So, I got the cooling system figured out and while confident everything is good to go, I get stranded with what I could only deduce was a bad battery (I used my multi-meter on the alternator and it was reading about 12.5 v) Got a new batt and was good to go for about a day. Had Oreilly's test the sys and found the diodes were dead. Confirmed by and got replacement alternator from NAPA as it was under warranty. Well, still not charging. Any ideas. FYI, I'm running a hot wire directly from the battery to the alternator. So I know it's getting power from the Alt...

you can run one wire from ground to the field terminal of the alternator, and then one wire from + of battery to other field terminal - if it shows charging of 14-16 volts, then it is either the regulator, or the wiring from the regulator. . .

None of us are Karnak

You know when you go to the parts store and he asks you year/make/model/engine?

You do understand there is a reason for that right?

if you want help you have to be more specific on your combo

Hello Carl, I'm always glad to help my Omaha brother! You mean you have a big jumper from the alt threaded "batt" stud to the batt positive terminal? that does not "energize" the alt, it just bypasses the ammeter/bulkhead & their associated wiring path/connectors for the alt circular route "output" path which are prone to issues (but not a dealbreaker) just clean EVERY terminal including ground paths, a seperate issue. If this is the '70 earlier system with the "box" reg/single field terminal alt, turn the key on & see if the "ign" terminal on the reg is hot & the fld" terminal on the other side of it is hot & the green wire from the reg "fld" terminal is hot at the alt field terminal. If this is the later "flat" reg system with the triangle connector & a 2 field terminal alt, turn the key on & see if the blue wire terminal in the triangle connector is hot & with it plugged into the reg triangle, stick a pin into the green wire terminal & see if it is hot & at the alt see if the green wire field terminal is hot. confirm the reg is grounded, usually removing the mounting bolts & screwing them back will take care of that. that'll oughta tell us something.

My bad. 74 Challenger 318/904 Stock wiring system

OK if you are only getting 12.5v from the Output of a known good alternator then you have a wiring or Voltage regulator problem.

If you have an Ohm meter.
Disconnect the battery to protect the ohm meter and disconnect the field wires (small wires) to the alternator and the Voltage Regulator. Check for 0 resistance from the blue Voltage regulator wire to the Blue wire on the alternator. Also check for 0 resistance from green to green. Then check to be sure the blue and green are not shorted to each other.

If you have an open wire check the mass connector on the engine wiring harness behind the passenger side valve cover. Commonly burned out on the 73/74 E-bodies. If that is OK then peel the wiring along the passenger side valve cover from the alternator to the bulk connector and look for burnt wires. Very common on all Mopars from the 60's through 80's.

If the wires are good replace the Voltage Regulator. A functioning alternator & Voltage Regulator should throw about 14v at the alternator terminal when the car is running. Also 14v at the battery when running.

If you do not have an ohm meter you can use a battery powered test light to check for continuity/shorts.

I also suggest adding a Fusible link on the output of the alternator, a runaway charge condition from a bad voltage regulator can fry wiring as quick as short will.

Hope it helps

Thanks Robert, others. I'll make those checks

When I first started getting this car running, I did discover some burned blue wires on the run along the pass side valve cover. I had to replace some wire then. May have to open it up again and check for more burnt wires again...

Are these wires right? Imagine that the yellow wire is green. (Had to replace some wire and couldn't find green in my wire stash😁) Could I possibly have these switched? Would this cause a no-charge condition?
-Scott

all good there. turn the key "on" & the blue wire there at the alt & at the "top" blue wire terminal in the triangle reg connector (with it facing up like a pyramid), some are sideways, but the blue & green are always the same, should also be hot. confirm the yellow wire has continuity between the other alt field terminal & the "side" terminal at the reg (likely it is OK) & confirm the reg is grounded (unscrew/screw in the mounting bolts will do it) then all that is left is bad reg.

Thanks Robert. Hopefully get a chance to check things tomorrow. Pretty sure ground is good, I even have a dedicated ground wire from reg mounting screw in bulkhead to rear of cyl head

As for the switched question. You can switch them and it will still work fine.

The blue and green (yellow in your case) generate a magnetic field inside the alternator's rotor. Reversing the wires reverses the direction of the field but it doesn't effect the way it works. The rotor is rotating and the rotating magnetic field "Drags" electrons along the coils along the outside of the alternator creating electric current in those coils. The rotating field creates alternating current that cycles from positive to negative, hence the name alternator. A rectifier bridge (set of diodes) at the end of the coils "directs" (not the best word here) the alternating current to push the positive current to the output terminal and the negative current to alternator body (engine ground).

