Alternator Not Charging 1971 Roadrunner

Ok so I’m looking for some wisdom/advice on how you guys would proceed.

Car is a 1971 Plymouth roadrunner with a 440 six pack. I purchased a new round back alternator to replace the one that was in the car. The previous one was a low output (48 I think) and I went with a 90 amp power master 2 field round back. After connecting it, there is no charge coming from it. The wiring harness is new (less than 10 years old) and I have replaced the voltage regulator and made sure it had a clean good ground on the firewall. I have checked that both field wires have a good connection on the back of the alternator and the black wire is tight and sandwiched between two nuts on the output stud. I have checked the full wire going into the ammeter and it is clean and the ammeter looks good and is reading a slight discharge when the car is running (drops lower when I turn the lights on). What else should I be looking for?

Thanks ahead of time!

Are you getting 12v to the stud on the alt?

How would I go about checking that?

First you need a multi meter (even an el-cheapo from HF ) put it on the DC volt scale and check voltage at the large black wire on the alternator. Did your older alternator work at all? Did it keep the battery charged?

Gus

Or use a test light.

My last alternator seemed to work just fine, although it suffered from the typical mopar problems of low output at idle and such but car has not been driven much in the last several months so I’m not sure if it was working or not. Ok so with the power off Check and see if there is 12 volts at the alternator output post. What will this tell me?

If you have the original green and blue wires to the fields, check that the blue wire is providing 12V when the ign. sw. is ON. If good, remove the green wire and temporarily ground the green terminal...this should sent the alternator into max charge.

Ok so this will tell me if the alternator is good?

The 12v at the stud tells you the alternator can be excited. Necessary for it to charge. The grounding of the green terminal will tell you if the alternator is good. Just don't allow it to over charge for very long.

You alternator has three wires hooked up to it.

the fat black one you mentioned, that is the output, it goes to eh battery via the ammeter.

A blue wire, this one comes from the ignition switch and provides a switched 12V to the alternator so it can operate.

A green wore that comes from teh regulator, this is what controls the alternator output.

The black wire should have 12V on it all the time
The blue wire should have power on it when the key is in run.

If neither of those is true you have a wiring issue.

The green wire will have a variable voltage on it is it's controlling and the key is on. Grounded turns the alternator on to max, 12v turns it all the way off, somewhere in between it's putting out something more than nothing, but less than max (sorry no output chart that I know of).

You can temporarily remove the green wire from the alternator connection and replace it with a jumper wire, if you ground that jumper with the engine running and the other wires all hooked up it should make the alternator charge. If it doesn't the alternator is bad. If it does you have a wiring issue. Just connect the jumper long enough to verify charging is happening. You meter can tell you that or you can take a quick look at the ammeter.

Tell us what you find.

It will move the amp gauge it its max charge unless your gauge is burnt out

1971 should have a "square back" alt with 2 field brushes, and with an electronic regulator. using a round back replacement may be your problem!

Ok so there is no power at the alternator post with the car off and key off. This should mean that my ammeter has failed and I need to bypass that. Am I correct?

I’ve been running a dual field (2 brushes) alternator with an electronic regulator for 50,000 Miles now without a problem. I’m wondering if my ammeter gave way at this point and if so, what’s the best solution?

Might be the ammeter, might be the wiring.

What's teh vvoltage ont he stud with the engine running? That will tell the tale.

If the starter turns over then the ampmeter is ok all the ignition power goes through it.

id be looking at fusible links..... going thru something similar on one of my trucks.

Ok everybody here is what I found-

I traced back the issues as y’all suggested starting with the post on the alternator. I found that I had no power at the alternator stud but I had full power all the way through the ammeter, but somewhere between the alternator stud and the back of the ammeter I had a short in the wire. I replaced this line with new 12 gauge wire and that solved my issue! As always I appreciate the advice and wisdom of you guys!

Have you inspected both brushes to make sure they are spring loaded properly and engaging? one or both may be hung up?....

Thanks for the suggestion! I will be sure I give that a check next time as well! For now the alternator is charging as it should

If you had a short on that line your wiring harness would have gone up in smoke, BTDT on a 64 300, had to hand make a wiring harness, no reproductions then.

You had an open, which means the wire was broke somewhere.

Good job sorting it out.

You had an OPEN in the wire, if it would have been a short the smoke would have lead you to the source.

when the "magic smoke" appears, "magic words" will come from you !