Voltmeter reading low, alternator and batts good

What is your volt gauge doing in the dash, is the needle dancing around and or spiking when you are going down the road? Alternators do fail.

 

As someone said a lot of good info here. I'v read through a couple times . There are really only 3 possibilities. Can be 1 of them or also a combination of any 2 or all 3. Batteries, alternator, and circuits. The first 2 are the easiest to verify. Load test all the batteries. A bad battery or 2 may cause issues you describe. Make sure that the alternator is putting out a strong 13.8 - 14 volts. A weak alternator can produce a few amps but not enough to handle the loads. If you don't verify these 2 you are chasing your tail with circuits, connections, wiring, and electrical loads. If the first 2 are confirmed good then you know you'v got a different problem someplace and go from there.

 

If a guy has the proper tools, you can actually put an amp load equal to the alternator's rated output on the batteries. Put an inductive clamp around the alternator's output wire and see what its actually producing.

Shorted diode or open diode in the rectifier causes you to lose IIRC 66% and 33% amp output respectively.

#1 cause of failed alternators are guys who like to spark the jumper cables together when trying to boost a vehicle. That's a huge no-no.

 

Unfortunately a visual inspection can only tell you so much when dealing with electrical. I just hate to see guys just throwing parts at a problem. That can start costing big bucks real quick.

I had a buddy with an old Z24 Cavalier. He bought a new alternator to put in it because it wasn't charging. Opened the hood and found the alternator mount bolts were loose. Tested alternator output before and after tightening the bolts. He gained 30-35 amps of alternator output.

Another friend with a charging problem on his car. He replaced the alternator and made no difference. We screwed around with it for a bit and we eventually found a poor crimp on a ring terminal at the starter solenoid. The repair took less than 5 minutes and cost him a nickle but his wallet was $200 lighter because of the alternator he didn't need (and couldn't return).

Not saying your alternator is ok or failed but just suggesting you do your best to verify that its faulty before you spend your hard earned coin on it because there are multiple issues that can mimick a failed/failing alternator.

 

Take the alt off and go to a good electrical shop and have it tested.

 

Yup back when i was with USX my voltage gauge would spike at 16volts on the high & 8volts on the low. After 3 sets of batteries and 2 alternators, it was found to be a chaffed up wire going to the starter. The tech at the dealer got 3 different readings off my truck in the parking lot that day. The batteries, alt, & dash were the 3 spots he checked. What a fiasco