2004 Mercury Grand Marquis with auto air condition. Blower does not come on AC / Heat still works but no air. I have replaced the blower motor, relay and fan control module but with no luck. Am I missing something or could it be more complicated than I expected. Thanks for any assistance. Damon

SOURCE: Location of blower motor switch resistor on 2004 Grand Marquis

The blower speed controller is located under the hood on the right or passenger side on the heater housing just to the side of the heater hoses near the very back of the right cylinder head.
Yes that would cause the concern you are having.

SOURCE: location of blower motor switch resistor on 2004 Grand Marquis

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Enter Support Key 298-118-364

I will be able to chat with you for free and we can solve this very quickly.

SOURCE: 2000 mercury grand marquis air conditioning won't work

1st Thing,
Located Someone With a Set Of A/C Gauges 134A Freon,
Make Sure Your A/C System Has Enought Freon In It, There Is a Low Freon Pressure Switch
In The Electrical System, Usually, 50-60 PSIG or Less On The Gauges, Will Indicate Low Freon, When Freon Is Low The Compressor Is Dis-engaged To Prevent Compressor Damage.
1] Low Freon
2] Bad Low Pressure switch
3] Blowen Fuse[s]
4] Electrical Problem[s]
[a] Digital Climate Controlled ?
Climate Control Self Diagnose From Button's On Unit, OEM Manual Explains This,
If Yor Auto Has Digital Climate Control.
5] Bad Compressor DC Coil

Note: Does The Compressor Engage, Click-On When Button Is Pressed, But Getting
Only Warm Air?
Is One Of The Compressor Aluminum Tube's Cold, Be Careful Other Is Really Warm/Hot,
If Cold and or Sweating, Dripping Water [Normal], You Are Blending Hot Air w/Cold
Air At The Heater Blend Box, Making Warm Air.
If This Is So, If A Vaccum Controlled Temp Unit, Check For A Vaccum Hose Bad,
Should Be Either 1or 2 Vaccum Lines Going Into Firewall To Feed The Controller
Unit, If OK, Might Be a Bad Blend Box Diaphram, [Control's A/C air, Blocks Off
heater Coil Hot Air].
If Your Auto Is Vaccum Controlled, Can You Changed From Bottom, Dash,
Defrost Air Flow?, Yes, No Vaccum Lost Feeding Controller, Still Might Have
Bad Diaphram Blending Hot Air w/Cold If Your Have Cold Alum-Tubing In
engine Compartment.
If Digital Climate Controlled, Might have Diaphram's, Vaccum Controlled or
electrical Servo's Controlled vs Diaphram [Vaccum] Controlled.
This Might Help, Hopefully, This Is Just For Starter's, There More To Some System's
and Without The OEM Mgr's Manual[s] You/Be Would Be Lost, especially w/Electrical.
Later, God Bless

SOURCE: mercury 2003 gand marquis heater

One of 2 things could be happening. #1, the valve that allows coolant to circulate thru the heater core is not functioning. #2 the door that is actuated to allow cabin air to circulate around the heater core is not functioning properly. Possible causes, linkage to the valve, or actuator not working. Also a broken valve to the heater core. A vacuum leak, or faulty actuator would cause the door not to function. You can narrow down the problem somewhat by making sure that both heater hoses have hot coolant in them. You can do this by simply feeling the hoses. If they are both circulating hot coolant, then the problem is the misdirection of air flow. To repair this, the cause will have to be diagnosed, could be anything from your dash mounted controls to an issue with vacuum or the ductwork or the door or actuator. If not in the controls, the dash may have to come out. Good Luck, I hope this is helpful!

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SOURCE: where is the lighting control module on a 2005

it is located under steering wheel to your right