The other day my A/C started blowing thru the defroster vents, after messing with the dials it started working again. This has happened a couple times now and it seems that the fan speed dial is the one that gets everything back working again. After doing some research it seems that I have a vacuum leak causing the problem. Now I have a new problem, where to begin looking for a leak in the "Jungle" of hoses and connections of vacuum components in my truck! Any ideas or suggestions would be very helpful,thanks.

 

Which truck, what year, and what kind of options? For instance: If you've got a 4x4 with auto-hubs - the vacuum lines going to the hubs seem to be a common culprit. If it won't shift to 4x4 mode, it will also mess up the defrost diverter door.

 

HVAC blowing through defroster vents only is the manufactured default in the system when there is a leak in the vacuum control system. Typically the leak can be found down on the front axle. Look at the lines feeding the ESOF hubs or the hub seal. Search here or google as its been addressed a thousand times here.

 

I'm sorry, 2005 F250 XLT V10 4X4 w/auto-hubs. I haven't tried to engage the 4X4 so don't know if it will or won't work.I was driving on the highway pulling a travel trailor when it started happening and it does it intermittently. Since it has been addressed many times I was hoping for a short list of culprits to narrow my search area and save alot of time,thanks.

 

My '01 just did this (intermittant AC going to defrost mode after driving for a few hours), and I tested the vacuum pump, the reservoir, and the lines to the hubs and the HVAC. A vacuum gauge and a piece of tubing are helpful. I could find no serious leaks, but my vacuum pump seemed to take a long time to evacuate the reservoir and shut off.

I ended up buying a $62 Dorman vacuum pump, and replacing the OEM one. I have not had the truck on a long trip to see if the problem returns. I sort-of replaced the pump because I could find no other smoking gun that implicated another part.

I did learn that switching HVAC modes only uses a tiny amount of vacuum, and it took about 20 'switching cycles' to deplete the reservoir to the low kick-in setpoint, where the vacuum pump came back on. So, if your vacuum pump is even working a little, it should have enough guts to cycle the HVAC many times.

So, don't have all the answers as to whether mine is fixed, but I have more knowledge, now.