PLZ HELP! Battery not charging, alternator tests good.

[swapmeet]

Quick overview.
07 V8, stock alternator, 120k/mi

After wheeling yesterday, noticed the battery light came on. Drove home on just battery for an hour. Tested voltage during my hour drive home with a meter. It was never over 12.5, and slowly dropped to 11.7-ish. Thankfully I installed an AGM battery, so it's more tolerant of being run that way.

I assumed it was the alternator, pulled it out (major PITA) took it to O'rileys just now and it tested good. Even had them test it twice. Good both times. Based on my research the Denso alternators are pretty robust.

I can see mud possibly fouling the windings, but it had an hour or dry off.

I'm out of ideas.

I don't know what else to check. The fuseable link in the fuse box appears to be in good shape.

Possible issues:
Wheeling yesterday, water up to rockers, mud etc fouled the alternator?
Also recently installed HIDs. (Shouldn't matter but mentioning anyway.)

Hope y'all can help, this is my daily and I have to get to work tomorrow!

 

[having fun]

Strange... Alternator produces the power and these I do believe have the voltage regulator built in them and it should have came up on your test if bad.

Just had the same issue with my big rig, alt was good, but, regulator bad.

Fuse linking/ box and connections at any point would normally be the other spots for problems.

Edit... including the ground.

 

[swapmeet]

Ok, little update.
I found that the short ground wire that jumps from the ground post connector to the body... The wire is broken.

I don't think that would cause a charging failure, I assume the ground wire terminates to the block or frame at some point. So the ground to the body is superfluous to some degree.

I'm about to do a continuity test between the three wire plug on the alternator and where it plugs into the fuse box. Still not sure if I'm chasing the right thing.

Y'all LMK if you thing that broken ground wire to the body makes a big difference.

 

[hoggy46]

sounds likely that could be the problem, alternator can't be functioning properly in the vehicle with the voltages you said you were seeing and that you drove home on battery means your battery has to be good

be weary of auto parts store battery/alternator tests, I haven't seen one use a real load to check one in a long time, I'm not sure how you can say it is good without putting a real load on it

ground wires sounds reasonable

 

[BlackWorksInc]

I have a hard time believing the alternator is good if it was providing no voltage to the battery. It should have been around 13~14v.

A broken ground wire to the body could be a possible cause, keep in mind rubber bushings support the drivetrain so there is a need for these grounds; but I would expect other issues to arise from that as well.

 

[earthrealm]

before pulling the alternator, did you check to see if any voltage is coming from the alternator?

 

[swapmeet]

Good news, I think... I reinstalled the alternator and everything works.
14.2V running, 12.6V when it's shut off.

I have a hard time believing the alternator is good if it was providing no voltage to the battery. It should have been around 13~14v.

A broken ground wire to the body could be a possible cause, keep in mind rubber bushings support the drivetrain so there is a need for these grounds; but I would expect other issues to arise from that as well.

I fixed the ground, but it's been broken for months. So that could be the problem... But I still doubt it.

before pulling the alternator, did you check to see if any voltage is coming from the alternator?

Only test I did was measuring voltage at the posts with it running. It was 12.1v when I left, and got down to 11.7v by the time I got home.


I think I forced mud or something of that nature into the windings. My entire engine bay was covered in mud (dried muddy water) but that has been that way for months as well. I washed the engine off before removing the alternator, so maybe that was the fix.

None of it really makes sense, but it works now so I'll let it ride and see how it hoes.

 

[swapmeet]

BTW, thanks everyone for their response.

Just took the truck for a spin and everything seems to be fine.