96 Ford F150 battery drain

canon1a

I bought a used Ford F150 a couple of months ago
and the battery was dead when I bought it so they
put a new battery in. Now, if the truck sets for 5
days the battery is dead. I had the dealer check
the battery and the charging system and he says
everything is okay. When I described the problem
to my brother-in-law, he immediately said there was
a bad diode in the alternator. Has anyone else
ever heard of this and if so, how do I test for it?
Can I replace just the diode or must I replace
the whole alternator?

Thanks;

Larry

don434

Trying to help but mainly have Chev. experience.
If you have a amp meter of say 0 to 10 amp range you could do a basic test.
Do not turn anything on or even open a car door after opening the hood. If you don`t want to blow the diodes yourself - don`t start the engine. Really no need to have keys in the car at all.

You probably knew the above but wanted to be sure. Simply remove the large red charging wire that runs from the alternator to your battery. Remove the wire from the alternator and put the amp meter in series with the red removed lead and the alternator terminal that you just cleared of wiring. Any current flow should be as a result of alternator parts only. I think that you should observe almost no current flow - certainly not an amp or two.
On my 4 Suburbans I could change out the voltage regulator or the diodes. Many new trucks have constant drains - may be 35 milliamps - because there is so many solid state devices that each need just a little juice.
Make SURE you reattach the red charging lead securely to your alternators output terminal before closing that hood. If you don`t say goodby to your diodes.
Remember also that the red wire that you have removed from the alternators output terminal is still connected to the battery and may not have a series fuse. Allowing this lead to touch ground is not a good idea - could smoke the wire. Be very careful please.

don434

Another very simple test. Place a amp meter in series with your positive battery and the positive battery terminal. I would certainly think that if you see a two amp drain or more you have a defective part. By going to your fuse block and removing one fuse at a time you can identify which circuit is the source. Two people would be nice. Have your wife or friend look at the meter while you remove and reinsert fuses. Hopefully the helper will yell at some point - HEY the current drain is now ZERO.
Maybe a mechanic on this board can offer simpler ideas.