Been having a problem for quite a while with the battery fully draining after a week or so on my '06 F150, which has a pretty new battery in it. Finally got the meter out to check some things out. What I found was that when my headlight switch is set to "auto" there is resistance at the battery terminals, when I set the switch to "off", there is not. This is without the key in the ignition. So I assume this is the culprit? The function seems to be working properly as the headlights will still shut off when they should when in the auto mode. I never had a problem for the first few years I owned the truck, and all of a sudden this starts up. So can anyone tell me what to make of this?

 

Not too sure why you are checking resistance, what you should have been looking for is an AMP draw after the battery saver relay times out.

when the headlamp switch is in the auto position Fuse #40 ( hot at all times ) is switched to the VSM. The VSM uses this input to ID what position the headlamp switch is in ( if it should be activating the headlamps & parking lamps ). Think of it as a positive trigger input.

The VSM is only active ( for the autolamp section ) when the key in the start / run position via Fuse #18 ( R/S Input to VSM ).

Start with are there any modifications to the truck ( lights, audio, etc ).

"Pretty new" battery does not really describe it.
I have had BatteryPlus junk piles that are 10 months old, and dead as a doornail.

When the battery is dead, how do you recover it ?
- Charge is long term on an external battery charger or jump it and drive it ?

Have you tried hooking up a battery charger and after a long term charge, check that the battery will hold a charge ?

Have you checked the output of the alternator ?