Ok I got a pretty big problem that I am hoping i am just missing something stupid. I have a 99 F150 5.4L 4x4. I had a strange occurance. I was driving down the street (not fast or pushing high RPM's) and I noticed my battery guage move. Then the battery light went on. Then the battery guage droped like a rock, everything went dead. Got it towed home figuring maybe the alternator went dead or possible battery. I took the alternator out and had it tested. The test came back good. 15.6V output. Tested the battery. Was dead but after a trickle charge the battery tested good. Put everything back together and ran if for 10mins or so and BAM.. Same problem. I figured that the tests are not always good at kragen's for alternators so I went with my gut and bought a new alternator.(rebuilt for kragen) Installed it. The battery light stays on yet the guage does not move. Had Kragen Test it while in the vehicle and was running. I am getting a 11.88V output from the alternator. I disconnected all exterior power draws...ie Amp stereo ect. I cleaned the Neg cable and Pos cables at both connections. I also replaced the starter relay. I cleaned the connectors to the alternator. Still same problem. Tried jiggling wires to the alt. and the battery to see if that changed anything. To no avail. Im just about out of ideas and Im too stuborn to take it to the shop yet. (well not stubborn just broke cause I live in California)

PLEASe Help if you have any idea's.

Thanks

 

You might want to check for a master fuse problem if memory serves me right ther is a master fuse on the passenger side of the firewall. check fuse or connections in that area It might be in the solenoid area.

 

15.6V isn't good, it's overcharging, a sign of a faulty regulator. Eventually, that'd boil your battery.

Anywho, assuming the mega fuse by the starter relay is okay, and the 10A or 15A minifuse near to it is okay, more than likely your alternator has gone to that big parts store in the sky.

 

You are going to have to replace your alternator.

 

The battery light turns on when the alternator (99 F150) is not outputing. Two main areas that cause alternator to not output:
1) Alternator failure (didn't indicate orginial mileage, but regulator brushes may have worn out if greater than 100K miles)
2) Alternator is not receiving the correct inputs from the wiring:
Check the voltage inputs to the alternator with ignition key in run position with engine off. Backprobe the wiring regulator connector to measure the voltage:
a) regulator pin closet to alternator B+ stud should equal battery voltage (Battery sense line), High resiistance (corrosion) in this circuit will cause alternator to regulate at higher voltage.
b) regulator pin farthest from the alternator B+ stud should be approx ~ 1.5 V (Igniton Line)
c) alternator B+ stud voltage equal battery voltage

If voltages are ok, then you bought a defective replacement alternator. If voltages are not ok, then you have a wiring problem.