• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Low Alternator Output

gary h

Well-Known Member
Local time
12:11 PM
Joined
Aug 30, 2015
Messages
679
Reaction score
595
Location
USA
I had my '71 GTX out today and noticed the Alt.gauge pointer going up and down abnormally.
Getting about 12.9 V at idle and 14.0 V revving it. Battery tested ok. Charging system Low. car has 10 Year old rebuilt Stock Round Back Alt.with Two fields.All the parts stores Have the "Square" back style as a replacement. Any thoughts
 
use the full field test to see if alt or voltage reg is working properly. If you are not familiar with this 'search forums' for "full field test". That's how I found my voltage regulator was not working. good luck.
 
Unless you're trying to stay original I bought a Powermaster five years ago. Works great.
 
It could be a diode went bad

Squareback alt will work, fit and conect the same than your dual field roundback. If the revised squareback version, could fit a bit tight thought but sure will have more output to source, which is better and healthy for your system.
 
Your ammeter should be on the discharge side if there was a problem. Factory low output alternators do not put out much at low RPM's and a tester may tell you low voltage at low speeds. 13.8 to 14.2 is the normal range. If the battery is not going dead leave it alone.
 
I had my '71 GTX out today and noticed the Alt.gauge pointer going up and down abnormally.
Getting about 12.9 V at idle and 14.0 V revving it. Battery tested ok. Charging system Low. car has 10 Year old rebuilt Stock Round Back Alt.with Two fields.All the parts stores Have the "Square" back style as a replacement. Any thoughts

IMO...
It is possible that you have an open diode in the 3 phase full wave rectifier circuit. A 3 phase full wave bridge rectifier circuit will have 3 positive and 3 negative diodes. An open diode will present it self as low output voltage at idle but at higher RPMs, appear to be OK but with light loads. If system load is increased, like: headlights, defroster, wipers, the alternator will be unable to supply the additional current. A failure of one diode, albeit a positive or negative diode, will reduce the capacity of the alternator by half of the rated output (current). Alternatively, if the voltage regulator is faulty, it usually presents as low output voltage or high output voltage. If the brushes are worn or the slip rings are dirty, the alternator presents as low output voltage and current.
The check for diode opens or shorts, the alternator must be disassembled and each diode tested separately using a Digital Multi-meter. An open diode will show no continuity (infinity ohms) in each direction; a shorted diode eill show zero ohms in both directions. Hope this helps....
BOB RENTON
 
use the full field test to see if alt or voltage reg is working properly. If you are not familiar with this 'search forums' for "full field test".
I did some Searching and came up with the full Field test for the single
field Alt. can someone direct me to a Dual field Test? Thanks
 
same, but one to - and the other to +

on this case if you are full fielding on car, blue wire comes from ign switch being constant 12 volts on RUN. You will need to full field the other brush to ground. Green wire on laters are regulated to the ground side, oposite than earliers where the reg field is regulating the + side while ground is constant.
 
Had it Bench tested today, 13.1 Volts.
 
Had it Bench tested today, 13.1 Volts.

OK.....BUT....what was the CURRENT (amps) at the 13.1 volts. That voltage seems low, as the alternator is capable of 14.5 - 14.8 volts, while delivering rated current (37 to 60 amps) depending on the specific model. HOW was the rest performed? What test RPM was used? Was each diode tested individually? Was the field voltage applied within the FSM specs? Just asking.....seems like the rest was inadequate.....just my opinion of course.
BOB RENTON
 
Regulator.... look forUS or NOS don't get the Chinese junk
 
I looked at a Squareback today at Advance auto parts and it had a sticker reading rebuilt in CHINA.The rebuilds in the last 10 or so years were done in Mexico. I have located a old school re builder and i will visit him this week.
 
I looked at a Squareback today at Advance auto parts and it had a sticker reading rebuilt in CHINA.The rebuilds in the last 10 or so years were done in Mexico. I have located a old school re builder and i will visit him this week.

IMO....when purchasing a "rebuilt" alternator (or starter or anything electrical), the seller almost NEVER discloses what was replaced or reconditioned during the "rebuilding" process. To this end, how do you, the purchaser, know what was replaced or reconditioned, UNLESS you ASK? At the very least, the unit, would or should, have had the bearings (both) replaced and new brushes and brush holders replaced, and the rotor checked for proper balance. The stator windings and rotor should have been tested (and documented) for shorts, continuity and hi-pot tested to ground, to check insulation integrity. If your re-builder does not do these basic services, find someone else who does. Just because the exterior case is blasted, does not mean the internals were checked. For my money, I'll test and rebuild the unit myself, using NOS parts or made in USA components. Alternators are simple devices and if you are marginally proficient with tools (installing a new cam or clutch, or water pump, etc), you can do it yourself. WHY pay someone to do what you can do yourself? Just my thoughts.......
BOB RENTON
 
Hey Gary, don't know exactly where you are in troubleshooting this problem. Found on youtube a full field test for mopar alts with 2 field connections. " Testing a Mopar alternator and voltage regulator on the car". step by step instructions. This may help before throwing parts at it.
 
Well after i had the Alt. checked at several parts stores (All Failed it).
Went to the City to an Auto Electrical shop and had it tested and it tested good.So Reinstalled it and get 13.6 @ idle. So after chasing my tail, it appears all is good.I will drive it and see what happens.I want to Thank Everyone for their help and I have Bookmarked all this info for the Future.
 
Well after i had the Alt. checked at several parts stores (All Failed it).
Went to the City to an Auto Electrical shop and had it tested and it tested good.So Reinstalled it and get 13.6 @ idle. So after chasing my tail, it appears all is good.I will drive it and see what happens.I want to Thank Everyone for their help and I have Bookmarked all this info for the Future.

FYI.....the 13.6 volts is only half of the information.....how much current (amps) is being produced at the 13.6 volts? You mentioned it was tested at idle RPM...engine RPM or alternator RPM? All "appears good", to me, is an unsatisfactory answer. Your unit MAY be producing 13.6 volts, but is it producing enough AMPS to keep your battery charged and supply the car's electrical requirements. For your own satisfaction, you should find out what is the actual VOLTAGE and AMPERAGE your unit is capable of producing. Just my opinion of course.
BOB RENTON
 
Auto Transport Service
Back
Top