1991 ford f-150 new battery over night battery goes dead-why?

Answer 1

I own a shop, and I can tell you this is a short somewhere. A lot of times, this is due in part by a bad alternator. Also; check to see if there is a light that is staying on, such as the glove box not closing good enough to turn the light off. Nine out of 10 times a bad diode in the alternator is the culprit. Even though it charges OK, it doesn't mean it isn't bad. Remove the battery cable, and arc it to the battery post where it goes. If it has a pretty good spark to it, then there is a short. A sound system installed incorrectly is probably the next best thing to look at. A lot of times some people will route power wires through places that pinch the wire causing it to drain the battery. If you use a process of elimination, and use the spark thing on the battery post, you may find it pretty easily. Have Auto Zone, or any parts store check the battery, and the alternator before you try anything else. This will rule out the obvious, and may save you some time and effort.
Glad to help out, Good Luck!!!

Answer 2

Before checking for a short (which can be a needle in a haystack), observe the truck's lights closely or at night with a good battery charge to make sure everything is shutting off normally. Some later model Fords are notorious for door switches sticking open leaving the interior lights and cargo lights on.

Answer 3

You may have a defective alternator. The battery will drain through a bad diode in the alternator.

Answer 4

short in system or defective NEW battery. disconnect terminal, charge battery, clean terminals, when reconnecting terminals look for spark, small spark okay, big spark not okay, indicates short in system. Still not sure, disconnect charged battery overnight, then try it again in the morning, if it works fine, system has short.

Answer 5

My 94 Ranger used to do that and I had a short in my trailer light connector