how do i manually switch air flow from defrost to vent in 2004 merc grand marquis
all air flow is coming out from the defrost vents in front windsheild . how do i manually
change it to the vent position
You can't. What's going on is a loss of vacuum to your HVAC controls. It's likely that you have a vacuum check valve, vacuum line or vacuum reservoir that is leaking or that has failed. Just left of center, under the hood and attached to the Firewall is a vacuum canister and a "T". Usually, the problem is the check valve (the "T") but could be a broken vacuum line.
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