I own a 1999 Ford F250XLT Super Duty 4x4, V10 engine. Last year I had a problem where my heater was running when I first got into my truck on some mornings. It kept wearing down the battery. I noticed that this happened only when it rained (got wet). Also the "check engine" light, ABS light & other lights came on when not running. In 1992 Ford ran many diagnostic tests and finally came up with the Speed Sensor having to be replaced $16.00, then the ABS Module failed ($700). It was okay for a week or so then started having the same problem. I was told to unplug my rear electrical harness for my trailer hookup. I did and the problem seemed to go away. All winter long I had no problem (when it's very cold in NY).

Now it's starting again. Today it rained and I used my heater, the "check engine" light came on and the heater runs with the key OFF. Also whenever this happens, the engine runs hard and the automatic transmission shifts hard.

Any ideas?

 

Hi and welcome to FTE

The most common cause of the heater fan staying on is water in the fuse box from a leaking windshield. The water shorts the terminals inside the fuse box so there is power to fuses at all times when they are only supposed to have power when the ignition is on.
Sometimes you can blow out the fuse box with high pressure air but usually you end up replacing it. Dont do anything though until you find out where the water is coming in and fix that.

 

Same thing happened to me, what a drag... Water was leaking in through the top of the driver's side of the windshield, running down the a-pillar and dripping right on to the cpu. I had to replace the computer and fuse box, 900 bucks out the window. I'm still sore Good luck, Judd