AC air blows through defrost vent only on Ford F-350 6.0 Diesel Engine?

Its a vacuum related issue.Noticed by many other care drivers too.



This is how your system works :----
Inside the dash there are vacuum operated motors that move doors in the dash to control air flow,the vacuum motors use vacuum to control them ,to open and close them.When the system has NO vacuum then it faults to defrost.So the problem is that you have no vacuum to the motors in the dash

Since you have a Diesel engine ,, it does NOT make vacuum like a GAS engine does, so a Diesel has an electric vacuum motor located under the hood by the right side fender

See the diagram below for better understanding:----

So check if this vacuum motor is working .For that turn the key on but do NOT start the truck,if you hear the vacuum motor running and will not stop running then you have a vacuum leak some where.
But if you do not hear the vacuum motor run,then pull off the vacuum hose on it.
If it does not run when you pull off the hose,then there is a two wire conector to it ,you should have power on one wire and ground on the other, if you have both ,,,then the vacuum motor is bad and needs to be replaced.

This detailed instructions will help you.

Thanks.

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AC blows cold air through defrost vents.Many car drivers notice this problems.
Basically this problems are related to cracked vacuum line or loose vacuum line or loose worn out vacuum line hoses.
Inspecting it properly helps to confirm the problem.

If it will only blow through the defrost vents, then its a vacuum issue, as the vent selection is vacuum controlled.

But if you were only getting hot air, or cold air, but could get it out of any vents, then that would be the blend door.

But as per your problem described,your problem is vacuum-related. The floor-heat-vent-defrost selection is vacuum-controlled, and in the absence of vacuum from the engine, the heat-A/C control doors revert to the "Defrost" mode as a safety issue. Look at all your vacuum lines, particularly those on the vacuum reserve canister under the fender.

The vacuum line gets overheated and melted in some cases,some times lines gets cracked or loose .

There are two red vacuum lines behind the upper manifold inspect both line properly.

This basic details will help you thanks.

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