Hey guys, I'm new here but I am having serious problems with my truck and any help would be appreciated.

I have a 1992 F150 with a EFI 4.9l Straight Six and a T18(i think) 4-Speed. It is 2WD.

My F150 has been burning through batteries for about 2 years now, and untill now i have been fine replacing batteries more than usual. However, recently the truck has been dying whenever i let it sit overnight. I thought it might be the alternator, so i took it to an electrical diagnosis shop to have it diagnosed. They thought that it was the alternator so they replaced it. However, the next day i noticed my battery light would come on whenever i hit high rpms (only about 2800 rpms on the big six.) My battery was dead the next day. I suspect that some of the diodes on the alternator might be dead, but would that cause a parasitic draw enough to drain the battery overnight? I am sick to death of paying people money to not fix my problem so i'm gonna try my hand at diagnosing the problem with a voltmeter tomorrow. Any ideas would be greatly appreciated.

Thanks for any help in advance.

 

Dead short, enough to drain the battery overnight, not enough to burn down the truck (yet).

Got a "circuit tester" laying around? It looks like this:


take off the negative battery terminal, attach the clip to the terminal, and jab the awl end into the post (or lay it on top)

If the light comes on, great! You've got a short somewhere.
If the light doesn't come on, damn, you've got a charging issue.

With the light on, have a neighbor/friend/kid/old lady/whomever watch for the light to go off as you start pulling fuses 1 by 1. When you pull the fuse that cuts off the light, you have successfully found WHAT is causing the short.

From there check all wires and connections of the suspect device that the fuse belongs to, making sure the wires and connections are secure, un-damaged, and the +'s are insulated from the grounds.

Redo the light test to confirm you didn't cause another short.

 

The three shorts:
1. short to power
2. short to ground
3. short circuit

 

I had a 1993 Dakota that had a similar problem, drain the battery overnight, or while at work all day. A local shop ran some tests and concluded the alternator was causing the drain on the battery. I replaced the alternator, but the problem continued. In the end, in was the voltage regulator that went bad. $10 later(not including the alternator), the problem was solved.

 

I decided to take it into the shop again and see what they say. Maybe they just screwed up the alternator rebuild and a new alternator might do the trick. Thanks for the help guys.

 

I took it to the shop and they found out that it was just a bad replacement alternator. They replaced it for free and the truck runs great now. Thanks guys.