I have a 2004 Grand Marquis with ATC. Recently, the air has started blowing mainly out of the defroster vents. If I leave the control on regular "A/C" for a while, it will eventually start coming out the dash vents, but as soon as there's a load on the engine, it switches back to the defroster vents. Should I suspect the ATC control head, or some vacuum operated device?

 

May just be a loose connection on the back of control head where the vacuum lines attach but it's probably a poor signal from the control head. Also make sure the vacuum lines are attached to the servo motors as well. There may be a leak in a line not allowing full vacuum to get to the servo. When you accel it lowers the engine vacuum making the doors awitch. Also check the vacuum reserve tank & any one way valves for proper operation. The reserve tank is supposed to handle a momentary drop in engine vacuum & it may have a leak. I've had a control head do this same thing though because the vacuum leak was in the control head. Had one where the line was loose on the servo motor. I've heard of leaky tanks but haven't experienced one.

 

Thanks for the feedback. I finally got around to pulling the control head out of the dash for testing. I couldn't detect a leak at the head and all the servos work properly when vacuum is supplied by the black vac source line. I'll have to replace the control head.

 

I finally dug deep enough to find that the o-rings inside the vacuum solenoids were worn. The new o-rings were a bit snug, so I barely touched them with a little "Pam". The snapped in perfectly. I re-assembled the vacuum solenoids, re-installed them in the EATC control head and tested. It works perfectly now.

 

My ac was comming out of the defrost vents when accelerating. I have an 08 GM with 70k miles.

I almost changed the control module at $500 to $800.00. In a repair database at the library I found an answer. I cold booted the module and it started working right. The actuators are re initialized and calibrated.
To cold boot you turn off the ingition, unplug the two electrical connectors for one minute, then re connect and turn on the ignition and push any button on the module except the off button and let it re initialize for thirty minutes. It will appear that nothing is happening but when finished try it out.
The woodgrain trim above the module just clips on I got mine off with my fingers. Take out four mounting skrews and ease out the module just far enough to unplug the wires (be sure to press the release on the plug). Saved myself big dollars and torment.

Georgie

 

I had the same issue on my 05 Marquis-after taking the control apart still installed-it just flips open after 2 screws-come out- i heard slight vacuum around one of the solenoids and if i wiggled it a bit it switched instantly to dash vents off the defrost- using icusteal post pics i took it totally apart and installed new o-rings and a dab of black silicone on each o-ring-let it sit over night--no problem since-the fix wasn't hard with simple tools-the worst part was bending the tabs back tight after i reassembled the solenoids

 

SAVE YOURSELF $700..... This repair will take less than one hour. Pictures of the repair are on page 35 fo the below link. Page one has detailed instructions.

http://www.mercurymarauder.net/forum...ad.php?t=36922

If anyone wants me to send them the O-Rings just send me a PM with your address and I will send you four of them. I bought the bag of 100 from McMaster-Carr Part Number 9396K12 for less than $10. But now I have 96 extras.

Sorry to post something from another forum but it is a Mercury and it does fix this problem.

Rob