Hi Forum, I have a problem I hope ya'll can shine some insight on. When I turn A/C to Max or A/C, I hear the fan running full blast in dash, but it is coming out defroster vent, then without touching anything, it will start blowing out vents, then back to defrost vents. It seems to be cycling between the 2. I thought maybe A/C was low, but it is blowing very cold, just keeps cycling between vents and defroster...any ideas what I need to check ?? Thanks

 

Check the little hard vacuum lines behind the battery for cracks.

 

Thanks, yes it was broke clean in half, I tried to take a piese of hard line asnd push in each end but it didn't work, still doing it, looks like other end goes behind wheel well so I guess I will replace whole line with a rubber line, I CAN use a rubber line right??

 

As long as its rated for vacuum.

 

Cut it clean on both sides of the break and bridge them with a rubber vacuum hose.

 

Thanks, will try that, I was thinking of going from whatever is in the wheel well to the T

 

I replaced that piece with a vacuum line bridged between break..still doing it, any ideas what else to check??

 

You probably still have a vacuum leak in the system.

 

Where does the other end of that line goes?? Looks like it goes down behind the wheel well...do I have to remove the wheel well to get to whatever that line hooks to?? Is there any other lines to check INSIDE the truck??

 

The line goes into the fenderwell to the vacuum reservoir. It also goes through the firewall to the HVAC control unit.

A Mityvac pump is invaluable for troubleshooting problems like this.