I have an 88 Ford F250: I cannot seem to keep the battery charged. I've replaced the alternator (twice); replaced the battery; replaced wiring harness for alternator. I had a mechanic look at it, and he was unable to find the short.

If you charge the battery it will charge fully; It can sit for days and start up fine every day with no problem. As soon as you put the truck in drive, and literally drive 100 yds, turn the truck off. It will not start again. The battery goes from 14 volts to 12 volts in that short amount of time. What in the world is killing the battery after driving 100 yds. Please help!

Thanks

 

Nothing is killing the battery in 100 yards. That's not even possible. You say it goes from 14V to 12V. Is the 14V when running? I would certainly hope so. Is the 12V after you've turned it off and tried to start it? You do know that a 12V battery that is fully charged runs 12.6- 12.8V, right. 12V is essentially at 1/2 charge or less, not fully charged, and even then, to know what it is actually at you would have to do a load test. It may well be below the 12V and essentially dead. Also, voltage means nothing if you don't have the cranking amperage to get that voltage out of the battery to the starter. many people think becuse their vehicle starts fine that their battery must also be fine. Nothing could be farther from the truth. You need a good auto electrician to start from scratch on the diagnosis. I have my personal doubts that you have a short and if he didn't start with charging AND load testing the battery (and that means again in 2,4 & 6 hours AND the next day) then I wouldn't waste any more time or money on him. He also should be able to pick up any parasitic or excessive drain with a Fuse Buddy or an Amp Clamp.
There's a lot more to diagnosing, but it's NOT rocket science. You have to find someone who KNOWS what they're doing from the get go.