I have a 2009 Mercury Grand Marquis Ultimate Edition and the air blows in different places when ever it wants. It may go from the floor to the panel and then to the defroster constantly moving when it

Asked by May 20, 2017 at 07:33 PM about the 2009 Mercury Grand Marquis LS

Question type: Maintenance & Repair

I have a 2009 Mercury Grand Marquis Ultimate Edition and the air blows in
different places when ever it wants. It may go from the floor to the panel and
then to the defroster constantly moving when it wants to. Has anyone ever
seen this?      Tyrone

1 Answer

69,385

This is a faulty check valve in the vacuum portion of the climate control system. It's called "mode shifting". The climate control system is hooked up to intake manifold vacuum. When at idle intake manifold vacuum is highest. When you accelerate intake manifold vacuum drops. To maintain a constant vacuum supply for the climate control system there's a check valve and vacuum reserve tank incorporated in the vacuum part of the climate control system. If the check valve goes bad you no longer have a steady supply of vacuum and with the fluctuations in vacuum supply you get mode shifting. HTH. -Jim

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