I just bought a 1988 5.0 and the battery drains over night i already tested all the fuses, installed a new alternator and belt yet the problem does not go away and the battery will drains overnight. does anyone have a suggestion as to what could be the problem or how to fix it?
Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.
The ideal method is to disconnect the negative terminal, and connect a Digital Multimeter (DVM) between the negative terminal on the battery and the negative cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.
Using the negative side prevents nasty accidents like shorting the wrench to ground while disconnecting the battery cable. Electrically, the test will work with either positive or negative battery cable.
Do you have a digital multimeter that will read amperage up to 10Amps? A lot will rear DC amps but only to 1A or so. You'll need to go a bit higher if the drain is that bad.
Hook the meter up in series and see what you have. You'll want as low as possible...1/2AMP or even less. A big drain would be 2-3 amps to draw the battery down that fast. With the meter hooked up, start pulling fuses. The power windows/doors work off a circuit breaker on the solenoid so you'll want to disconnect that as well and see what you have if you can't find it.
My draw, years back, was the vanity lights. I leave them disconnected as I can't trust them to stay off