Ford Repair: 1986 Ford F150 Electrical Problem, negative battery cable, fuel gauge

Question
I own an 1986 F150 4x4 w/ a 302 and A/C.  The problem is I noticed I wasn't getting good cranking when I was starting the engine and within a few day may battery was dead.  So, not really worring about it, thinking I may have left something on, I jumped it and checked to see if it was charging, it was, so I went to  work.  Eight hours later the battery was almost drained again....jumped it to get home, it charged again but dead by morning.  I assumed the battery would no longer hold a charge and replaced it w/ a new one.  Truck popped off first try, just like it always had, next morning it was dead again.

I know there is something draining the battery like a short but where do I start to look for it?  Does the fact that it drains the battery with the key switch off and nothing running or on mean anything or narrow it down?

Only repair I have done is changed the break master cylinder, one day before all this started to happen.  And the only electrical problems I have noticed are #1-fuel switch doesn't work, stays on front tank, always has for my six years of ownership and #2-very recently the fuel gauge will stop working and then start working again.

Thank you for your time,

Darwin Csae

Answer
Darwin:

Diagnosing a voltage drain can be a long and tedious process.  Disconnect the negative battery cable and attach the ammeter in series with the cable and battery ground post.  With all turned off there should be a draw of no more than .5 amp.  If draw is greater than .5 amp remove one fuse at a time until the draw drops.  This will identify the offending circuit.  At this point follow normal wiring diagnosis to find the shorted wire, switch, or component.  

Hope this helps

Patrick