Took the truck to the store for some stuff and everything was normal. A/C was on auto and set for 73 and outside temp was about 45. Got my stuff and headed to work and I noticed that the display was blank and the A/C wouldn't work. When I got to work I turned the truck off and then cycled the ignition back to the ACC position and the display remained blank. After work I got in the truck and when I started it up everything was back to normal. I had both batteries tested and the charging system load tested. Both batteries had over 800 CCAs and the alt was good (13.3v at idle). I'm pretty sure that the batts are the originals since they're both motorcrafts. The truck takes about 2-3 seconds to start pretty much regardless of the temp so far (32-85). Normally once a week I hook my charger up to the batts overnight and charge overnight at 6 amps. It's fully automatic and shuts off when the batteries reach ~14v (not sure exact number). The passenger side batt has the red ball showing and the driver has the green one. If anyone has any ideas they're much appreciated. I think I picked up a gremlin and hopefully he jumped on the dodge I parked beside today.

 

Perhaps the alt isn't functioning as well at high speed? When the motor is turning 2k rpms and the alt should be putting out 100amps, perhaps you have a couple dead diodes or they over heat and your only putting out 40-50 amps. The lower output means your electronics are getting too little voltage and shutting down.

 

You didn't say what kind of truck you are talking about but your profile says you have a 2005 F250. They way you are talking it sounds like you have a Lariat with the Electronic Automatic Temperature Control (EATC) system. If so this is a somewhat common problem. I guess the relays inside the units have the tendency to get stuck and it causes the unit not to function properly. A lot of the time if you turn your headlights on and off, or shut the truck off and restart it will start to work again. However once it starts to happen it will begin happening more and more often. If you are still under warranty be sure to get it in to your dealer and get it fixed. If you aren't you may want to hold off on replacing it until it starts to act up a lot, as they are pretty expensive ($300+ if I remember right).

If you don't have the EATC system ignore everything I just said

 

Didn't even think to post the truck info. You're right. It's a 05 F250 Lariat with the EATC. I'm within the 5/100K but not the 3/36K warranty. Guess I'll have to look into a relay eventually. Thanks for all the info everyone.

 

The relay is internal to the EATC unit. It can't really be replaced, you have to replace the whole EATC unit.

 

Thanks for all the info everyone. The problem hasn't popped back up since then so hopefully it was just a fluke.