The blue wire gives energy to the field coils in the rotor and the voltage regulator. This is the reason to check for voltage at the blue wire. It actually splices into the "Run" portion of the main harness inside the wiring bundle along the passenger side valve cover. You could have positive voltage to the terminal on the voltage regulator and still not have voltage to the alternator if the wiring is bad so check for voltage at the regulator AND the alternator blue wires or check either and then check for proper continuity between the two terminals.


The green wire is a ground passing through the regulator. The stronger the ground connection the more current flow, the stronger the field strength, and the more current output. This is the reason to check the Voltage regulator for a good ground.

Lastly there has to be a good circuit from the Voltage regulator to the alternator and back with no breaks or shorts or nothing works.

I hope that helps because for me understanding how it works helps me to see the problem more clearly.

Update: bought a new regulator. Problem solved. Albeit, my volt meter says its charging a might high, 16v(?) But lights dont appear overly bright. Volt meter going bad? Oddly, my brake light come on while started now where it didnt before. Thinking unrelated issue.

Without an ammeter in the charging circuit, its only a guess to what the charging current is.
However you'll damage the battery for sure if its seeing 16 Volts.
That's a very specific statement. If the voltage across the battery is 16 volts.
If the voltage across the battery is 14.5 Volts, then it will be OK even if the voltage at the alternator output is 16 Volts.

Even so, you still need to find out why there's a voltage difference. Often its poor connections or wires between the alternator output and the voltage regulator's 'sensing' connection. If there are voltage drops between the alternator out and the regulator sense, the regulator is operating correctly but being given incorrect info. The regulator keeps upping the current allowed through the field until it sees 14.2 - 14.9 Volts.

Schematicly your charging system looks something like this. Or did before it was altered.
Actual colors and connection points you'll have to get from a '74 factory service manual. To find the culprit(s) check the voltage at each accessible point between the alternator output stud and the regulator. Alternatively (pun intended), instead of measuring voltages to ground, measure voltage drops in the circuit itself. Also check there are no voltage differences between the ground system from body, engine, battery and alternator.

Confirm the battery is charged up (slo charge it overnight if need be). What Mattax said: 16V is sky high & will fry the system. at a fast idle (with a good meter) see what you have at the batt posts & work outward from there. If 16V stop the eng & do some checking as much as you can WO it running to see if you can spot the problem (something is dead shorted/full fielded/wired wrong or bad reg). keep run time to a BARE minimum. unplug the reg & see what it drops to. EDIT & keep all lights/accessories OFF as 16V will fry em.

Its very hard to track a voltage drop without having equipement running.
Current has to be flowing for there to be a voltage drop.

Before shutting down, measure the voltage at the alternator output, and regulator sensing wire. Probably the closest two locations for that will be the ignition run wire (blue) at the ballast resistor and the field connection on the alternator.

With the engine off, you'll have to set up tests using the battery to supply power through a couple different circuits. Not that difficult.

IMGTX's post is very good. Read it again if needed.

Below is normal paths of current flow when a battery is fully charged and the engine is running.

Agreed, if it ain't flowin it ain't droppin but I would try & do as much as you can ahead of time (eng off). Ohm the alt male field terminals to ground (shouldn't be)/ohm green wire to ground (should be none)/confirm the field circuit wiring is correct (I'm assuming nothing was rewired- it just started overcharging). My gut is telling me that it is the reg (even tho it is new) IF the alt field terminals are isolated. A good precaution, pull a batt cable before switching the meter to ohms.

It seems to me you might have the blue melted elsewhere in the harness. Check under the dash too. I had a similar problem and it was melted in multiple places. The blue wire was pinched near the column. My Optima battery was toast after that.

Easy to do a rough test of the suspect circuit. Get a headlamp bulb that pulls some current. Connect one end of the suspect circuit to ground. Connect the other end of that circuit to the bulb. Connect the other end of the bulb to battery positive. The bulb will load the circuit.
Doug

Sorry for the lag in responding. Been a long winter.
So, I re checked all the connections, and eliminated issues there. Did have a questionable connection on the output post. but discovered I had a faulty alternator. Went from over charging to nothing. New alt and runs great! Need a new battery as it won't hold a charge for more than a few days, but will start and run nice (electrically, still have a littel carb issue) if drive it every other day or so

Scott