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)
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.