I recently got my first Ford ever. I'm coming over from over 10 years of driving Jeeps. Got a 97 f150 4.6. The first day I had it, I had the truck into the mechanic getting some things fixed up that I didn't have time to do myself, and the first thing he noticed is that the battery in it died overnight. While he was working on it, he had the interior fusebox completely disconnected and sitting on the drivers seat, and the battery still died completely overnight.

So when I got it back, I hooked up my multimeter in series between the battery cable and battery post. I got a steady reading of a 40ma draw. To me that seems pretty normal. But just for fun I went ahead and pulled fuses one at a time anyways to see if there was any change. None of them made any difference. So to me this said the battery is bad. Brought it into the shop, had a draw test put on it, it tested ok. But I went out and bought a new one anyways. Put the biggest one i could fit in there, 850cca, has to be wedged into the battery tray hahaha I can now get about 24 hours before the battery is completely dead, but this still isn't normal.

So what can cause 2 good batteries to go dead in under a day with only a 40ma drain when theoretically they should last weeks at that? It has to be something under the hood because with 100% of the power cut from the interior it still does it.

 

Have the alternator tested.

 

Not in 1 day, but a week or two - had that happen. Turned out to be the glovebox light - the switch that turns it off when the glovebox door is closed was broken so the light stayed on constantly. This happened in my car, which was at home, several hundred miles away.

I was OOT for a few months on a contracting job, at that time didn't find it so I had a mechanic change out the fuse in the 'under the hood' circuit box with a relay and wired it to the switch circuit, this effectively killed the circuit that was causing the drain. Key is off, that particular circuit was dead. Took out the radio, interior lights etc till the key is turned on. Found the cause later but left the relay as it was, just in case something else goes bad in the future.

Good luck.

 

00gstang - alternator was just replaced before i got it. It's nice and shiny under the hood. Could a faulty alternator cause drain on a battery when not running? I know a faulty one could cause it not to charge fully, but I charged the new one with a battery charger before installing....

hcbph - i know for sure it's not something inside because 100% of the power was cut from the interior of the truck and it still drained. So it's definitely a circuit under the hood. What's got me baffled is my meter only shows a 40ma draw which is well within reason. But thanks for the idea

 

doesn't sound like the alternator then ... do you have one of those underhood lights?

 

Have you charged the battery, pulled both cables, let it sit for 24 hours or so, and checked it again? I have bought bad batteries before. Eliminate the easiest before you start pulling your hair out. Make sure the battery will hold a charge.

Clean your connections and grounds. The battery may not be going dead, just seems that way due to a bad connection somewhere,or may not be getting charged.

Went through something similar with my boat a few years back. Turned out the starter was going bad. It would still work, but drew so much current it would kill the battery. Made it seem as if the battery was bad/dead.

 

the bulb for the underhood light is not installed.... which to me would mean there is not a complete circuit and would not have any draw.

If I unhook both cables from the battery it will stay charged overnight no problem. And it is definitely draining the battery completely and not the starter trying to draw too much because even just by putting the battery in and connecting it and not starting/not touching the truck, the next day it will be so dead it won't even turn on the dome light when I open the door. I also just upgraded the grounds over the weekend with 1/0ga wire and it did not make a difference in the drain. I also tried taking the battery out and putting it in my jeep while it was dead and it was too dead to start that too.

 

I just went out and put a fresh off the charger battery in it and unhooked the top main fuse which supplies power to the engine fuse block, etc.. etc.. the only thing hooked right now is the circuit for the alternator. This should essentially isolate the alternator, and if it's bad and causing draw I should have a dead battery in the morning, if it's still charged then I can eliminate the alternator. Until then i'm open for more ideas.

 

I will look in my handy dandy 97 service manual....

 

I would have disconnected the Alternator wire for a test, but either way let us know. Don't leave us hanging with an unsolved mystery.
I vote like the other guy on the Alternator. Sounds like a short in the voltage regulator or diode. Probably the diode that is supposed to only let electricity go one way to the battery is letting it go the other way into the Alternator and draining the battery. If it is not the alternator, UFO's must be draining your battery to refuel. I work out of town a lot. My Porsche would sit for a couple of weeks and have a dead battery. It was checked over and over again. Turned out they finally found the problem. The digital radio was turning on by itself and draining the battery. The service tech got into the car to do diagnostics and the radio was blasting away even though no one had turned it on. New radio, end of problem. Those power drains are totally weird.