I just got done load testing my batteries and load tested the charging system. Batteries were between weak and good, charging system was at 14.5. Hit the button and the alt kicked in but wouldn't get past 12.5v, pretty much in the red area.

 

Hmmm... For what its worth I don't think the alternator is bad... How much load are you dropping on it? (Amps??)

Double check you wiring for bad cables, bad connections, weak grounds, etc... Also do a voltage drop test on your cable to verify there good too...

Even my truck when the grid heater hit in the morning suck it right down to ~12.5-12.8 Volts with the engine running... Remember the grid heaters are 95 Amp per heater... There is 2 of them... I've even seen the CHECK GAGE light pop a couple of times on really cold mornings...

Funny part is I'm still on the stock batteries at 6 years old and still going strong...

 

Would short brushes cause the same symptoms?

 

Short brushes it will be erratic or nothing at all for a charge... The brushes are the contact for the field. So if the field is gone there is no charge at all or randomly...

 

I really don't want to pull the alt out and find out there isn't a problem with it. It should run up to 13v or better under a load, right? Or is it normal to hang at 12.5?

 

Well it depends on the load too... If its at rated load 120 Amp it should be able to hold it. But beyond 120A the battery voltage will fall with is normal because you alternator can't keep up with the loads...

So once again it depends on the load (how much) your dropping on the alternator...

 

Had a dealer check it for me this afternoon ($38). Said the output was 156 amps and the output on both of the batteries was 770CCA a piece. So I guess that means there is nothing wrong with it. For some odd reason, it still doesn't seem to want to turn over very fast. Maybe twice last week it whipped right over really fast. IDK, guess I'll look at getting the starter tested here soon.

 

how old are the batteries? Mine did this and it was the original batteries then I put in 2 new and what a big difference it made. any parts store can test your alt and batteries for you.

 

Usually on a alternator if you have charging trouble you just have to replace the Regulator its just a couple screws. Save lots of $$$.

Of course before you would do this take the belt off and see if the altenator spinns smoothly first.

If your altenator and batteries checked out makes me wonder how good your starter is. I have never had any started problums on any of my Rams. But for example this summer I could hardly get the old 6.9 ford diesel to start. New batteries, altenator tested good. Finaly checked the draw on the starter and it was drawing way to much. It would just crank slow, it progressively got worse.

 

I plan on having it checked out, maybe tomorrow. NAPA here has a bench tester, see if they can check it out for me. Seems like all I would need is a new field and brushes, IDK.