I have a 97 f150 with a 4.6. Last week the bearings in the alternator went out. I have tried 3 alt's with the same outcome. What is happening is the battery light is staying on when it is charging. When I first start it it charges at 14.4 to 14.7 volts after a few minutes it drops down to 13.5 and the whole time the battery light is on. After running for 20 minutes it will stop charging and the battery light goes out. I have 2 of the alt's tested and they are good, the original alt worked good so i replaced the bearings and it still does the same thing. Any ideas, I am stumped.

 

Did you check the fuse for the alternator field, and make sure the wiring from it to the alternator did not get damaged ?





jbrew has a great post i a thread, this is usualyl no power from Alternator, but worth as a starting point for checking.

The other question, did you check the battery ? Could be the battery is shot, and beyond the charging ability of the alternator, and needs to be bench charges with a trickle charger.

 

as far as i can see all the fuses are good. is there a relay?

 

As far as you can "see" ?

Did the fuse test good with a meter ?

The diagram is above, that is it.

 

The 2 megafuses are good, the small one i will check in the morning. If the fuse was bad it would'nt charge at all. the problem i am having is it charges with the battery light is on. then the light goes of at quits charging.

 

it turns out the 20 amp fuse was corroded where it plugs in. put on a new plug and so far no more problems.