alternator charging light on and not sure why.

[speed1975]

2004 lincoln aviator
AWD V8

A couple weeks ago I ran into a situation where my battery was dead. I jumped the truck and went on my way to work. I went out at lunch and found my battery was once dead again so I took the truck to have the charging system checked and was told the alternator was bad. I had them change the battery because it was still under warranty and went home to check with my fluke meter. I found that with no lights or radio on the charge voltage was only 12.8 volts. however the battery light was not on.

So I ended up getting a rebuilt alternator from a junkyard and replaced it. After I put it on I noticed that the battery light was on and would not go out. I checked the voltage and it was pushing 15 volts. After it ran a bit it is now down to 13.8 volts and the light is still on.

Now the truck runs fine all battery cables are in great shape. But I have no idea why the light would still be on????

With my meter black on the negative I get 13.8 on the positive battery terminal, on the large wire on the alternator, and both smaller wires...

Help.. haha

 

[Mrooney322]

if im not mistaken it sounds like the regulator on your alternator is going...

i think with the car on the alternator should be outputting around 14.2? volts.. someone correct me if in wrong.

 

[Mrooney322]

i would have someone press oon the gas and measure the voltage at higher rpm's, but if you are getting anything around 15 volts it could damage your electronics.

 

[speed1975]

It was only 15 for a bit. And it came right back down. 13.8 it very well could be just a bum used alternator however why is the light on at 13.8 when it was not on when it was 12.8????? that is the question

 

[Number4]

It'll run in the 13's regularly, low 14's when under a load.
At 14.9 you will have issues.

Don't know if using a scan tool to clear codes would help or not.

 

[shucker1]

Set your Fluke for AC voltage.

Any excessive AC voltage (over .5 volts) usually means a bad diode in the rectifier pack.

AC component in a DC circuit will smoke a battery fast.