Quick question and thanks for any info that may help me. I have a '02 S-crew, 5.4 with the automatic temperature control. If I am driving down the road and hit automatic and set the temp., the air starts to come out of the defrost and will stay there unless I hit the vent button. If I leave it on automatic with the air coming from the defrost vents, it will flip to the regular vents when I let off the gas and the truck is slowing down or idling. Once I hit the gas again, it switches back to the defrot vents. Thanks in advance for any ideas where to look first.
Michael Brown

 

vacuum leak or the check valve (10¢ item) that probaly costs $20 at a dealership.
A generic one from any parts store will work fine.

 

several years ago, and I'd suspect the newer ones are the same, but all the a/c doors were vacuum controlled. I'd look for a vac leak or something similar. Unless of course someone else comes along and shoots down this theory....

 

Dang, zziggy got the hole shot!

 

THanks alot guys. I was thinking along the lines of a vacuum leak but did not know where to start. This check valve makes alot of sense. Would it be under the hood or under the dash? Thanks guys,
M. Brown

 

It is under the hood, firewall behind and on battery side( almost under starter Selinoid The battery holder also has a vac canister ( plastic container ) that occasionally gets a hole or splits and leaks. I currently have the same prob, have checked vac and tested the canister. Next step is this weekend, testing the door with a vac pump to see if it is operating ok. will also test incomming vac. I still suspect a problem with the dash controller but will know more Sat afternoon

 

Well, I changed the check valve and one vacuum connector that looked dried out and was very brittle. It is better now but still not right. I am going to trace the lines from start to end and see what I find. Now, If I change the automatic air up to 72-80 degrees, it will change to the vents. After riding a while I can adjust it to 65-60 degrees without jumping to the defrost vents. Thanks guys,
michael

 

You still have a vacuum leak.

Check that hose located between the battery and the firewall. It is the most common cause.

Steve