Auto Air Conditioning & Heater Repair: Ford f150 a/c, ford f 150, c switches

Question
I have a 1998 Ford F-150, 4.6L engine.  The a/c works, but not as good as it has in the past.  It probably needs a recharge.  The problem is this:  the a/c works while the blower is on the 2 lowest speeds, but when I turn the blower up higher, the a/c starts blowing hot air;  if I turn the a/c off and then restart it again on the lower speeds, it works again.  Before the a/c switches to hot, I can hear something in the underdash duct work changing position.  I would like to know what the problem might be, or where I should look for the problem.  Thank You!

Answer
Hi James

Check your pressures you may be low on refrigerant.  On these vehicles we are finding a lot of problems with the heater door breaking.  Usually it will cool  or heat up at any blower speed. Taht's why you need to check your pressures it may be low.