I have a 2002 Ford F150 that the HVAC system heater AC comes on as soon as you turn the key on and blows on high even if the unit is off

 

My first thought would be that the blower is stuck on HI and your air conditioner compressor is actually not running. You can unplug the two prong on the compressor to prove that....


Im not sure, but I might guess that the resistor block that selects the fan speed has failed and it STUCK ( burnt) connecting the HI to the motor / ground and so bypassing the dash selector switch... it is also possible the dash switch is broken.


Lay on the passenger floor and look up under the dash at the duct work. You will see a 4 prong wire harness plugged into a small black box ( resistor block).. The box is screwed to the duct work with two small screws.. Remove the block and look close.. Pop off the cover and see if it is all burnt up inside.

 

It would help if you explicitly identified if you have the manual or automatic HVAC controls, we ain't psychic.

 

If it's the manual climate control, replace the blower motor resistor.

 

A blower resistor cannot cause the specific symptom as he described it.

He probably has an EATC system with a shorted BSC module.