Ok guys, I've already posted this in the electrical section, but I am desperate as we leave on a trip in 2 days. Last weekend while my friend was towing his race car the battery light came on in my 99 f-350 4x4 dually. Took the truck to autozone and they said the alt. was fried. 3 changes later and the alt. is still not putting out any juice. Had a local mechanic friend of mine check it out and he is also stumped as to why the alt. works on the testing machine but will not put out when hooked up to the truck. Any help from the great guys in the diesel forum would be appreciated.

 

Have you checked for any blown fuses or burnt fusible link wires? According to my service manual, there should be a 50A fuse (#20) and 10A (#6) in the battery junction box under the hood and a 5A (#29) in the central junction box under the steering column. The fusible links are located in the wires from the central junction box and the alternator to the battery cable at the starter.