Battery not charging

[JG1966]

Hello, before I yank my alternator and take it to some box auto parts store to be tested, here is the scenario
My battery is only charging at 12.7 when running at increased RPM. The battery is good and will hold a charge. I am getting "same as battery" voltage at the alternator stud (high 12s) with the car running. I grounded the green alternator terminal to get full voltage and the alternator charged the battery, but only to 13.2 or so. The voltage regulator is grounded and good.
One test that I did that I'm sure is pointing to my problem --- with key "on" I am getting "same as battery" voltage on both the blue wire AND the green wire at the alternator terminals. I'm certain that one is supposed to be "same as battery" and one is supposed to be much, much lower, like 1-2V. I checked the continuity on both the green and blue wires from the alternator to the voltage regulator and they are fine. What does it mean that both the green and the blue terminals at the alternator are giving "same as battery" voltage? Bad alternator or perhaps something else? Thanks for any help

 

[RJRENTON]

Hello, before I yank my alternator and take it to some box auto parts store to be tested, here is the scenario
My battery is only charging at 12.7 when running at increased RPM. The battery is good and will hold a charge. I am getting "same as battery" voltage at the alternator stud (high 12s) with the car running. I grounded the green alternator terminal to get full voltage and the alternator charged the battery, but only to 13.2 or so. The voltage regulator is grounded and good.
One test that I did that I'm sure is pointing to my problem --- with key "on" I am getting "same as battery" voltage on both the blue wire AND the green wire at the alternator terminals. I'm certain that one is supposed to be "same as battery" and one is supposed to be much, much lower, like 1-2V. I checked the continuity on both the green and blue wires from the alternator to the voltage regulator and they are fine. What does it mean that both the green and the blue terminals at the alternator are giving "same as battery" voltage? Bad alternator or perhaps something else? Thanks for any help

What is the YEAR AND MODEL of your car.? Sounds like a 1970 or newer but not sure. Possible bad voltage regulator or bad brushes or high resistance rotor windings......just a guess without further info.....
BOB RENTON

 

[JG1966]

What is the YEAR AND MODEL of your car.? Sounds like a 1970 or newer but not sure. Possible bad voltage regulator or bad brushes or high resistance rotor windings......just a guess without further info.....
BOB RENTON

Sorry Bob. It's a 72 Roadrunner with an aftermarket Powermaster alternator that's about five years old

 

[RJRENTON]

Sorry Bob. It's a 72 Roadrunner with an aftermarket Powermaster alternator that's about five years old

Not real sure what's inside a Powermaster alternator. Some were internally regulated, a few were not. If its a copy of the Mopar square back design, your issue may be a bad diode, either positive or negative type. Since, from your inquiry, you have Low output voltage and current, and "full fielding" by bypassing the regulator, low voltage and current output is indicative of a bad diode as all 6 are necessary (3 positive and 3 negative) a loss of any diode will result in low voltage and current output due to internal configuration as its a three phase full wave bridge rectifier circuit. Loss of any diode (open) results in a single phase condition and usually half of the rated output. Try fixing it yourself.....alternators are easy to repair.....parts are available from several suppliers....doesn't need to be "Powermaster" brand parts.....they just purchase the parts themselves.
BOB RENTON

 

[JG1966]

Not real sure what's inside a Powermaster alternator. Some were internally regulated, a few were not. If its a copy of the Mopar square back design, your issue may be a bad diode, either positive or negative type. Since, from your inquiry, you have Low output voltage and current, and "full fielding" by bypassing the regulator, low voltage and current output is indicative of a bad diode as all 6 are necessary (3 positive and 3 negative) a loss of any diode will result in low voltage and current output due to internal configuration as its a three phase full wave bridge rectifier circuit. Loss of any diode (open) results in a single phase condition and usually half of the rated output. Try fixing it yourself.....alternators are easy to repair.....parts are available from several suppliers....doesn't need to be "Powermaster" brand parts.....they just purchase the parts themselves.
BOB RENTON

Thanks Bob. It's externally regulated. Is there a way to test the diodes with the alternator either on the car or off the car?

 

[72RoadrunnerGTX]

The Powermaster Chrysler alternators are based on the square-back design. Testing the diode bus bars involves removing the alternator, pulling the back cover off, disconnecting the stator connections and checking each diode individually. The process should be detailed in the FSM for factory units. I agree the internal parts are likely interchangeable with over-the-counter replacement parts for the squareback, I’d check with Powermaster for replacement diode bars designed specifically for the higher than stock output rating if a diode bar needs replacement.

 

[JG1966]

The Powermaster Chrysler alternators are based on the square-back design. Testing the diode bus bars involves removing the alternator, pulling the back cover off, disconnecting the stator connections and checking each diode individually. The process should be detailed in the FSM for factory units. I agree the internal parts are likely interchangeable with over-the-counter replacement parts for the squareback, I’d check with Powermaster for replacement diode bars designed specifically for the higher than stock output rating if a diode bar needs replacement.

Thank you, 72

 

[PlymCrazy]

Ed Law at Powermaster. Had a great experience with him in regards to technical and repair support recently. He responds fairly quickly to emails too.

[email protected]

 

[RJRENTON]

Thanks Bob. It's externally regulated. Is there a way to test the diodes with the alternator either on the car or off the car?

As outlined, the alternator must be disconnected, removed and disassembled to test the diodes individually. The positive diodes are connected to the output stud connection and the negative diodes are connected to the case thru the two externally mounted screws. It has been my experience that one of the negative diodes will have failed open; positive diodes can fail but I've seen more negative diode failures....why?....likely, due to voltage spikes....or overheat conditions.....just suppositions.....on my part. Just my opinion of course.
BOB RENTON

 

[RJRENTON]

Thank you, 72

As a side bar.....I use to live in the 'Burg in the east suburbs, Monroeville. I know many Mopar people should you ever need help or parts. Perhaps you've heard of Jim Kramer, Wally Allen, Lou Hudachek, and the late john Borgen. PM me if you want...
BOB RENTON

 

[JG1966]

As a side bar.....I use to live in the 'Burg in the east suburbs, Monroeville. I know many Mopar people should you ever need help or parts. Perhaps you've heard of Jim Kramer, Wally Allen, Lou Hudachek, and the late john Borgen. PM me if you want...

Thanks Bob. I have met John Borgen a couple of times at different car shows. He has some beautiful AARs and TAs. I didn't realize he had passed away. That's sad. He was a nice guy.

 

[JG1966]

Hey guys. Just an update. I put on a new alternator and it's still not charging. I know the VR is good and grounded. I even tried it with two other voltage regulators I have that are good (before I got the new alternator) and that didn't work. I think I know the issue but not sure what it means. The blue wire at the alternator is 12.1 volts with key on, same as the green wire. I'm certain that one of them I supposed to significantly less voltage. Assuming the alternator and voltage regulator are good (I'm certain they are), what are the other possible culprits and what tests can I run. Thank you

 

[JG1966]

I think I figured it out. I checked resistance on my blue and green wires from the alternator to the voltage regulator. I was sure I did this before. But now the green wire is bad. The blue wire was good, very low ohms. But the green wire was reading infinity or open at the voltage regulator. I poked the wire about six inches before the VR and got a good n reading. The green wire is bad at the VR pigtail. If that doesn't fix it, I'll really be stumped

 

[JG1966]

Update -- I got a $10 voltage regulator pigtail from Napa and that fixed it. I know I had checked the green wire for resistance during my troubleshooting and it seemed OK. But it took some stabbing with the multimeter to get it to read and apparently it was bad. When I checked today, I was obvious it was an open wire. Thanks again for everyone's help.