I'm working on my mother's 1989 F150 5.0 4x4. The battery drains overnight. If someone would help me with some suggestions on how to find were the short is, I would appreciate it. Thanks, Russ.

 

Do you have any aftermarket accessories on it?

 

Get a 12 volt test light.

All you got to do is disconnect the positive cable from the battery positive terminal. Connect the 12 volt test light on the battery positive terminal and connect the other end of the 12 volt test light to the positive cable that you just disconnected.

The test light should turn on if there is a load. Small load light the car stereo with memory clock takes a little power and it will lit up the test light for a few seconds which is normal. However, large drain will lit up the test light steadily.

Start disconnecting the alternator wires. If the test light turns off then the problem is with the alternator and probably back feeding itself and needs to check the wires going to it or the alternator itself.
If the light is still on after disconnecting the alternator, start disconnecting one fuse/relay at a time. Once the test light turns OFF, you just found the circuit that is causing an excessive drain. Follow the circuit and find out what devices are connected to that circuit. BTW, there might be other fuse/relay in the engine bay aside from the fuse box inside the vehicle.

If the test light is still ON after removing the fuse/relay, try disconnecting the wires on the starter relay. Sometimes the starter itself is causing the huge drain when it is failing.

 

The only aftermarket item is a set of fog lights. I'll check them out. Thank you for the help. I'll be working on it an evening this week. 2 months ago, Dad bought a new battery for it, thinking that might be the problem. The parts store were he bought it said when they installed it there was quite a spark. I've got to find it and with the test light info you gave me I should be able to locate the short. The truck isn't usable as is. Thanks again, Russ.