okay, so heres the deal... ive searched and searched for days (almost a week) to no avail. i have a 1978 f100, and im having some problems with the hvac system. first of all, it only blows out of the defroster vents, which i do understand is most likely caused by a vacuum leak (probably local in unit or behind dash.) however, what has me perplexed is that the blower motor works just fine, every time just as nice and strong as you please with the key on and engine off. however; trying to run the hvac unit with key on engine running is a completely different story. with the engine running, the blower motor may work one in every five to seven times, and lo and behold, if you disengage the ignition switch slowly enough to kill the engine without cutting power to the hvac unit, the blower motor kicks in nice and strong once again... any ideas or suggestions to point me in the right direction would be very much appreciated.

 

total freaking derp moment, guys.... it was all in my idle speed