My 89 5.0 won't hold a charge. The battery won't even last overnight. I have started unplugging things that get power, but nothing has worked. Any suggestions?
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 positive terminal, and connect a Digital Multimeter (DVM) between the positive terminal on the battery and the positive 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.
Have the charging system checked out. If the alternator shorts out one of the diodes, the output will be low and it will drain the battery.