Why does a new battery drain overnight in 89' Lincoln Signature and is it the fuses or is something is left on that will use the battery when the car is turned off?

Answer 1

Check the light switches that turn on the reading lights in rear. There is one on each back door next to the power window button. If one is left in the on position it will run your battery down over night. Generally, this is the result of a bad diode in the alternator. I had this happen to my '88 Town Car, so it wouldn't be surprising in an '89. What happens is that the bad diode allows a certain amount of leakage current to ground -- not much, about an amp or two. This runs the battery down over a few hours. Note that once the car is started, the alternator is producing good power, the battery will charge, and within at least one or two hours, the car will start and run quite nicely off the battery. You can check to see if this is the problem by using a clamp-on ammeter (voltmeter won't be of much help) to see if there is voltage drain in excess of the normal 'standby' for the electronics. Selectively pull the fuses for things like the radio, if you like, to confirm that there are no other drains when you make the measurement. I believe that the diode trio in this alternator can be replaced separately but don't have a Motorcraft P/N for it offhand. Might be reasonable, though, to install a properly remanufactured alternator instead of fiddling.