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Q:

major drain on battery w/ key off on 1993 Ford E-150

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The drain is so bad that it ran down a new battery over night. Could this be caused by a bad ignition switch? For some time now, occaisionally the lock cylinder will rotate a full 360 with the key in the switch. Could this cause a short that would drain the battery? How can I check to see if this is the problem?
(1) Answer
Yes, but a diagnosis is needed first.
You need to get an ammeter and check the current draw. When you have a measure amperage draw above normal (.5 amps max) then start pulling fuses until the draw drops to normal.
The fuse that made it change is the circuit with the draw. You will then need to check everything on that circuit to see what is causing the drain.