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Chuck, ASE Certified Technician
Category: Mercury
Satisfied Customers: 2100
Experience:  33 Years experience,Ford Senior Master,ASE Master,L1 Advanced Engine Performance
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2004 mercury marquis auto. temp. control.

This answer was rated:

2004 mercury marquis auto. temp. control. air blowing from defroster vents only

Your problem is a vacuum leak. Defrost is the default postion(no vacuum).Start first by finding the small black vacuum line going thru the firewall near the heater hoses to the heater core. Trace it back to the vacuum reservior located near the relay module and from there to the intake manifold and inspect for leaks.Here or pics of the reservior and how to replace it if you find it leaking. If all of this is good then one of the hoses inside from the control panel to one of the vacuum door motors or one of the motors themself are leaking.



1.Remove the relay module



2.Disconnect the vacuum hose





3.From inside the LH wheel well, remove the nuts









Yes it still does use vacuum even on the electronic system just the blend door is electronic.


Control System Outputs-EATC

A/C Blower Motor Speed Control

The A/C blower motor speed control is located on the A/C evaporator housing, in the engine compartment.

  • The function of the A/C blower motor speed control is to convert low power signals from the electronic automatic temperature control module to a high current, variable ground feed for the blower motor.
  • Blower motor speed is infinitely variable and is controlled by the electronic automatic temperature control module software.
  • A delay function provides a gradual increase or decrease in blower motor speed under all conditions.

A/C Electronic Blend Door Actuator

The A/C electronic blend door actuator is located on the top of the plenum housing.

  • Its function is to move the air temperature blend door on command from the control assembly.
  • The A/C electronic blend door actuator contains a reversible electric motor and a potentiometer. The potentiometer wiper is connected to the actuator output shaft and moves with the output shaft to indicate the position of the air temperature blend door.
  • A 5 volt signal is applied to the ends of the potentiometer. The voltage available at the wiper indicates the position of the potentiometer. The expressed value of the actuator wiper voltage is sent to the electronic automatic temperature control module and is matched with the wiper voltage of the module potentiometer. The control module then drives the actuator motor in the direction necessary to make the actuator wiper voltage agree with the control module wiper voltage.

Vacuum Control Motors

Vacuum control motors are used to move the floor/panel and panel/defrost doors.

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