I have a 1994 Ford F-150 4.9L I6 -- it's usually identified with the "Y" VIN number. It has about 130K miles on it. I VERY stupidly crossed jumper cables after leaving my radar detector plugged in over night. That's one mistake I'll never make again.

I'm in rebuild mode. The started solenoid was fried, the battery was fried. Now I have a new started and a new battery, but there seems to be a problem with the charging system.

When I turn the ignition to run, the charging light does not come on like it is supposed to. Then, when I crank the truck, the ammeter shows a steadily dimenishing charge -- headlights dim, dash lights dim, etc. I haven't let it go all the way down yet, but I feel certain that if I tried to go anywhere in it I would be left stranded. Also, if I just turn off the truck, the battery drains overnight. Also, even though I'm not supposed to, I pulled the positive cable off the battery with the truck running. It went dead immediately.

I've brought the alternator in for testing, the parts house guy said it was fine. I brought it to another parts place for a second opinion. This guy said it was charging fine, the diodes were fine, etc.

After re-installing the alternator, I checked for a drain by hooking up my multimeter between the disconnected positive cable and the positive battery terminal. There seemed to be about a 1.5amp drain (but I may have been reading the meter incorrectly). After pulling each fuse individually, then each cable on the starter, I finally found the source of the drain.

When I disconnected the alternator fusable link from the starter relay (on the inside of the right fender well), the drain went to zero. I reconnected the fusable link to the starter relay and the drain returned. I unplugged one of the two plugs on the alternator (the one on top with the small wires) and the drain stopped again. I plugged it back in and unplugged the light green/red wire that runs to the charging light/ignition switch. The drain remained. Oh, I also replaced the starter relay when I replaced the starter and battery.

I called the parts guy back and asked him if his test would have diagnosed the problem I was seeing. He said, "Yes. We test the diode packs and that's where the drain you're seeing would be."

So, I'm stumped. I'm afraid to replace the alternator if that isn't the problem. I still haven't taken off the dash to see what condition the charging bulb/resistor is in. I've checked all fuses and they seem fine.

Any ideas would be greatly appreciated. Thank you.