skylardog123

So i have a 97 f150 4.6, when i get in the truck, ill turn the key on (but not start the truck) and only my defrosters work, but if i start it, all of them work, also, when i accelerate, there is a VERY loud hissing noise behind the dash under the defrosters or windshield, cant hear it in engine bay, ive checked climate controls and doesn't seem to do anything, please help

twoskinsoneman

The motors that change the direction air flows are pneumatically driven. Without the engine running there is no vacuum so the controls will be in the defrost configuration regardless of where the switch is positioned.

skylardog123

Well what about the loud hissing when i accelerate

OhioLariat

If the controls work while the engine is running, but you can't change the airflow direction immediately (or shortly) after engine shutdown, you could a have a broken or loose vacuum connection. The first thing to do is to pull the HVAC panel, and check the vacuum line connections on the back.

Hissing under acceleration? This shouldn't be a vacuum leak, because vacuum decreases or subsides with an open throttle.

twoskinsoneman

true but I can't think of anything else that would hiss in the dash