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

23,455

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?

6 out of 6 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.

23,455

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.

23,455

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.
10

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.

Your Answer

Add photo

Related Questions

Grand Marquis

Looking for a Used Grand Marquis in your area?

CarGurus has 1,801 nationwide Grand Marquis listings starting at $500.

ZIP:

Search Mercury Grand Marquis Questions

Mercury Grand Marquis Experts

#1 Tracy Hooks
Tracy Hooks
Reputation 5,280
#2 kbro
kbro
Reputation 2,050
#3 Js08016
Js08016
Reputation 1,660
View All

Related Models For Sale

Used Ford Crown Victoria
12 Great Deals out of 948 listings starting at $2,990
Used Lincoln Town Car
26 Great Deals out of 1,805 listings starting at $1,695
Used Chevrolet Impala
544 Great Deals out of 28,509 listings starting at $700
Used Chrysler 300
341 Great Deals out of 20,505 listings starting at $1,000
Used Dodge Charger
241 Great Deals out of 27,116 listings starting at $1,997
Used Ford Taurus
193 Great Deals out of 19,234 listings starting at $500
Used Ford F-150
2,403 Great Deals out of 210,248 listings starting at $895
Used Ford Mustang
333 Great Deals out of 45,576 listings starting at $2,000
Used Honda Accord
679 Great Deals out of 82,247 listings starting at $500
Used Cadillac DeVille
5 Great Deals out of 1,580 listings starting at $2,900
Used Chevrolet Tahoe
611 Great Deals out of 27,552 listings starting at $600
Used Buick LaCrosse
196 Great Deals out of 22,821 listings starting at $1,500
Used Chevrolet Silverado 1500
1,983 Great Deals out of 160,213 listings starting at $1,995

Used Cars For Sale

2011 Mercury Grand Marquis For Sale
4 Great Deals out of 41 listings starting at $3,900
2010 Mercury Grand Marquis For Sale
7 Great Deals out of 95 listings starting at $3,999
2009 Mercury Grand Marquis For Sale
3 Great Deals out of 95 listings starting at $4,299
2008 Mercury Grand Marquis For Sale
6 Great Deals out of 141 listings starting at $500
2007 Mercury Grand Marquis For Sale
4 Great Deals out of 152 listings starting at $1,900