up voltage regulator, the sensing wire must be the same as the power feed. The green wire connection is the ground side and the regulator controls hpow frequently to complete the circuit to ground. According to the MTSC, they attach as shown in the triangle. I don't have one in hand to confirm that.

Notice the ammeter only shows current flowing to or from the battery.
Pegging it either direction indicates a big problem.

Because you have EFI and pumps to support it, discharge during start will be higher than a normal car and rechanrging after start will be proportionally higher as well. How much draw from the EFI and pump should be in the documentation.
Adding a parallel wire from the alternator output to the ammeter's positive post will reduce the load on the bulkhead connection both during start and during recharge.
Another paralel wire from the fusible link to the ammeter bat terminal wouldn't hurt if you can make a good crimp at the link.

Remember, the ONLY ammeter load on the battery side of the ammeter is supposed to be the battery. Nothing else. I fyou have anythign else there it will show up as a discharge on the ammeter.

Update: I ran the ammeter bypass wire from the alternator to the starter relay, and wired in a relay for the sensor wire on the voltage regulator. This morning on the way to work, headlights on it was putting out 14.0 to 14.8 volts. It seems to be fixed, thank you for all the help!

not really sure what you meant. but just in case... anything being conected to batt post will show a CHARGE reading on ammeter when engine is running. When engine is off ammeter will stay centered because load won't be running through the ammeter. One fine example is the starter motor, which is sourced straight from batt and ammeter doesn't show that. It can be showing some discharge what ignition sucks or the starter relay trigger load because ign switch is between amm and alt hence into the ammeter reading from batt but not the starter motor load itself

We're all saying the same thing.
Electricians here call equipment that uses electricity "load."
So the load side of a device like a fuse box, or in this case the ammeter, is side with equipment that draws power.
The feed side of the same device is the side where the power is coming from.

The confusion with many cars is as you know, the battery can be both.
It is the power source when the alternator can not supply the power.
It is a load when it is recharging from the alternator.

You are correct, I mis-typed discharge when I meant charge.

Probably too much Bushmills

LOL

And also will show Charge althought is not actually strictly charging the batt just if alt is being enough to the rest of car demand, i.e. nothing being sucked on alt side of the amm from batt. If not, will be simply a discharge process being not really registered by the amm.