I have a 79 F-150. I pulled the 400 out and gave it to my dad. I pulled a 73 460 out of my dad's F-250 that i rebuilt nine years ago. The engine only has about 3000 on the rebuild. The engie ran fine and the battery stayed charged before the swap. But now, somethings wrong. The battery is always dead, I bought a new one, and it worked for about a week, then the same thing, hard starting, then no starting. Now, I can't even jump it, and it would not even stay running. I believe I hooked the alternator up correctly, they were a little different connectors from the 77 to the 79. I looked up the alternator in my dads auto service manual and it showed the black connector going to the center most post on the back of the alt, for the ground. The two other connectors were of different size, so I could not have mixed them up. I'm thinking maybe a bad regulator. Any help would be great. Thanks, Billybob

im not to familiar with fords but make sure that they both use the same type of regulator [internal or external] etc. also make sure that both alts. have the same amp rating. also double check all your grounds.if you get the truck running and disconnect the batt.cable the truck will stall if the alt. is not charging. good luck.

There is a chance the battery has gone bad and will not hold a charge. Take the battery to Wal-Mart or almost any of your parts stores and they will check it free.

check that you don't have any current leaks on your system
or take it to get it checked, i had the same trouble in my
89 F150, and found out that one of the cab lights was draining
my battery even when it was turned off

Doc here,

DON"T Do THIS!! Even if the battery IS Flat dead, It will act as a "Buffer Cap" between the Alternator, and Vehicle Computer, ECM, and any other semiconductor device you may have..If you disconnect it while running, you will spike and possibly destroy these devices..May as well power the car up with an arc welder..

If the Alternator has a bad Diode pack (as I suspect it may) about 1/3 of the output voltage will be AC..Semiconductor devices will not tolerate AC on their Source current line.

First check the output voltage, Running, warmed up, measure the battery voltage (after a full charge 6 to 10 hours at 10 amps) , It should read 14.4 volts..with no system devices on..If you don't get this, look for a blown fuse link, or maxi fuse on the main power line to the alternator. If that's good,

(does this have an external solid state regulator? I think Fords did..Not a Ford guy..) Check the power to the regulator, If that's there, have the alternator checked at the local parts house. If it's good replace the regulator.

If it's going dead just sitting, and the load test after charging up the battery was good, Your Diode pack inside the alternator is bad, reverse biasing (leaking backward, putting a load on the system just sitting. ) To test, pull a battery cable, set your DVOM for A X the highest scale reading you have, most are 10 amps. put one probe on the battery, and one probe on the cable, (be sure all acc is off, fuses pulled or lamps pulled on things like hood lamps, domes etc, it will give you a false reading).

On a modern Computerized Vehicle, with a CD Player, Clock ect..It should read between 0.03 and 0.08 amps..If higher, unplug the regulator, and disconnect the Alternator, read again..If it drops down to normal, one or the other is bad..plug the regulator in, read, if OK then it's the alternator or Visa versa..If both don't change, you have a branch circuit loading up the system.

Go to the fuse box, If it has a fuse layout on the cover, pull them all, make a reading it SHOULD be normal, Then, one at a time put the fuses back in monitoring the meter as you do..when it jumps up, that is the offender..track down that circuit, and replace/repair as needed.

In the engine swap, you may have lost the engine grounds...AT anyrate, you should set your grounds up in this manner:

Run a properly gauged Ground cable from the battery to any handy bolt at or near the starter, From that same bolt get a Battery cable at the parts store, that has two 3/8 ring terminals on each end and attach it there and to the Frame.

Next get some 10 gauge wire and ring terminals and run that from your bolt on the block to the Firewall, and another to the Alternator bracket or mount bolt.

If the Bracket and alternator is power white remove them both and their hardware, sand or pressure wash them until clean semi shiny metal, dry and reinstall.

You have electrolysis there..and it will also eat a water pump impeller up faster than any acid..The Alternator Bracket and Alternator housing is Aluminum and the block is Steel..(what were they thinking?)

When you go to reinstall the brackets, go to the hardware store and get some aluminum wire compound, and coat everywhere the mount hardware meets steel (bolts, mount surfaces Ect..)

Next get some Wire Braid, (Radio Shack) and install braid from the radiator support to the frame, Fenderwells to frame , hood to firewall, Doors to door posts, gas flap to body, tailgate / Trunk to body.

At each point the wire is grounded, Burnish ALL the paint and grease off to bare metal. Use a proper star-washer and lock. Use sheetmetal or Tech screws where no screws are available.

It sounds like a lot of work, but after you assemble all the parts, it's only a few hours to do..and you'll end up with a system that will work reliably for many years to come..and can eliminate that from your troubleshooting list.

Remember: A CHARGING SYSTEM is not designed to bring back a flat dead battery, It is only designed to replace a surface charge blown off in start or other high current modes, AND support the vehicle system..Nothing more! IF THE BATTERY IS DEAD...properly charge it up with a charger!

Doc

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