Where is the first place to look for vacuum leak on A/C heater stuck on defrost?

Asked by Mar 20, 2014 at 12:17 PM about the 2006 Mercury Grand Marquis

Question type: Maintenance & Repair

My Grand Marquis ac heater is stuck on defrost. I've read I should look for a vacuum leak? Where should I look? Do I need to pull the dash out? Replace vacuum switch? Do I look under the hood?

6 Answers


There's a vacuum reservoir under the hood that is supplied vacuum through a "T" and a check valve. I'd check them first since they're under the hood and easy to access. You don't need to pull the dash to check out the controls. You can just pull the Glove box and check the lines. Do you hear a hiss coming from the dash? If not, I'd keep looking under the hood.BTW - Manual or Automatic Temperature Control?

5 out of 5 people think this is helpful.

I'm not sure if it's manual or automatic. There is a red button to warm and a blue button to cool.


Sounds like the Automatic version. Anyway, if you don't hear hissing from the dash area inside the car, start looking under the hood.

3 out of 3 people think this is helpful.

Thanks. I'll try it.


The reservoir and check valve are almost in the center of the firewall, just under the wiper housing. The check valve is a small, black and white vacuum valve with 3 ports on it. The reservoir is the black plastic box attached to the firewall by 2 nuts. I happened to have an '08 Crown Vic come my way tonight with a similar concern: the AC switches from the dash registers to the Defrost vents when accelerating (like cops love to do). Bad vacuum check valve.

9 out of 9 people think this is helpful.

My 2006 grand marquis is also doing this. While accelerating on the highway it blows thru vent then goes back to defrost

1 out of 1 people think this is helpful.

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