Can anyone give a idea on where to start looking. y a/c will blow good until i drive for a while on the interstate, Then the blower is still blowing hard(you can hear it ) but the air does not come out of the vents. I have replaced the blower resister and the relay under the hood. Any help would be appreciated greatly.

 

Sounds more like a fan/resistor motor than an A/C issue. If fan is blowing hard and no air discharge, seems some type of door in the ducts are suck or closed off.

Please clarify if you have an A/C issue as well (meaning your air compressor quits cycling). Or maybe the A/C continues to run but the blend door gets confused and starts mixing warm air.
Are you able to read for codes?

It is an unusual question because usually this time of year the universal "heater quit working" question is asked when they mean a fan problem. A/C issue questions start up again in spring.

 

As above, probably a vacuum problem, but you didn't say whether air was coming from the defrosters,
floor outlet, or none of the above, but here's a video that might help.

 

no other issues. When first start truck air runs good.

 

Could your evap be-freezing due to low charge because of a leak?

 

Yes it sounds like you are saying no air comes out of the vents?
do you have auto climate control or manual?
if no air is coming out there's a flap that directs air between the defrost, vents , and floor. Maybe it's stuck on defrost?
Or like mentioned your freezing up the evaporator with the ac on. Are you in a humid area?


Or if air is coming out,but it's not cold it's probably your ac compressor clutch needing a shim taken out to reduce the air gap.

 

It helps if you identify what you're working on. We're not psychic

2004 and newer (except Heritage) F150s do NOT use vacuum to direct airflow.

 

Could be the compressor clutch. I replaced mine. Air would blow cold and then after the motor was heat soaked(operating temp), the air would blow warm. There are many more reasons...maybe condenser...not enough charge...but when the truck is hot see if the compressor clutch is still turning with the AC on. If it is, ya better break out the gauges lol. If not, likely the clutch is getting hot and shutting off. The part is about a $100 and you'll need a puller to get it off

 

The compressor clutch can NOT cause the OP''s reported issue. Read the symptoms.

 

Yeah, true dat....I didnt read the post fully. is the air on your feet....or being directed to the defrost??