this is on a 2003 f150 5.4l FX4

heater blows only of defrost, I know this the default for a vacuum leak, which vacuum line should I be looking at? is it the black one that goes through the fire wall?

 

i would assume its that one but im not sure might want to run to an auto parts store and see if they have a diagram, also might be your blend door

 

wouldnt the blender door only control the hot/cold ratio?
I have a have a hand held vacuum pump that I will test the line and see if it is leaking. just want to be sure im testing the correct line.

 

ya im not positive on the line, but not neccesarily, my buddy has a ram and it has dual ac/heat passenger and driver and one side blows hot on heat the other just keeps cold on heat but his defroster blows straight heat.

 

Blend door controls the temp and vacuum controls the output location. Pretty sure you have a vacuum leak, it can be almost anywhere. Do you have a CEL?

 

Go to the heating and air conditioning sub-forum. Your problem is covered in the FAQs posted there.

 

[quote=Joe Finn; Do you have a CEL?[/quote]

Joe Finn no CEL

 

Does the heater blow on any other setting? If so check the diod-trio in the heater/ac case. If the heater only works on hi, the trio may need replacement. The default on it is to HI only. Check for vacuum also.

 

I had a crack in the vacuum canister inside the passenger fender. Removed and replaced now heat is good as new. I will do some looking and find a diagram for you if I can.

 

your looking for a vacuum leak on the right side of the truck , under the hood, between the battery and glove box. Look at the firewall and see a smal TEE and couple black vacuum lines connected and run thru the firewall. check the TEE and hoses for cracks. As mentioned, one hose runs thru the right front fender to a collector (vacuum canister). It can be cracked or the hose fallen off...... Its not the blend door and its not the heater motor resistor block.

 

there are no holes or cracks in the canister thing behind the battery, both of those tanks held a vacuum, from there the line that goes to the T is fine, the line that goes down into the fire wall is fine, the other side of the T I can blow through, but not sure what elese to look at.
(steve)ill I have read so many of your post on this topic but I cant find the problem, I looked at the #28 10amp fuse and it was fine.
where else should I check? would the 4x4 be the cause?

 

could this have something to do with the actual controls? The A/C is turned off, I push the automatic button, the temp is set at 65 and blows out the panel vents, as I keep turning the temp up to around 79 it automatically redirects the air to the defrost, still a vacuum leak or something else?

 

It's the vacuum line running from the reservoir behind the battery box thru the firewall. Defrost is the default position when there is no vacuum. Could be the temp control switchi itself. It is easily and cheaply replaced.

 

Remove the EATC control head and verify that you have manifold vacuum at the black hose in the vacuum harness behind the control head with the engine running. If you do have vacuum you may have a bad EATC control. I have seen several of these bad.

 

the black hose has a vacuum when the truck is started, so then I tested the red hose which opperates the the air directional door and it also had a vacuum when the truck was started, and I watched it operate the door. but as I turned up the temperature on the EATC at 68-70 it would change to defrost. what should I look for next? what do the blue, white and yellow hoses do?

let me also add that the vacuum hose through the fire wall holds a vacuum, and my 4x4 works.