Ihave 91 f150 xlt that just started to drain the battery when it is shut off. I replaced the battey (it was old) thinking it would not hold a charge anymore. The new battery was dead the next day. I charged the battery and now disconect the battery cable every time I park the truck and there has been no problem as long as I disconect the battery cable after I shut the truck off. Any ideas as to what the problem might be? Or where th start looking for the problem?

 

That sucks - I had a '71 Bronco that did the same thing. My first step would be to disconnect the positive battery cable and hook up an amp meter between the battery positive post and positive cable with the key off. If something is hanging on, you should be reading something on your meter.

Start pulling fuses until the reading goes away. Once your meter zeroes, you've at least narrowed it down to which circuit is hanging (the last fuse you pulled).

Probably not going to be that easy with all the electronics on the newer ones, but you might get lucky.

 

With the keys out during this whole test unhook the negative battery cable from the battery. Take a meter that will read milliamps and hook it between the negative cable and negative battery post. If you are reading anything above 25 milliamps you have a parasitic draw. Now you need a second person. One of you hold the meter there and keep reading the milliamps while the other goes to the fuse box and starts pulling fuses one at a time. When the one that is drawing the power is pulled the milliamps will drop on the meter. This will be your culprit whether it be some kind of light or what have you. There should be a fuse map on your fuse box to tell you what each one is for. Your meter will not zero because there are always small things using up small amounts of power when you have the vehicle off like clocks and stuff but there will be a large drop when you find the one that is pulling enough power to drain your battery. If you have more than 25 milliamps draw your battery will be dead or almost dead after sitting all night.

 

Check the plug that goes into the alternator.
I had a short there that took me two days to find.
The 2G alternators are known for this and a replacement plug is readily available in the aftermarket.
(they're also known to start fires)

See this post: Ford Truck Enthusiasts Forums - View Single Post - electrical draw

 

change the relay

 

i have a 89 fi50 that keep draining the battery dead.it was the relay on mine

 

Now that is a handy piece of information

 

I have had repeated problems with the voltage regulator failing and creating a battery drain on my truck, I can find nothing else wrong with it so I'm chalking it up to the cheap regulators used on rebuilt alternators these days.

 

Paul,

No doubt the regulator can fail to ground, and it is a good place to start!

I had tried unplugging my alternator overnight when I had the mysterious drain.
When that didn't work the decrepit plug was changed and solved the problem.

Now I have a 3G, and don't worry about it any more.

 

thanks! guessing my recent dead batteries have something to do with this half melted, rusting out plug!

 

13 years later....

The 2G alternator was known as a fire hazard. That melted plug is all too common and many times leads to an engine compartment fire. There is a service bulletin from Ford to replace that plug anytime you remove it from the alternator. The real fix is a 3G upgrade to eliminate that problem.

 

Plugged back in that melted plug so I could make it to the parts store without stalling again..guess that was a bad idea..when I tried unplugging it to install the new one I discovered one of the leads got welded to the alternator end making it difficult to remove and then clean up to plug the new one in (had to take the alternator out and carefully pry at it). Reinstalled everything and it seemed fine..appeared to be charging but did the amp drain test and read 3.5ish amps being pulled with the engine off. Eventually unplugged the wires going to the voltage regulator and no more drain. Guessing I either need a new regulator or just get an alternator since the other plug socket looks pretty bad now. I'm wondering if I screwed up the new battery too in the process because the meter on my dash would sometimes go to the max at 18v with the battery warning light red.

The genesis of all this pain must've been getting a jump from someone's newish car at night and accidentally reversing the cables cause I couldn't see any + or - symbols just a buncha wires on both..

long story short: I'm an idiot but learning! Now gonna research this 3g alternator swap and hopefully install tomorrow