HELP! I am trying to get my truck running. It is a 1990 F-150 I have replaced the fuses, battery, and checked the wiring. I tried to start it a few days ago after driving it for a couple of weeks and wouldn't start. After jumping the battery it started with no plms. I thought maybe I left my door ajar overnight so I went on with my errands and wouldn't start after the stop at the store. Once again it started without fail and I went on driving and then the lights went dim and she started sputtering like being out of gas then nothing. Borrowed a battery charger from my pap and got it home. I read somewhere that I could full field the regulator and see if it was the alternator or just the regulator. How do I full field the alternator?????

 

the alternator has a built in regulator. You cant really check these things with out taking them off and having them tested. I would fully charge the battery and have it tested. You should be able to start your truck then go out and and take the battery cable off and the truck should stay running. If not the alternator is bad. Once the truck is started the alternator will completely support the truck and all the accessories.

 

It's your alternator! Your referring to a different type of system. "Full fielding" sounds official & exciting, but not applicable!

 

i had the same exact problem! when you turn the key over to the on position (but not actually starting the truck) do the check lights all come on? what happend with mine was i would turn the key to on, the check lights would all light up like normal, but the second i went to fully turn the key over and start the truck, everything just cut off instantly...no power at all. i jumped the relay studs and then itd start up no problem but if i stopped anywhere like a store or whatever and shut the truck off, itd do the same thing when i wanted to start it again.
honestly, all my problem ended up being was a loose negative battery cable (lol embarassing). check your cables, i mean idk where you live but i live on a dirt road and my cables just got rattled loose over time. and yeah it could be the battery going bad, i agree with what "the 89" said. cant hurt to have it tested and youd be way better off if it were the battery rather than the alternator!