This has to be a common problem since this happened to my brother-in-law on his '94 F150. First of all when switched to "MAX A/C" the blower speed no longer goes faster than when it is set to "NORM A/C". Also when set to MAX, the air will not blow out of the front vents during heavy exceleration. It starts coming out of the defrost vent instead. Once at a crusing speed it starts coming out of the front vents again. Anyone heard of this? Any idea what could cause it?