Why is my battery going dead overnight in my 201l Mercury Gran Marquis (new battery) with nothing on and my ignition key out?

Remove the radiator fan fuse and see if the battery holds a charge.

Somewhere around 2006, Ford decided to eliminate a diode in the AC clutch circuit causing the AC Relay to overheat and stick closed. In other words, the AC System is powered even with the car off. To check: I like to mark the AC clutch with white or yellow paint - just an easy-to-see spot, turn off the AC, start the engine, and see if the compressor's engaged. If it is, It's the AC relay stuck closed. Another indicator of this may be a rather large puddle of clear water forming under the passenger's side of the car when parked for 10 to 20 minutes after driving for a while. The AC relay is in the fuse box under the hood and fairly inexpensive. I'm considering studying an old system to see where and what type of diode to use and retrofit our Patrol Cars. We've had lots of dead batteries and a couple of ruined evaporator cores because of this one.

You might also have someone check the voltage regulator. If it fails it can drain your battery.

Eric the car guy will take you through what it's gonna take to find your parasitic drain..probably the make-up light on the passenger's visor, the courtesy light in the trunk underhood or in the glovebox~....good luck~ http://www.youtube.com/watch?v=KF1gijj03_0

I had a '97 Jeep Grand Cherokee that kept losing battery charge. Finally found out that the switch on the dome light over the rear liftgate had been left in the "on" position. Since I usually came home at night and the lights remained on for a few seconds after the doors were shut and locked, it went undetected for a long time.

The battery go down when it set overnight 88 grand marquis