How to fix vent from turning to defrost?

Asked by Jul 09, 2016 at 09:26 PM about the 2003 Ford F-150 Lariat Crew Cab 4WD SB

Question type: Maintenance & Repair

Dash vent works when at idle or real low speed, accelerate and it goes to
defrost.  If the panel floor button is pushed, then vent works and floor portal
freezes your foot. How can you determine if it's the vacuum lines or the dash
box controller?

1 Answer

72,435

This is going to be a vacuum problem. Intake manifold vacuum is strongest at idle and decreases as you accelerate. There's supposed to be a check valve and possibly a vacuum reserve tank in the system to combat mode fluctuations. The check valve is usually in the vacuum line that runs from the engine, through the firewall, and to the climate control panel on the dash. It will look like a little plastic disk. If you have a vacuum reserve tank (some systems do and others don't) it'll most likely be in the engine compartment and look like a plastic ball or box with 2 vacuum lines connected to it. It may even look like a coffee can with 2 vacuum lines connected to it. HTH. -Jim

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