Fix Always-on Defrost + Floor for Ford/Lincoln/Mercury EATC Climate Control

So there we were: driving from Portland to San Diego, car full of apartment supplies and heading into the desert south of Eugene, OR on a 95F/35C day. When we hit the first hill, the AC makes a sound and switches from Max to Defrost and looses all efficiency. Ugh.

I had a hell of a time getting the air conditioning on my 2000s Mercury Grand Marquis to blow anything but Defrost/Floor. It started with family smelling exhaust when it was on Max AC/Recirculate. Then it would switch to Defrost when going uphill. Finally, it stopped sending air through anything by the top vent/floor.  Turns out the vacuum-actuated doors run through the notoriously leaky EATC HVAC Control Head. The troubleshooting advice below should apply equally to any vacuum system Ford/Lincoln/Mercury AC units.

Initial Troubleshooting - O-Rings

If you're here, you have probably seen the archived MercuryMarauder.net thread with tons of advice, pictures, and fixes to help you get started. The best fix to start with is to swap out the o-rings inside the solenoids. The fix is a few dollars (007 O-Rings from any Ace should work, or get the silicone ones from eBay from sellers that have them specifically labeled for this fix). It can be done in an hour with the proper tools, and there are a ton of YouTube videos detailing the fix.

007 O Rings at Ace hardware

 

Next Step - Leaking Valve Body

Sometimes, the o-ring fix doesn't work. Farther down the thread, fastblackmerc details how to fix a leaky valve body -- the part that connects the hoses to the solenoids:

1. Disassemble the EATC Head. Take off the solenoids completely so it is just black plastic.
2. Plug the solenoid tubes with 5/8" vac caps (made mine out of kinked tubes)
   
3. Plug the vac hose tubes with 1/8" vac caps (connected these to each other, see below)
   
4. Check to see if the body is holding pressure: Hook a length of spare tube up to one of the tubes and blow with your mouth. If it's not holding pressure, you will hear a hissing from the tubes or body.
5. Check where the leaks are coming from: Submerse the entire valve body in water and blow again. You'll see bubbles rising from the micro-cracks letting out the air.
6. Dry off the valve body and cover/fill up cracks with 2-part epoxy. You can also use the same tube you used to blow in air to subject the valve body to a vacuum and draw the epoxy into the cracks. Careful! Don't get too aggressive with this or you might plug the pathways for vacuum.
   
7. Wait for the epoxy to cure and repeat the check for cracks/fill steps until the valve body is no longer leaking.
   

For me, I took his advice and it still didn't work. I had leaks elsewhere. But it might work for you.

Valve Body with all it's holes plugged

Desperation Move - Bypass Valve Body

If the fixes to the valve body are still not working, you might needs to bypass the head completely. This can be done as easily as hard-wiring 1/8" vacuum tubing to the two mixer doors that control air flow. Or you can follow a simple extra step to ensure you can restore Defrost as needed.

I followed Victor's video about the bypass. He gets under the dash (two plastic panel shield retainers hold it to a metal bracket) and is able to access the vacuum coupling behind the EATC Head.  1/8 hose and T connectors from AutoZone helped me get the bypass and Recirculate working just like him.

To take it a step further, I bought a 1/4 sprinkler valve from Lowes and patched it in with an additional T. Now I can turn the valve to open on cold mornings, let the vacuum out, and get Defrost/Floor when I want. I left the hose extra long and ran it up into the glove box. Easy access but still hidden.

Bodge under the dash for Recirculate

Valve that an be opened at will for turning on Defrost/Floor

Side Step - Check that the Door Actuators Still Work

If this isn't your first crack at the fix, you might want to make sure the door actuators are actually working. If the issue started occurring for you gradually, and all mix doors won’t respond, this is unlikely to be your problem. However, it can help to rule out all possible causes if it is driving you crazy.

To do this, you will need to access the vacuum lines (detailed above) and a length of vacuum hose and some fittings to get a good seal. You can either create your own vacuum with a hand-pump of the type used to bleed brakes, or pipe the existing vacuum main line from one line to each outlet and listen for the doors as they actuate beneath the dash. An extreme solution is pictured below. Using sprinkler fittings, I bodged together a 4-way plug to make sure I was getting Recirculate mode activated.

The hand pump vacuum is holding pressure. The EATC is probably at fault.

Fixing Ford AC Head Controller Vacuum Problem

The AC on my land yacht (2009 Mercury Grand Marquis) has been in the fritz for a while. Last winter, it gradually stopped switching from max AC/recirculate (a necessary in Vegas), then got stuck on norm AC until it rested on Defrost/Floor. I was able to fix it with some basic troubleshooting, YouTube sleuthing, and two bucks in o-rings.

This shaky yet informative video by Ian Smith helped me diagnose it as a problem with vacuum only. The AC itself was fine. It blows cool air all day long. It just did so at the windshield. It couldn't be the blend-door actuator.

The same video showed me how to diagnose the vacuum problems. The black hose providing vacuum from the engine seemed fine: I was getting 20 inches of vacuum with the car turned on when I hooked up a bleed pump with a gauge (mine came from Harbor Freight, shown in the video). To test the actuators, all I had to do was hook a 'jumper' pipe from black to the other pipes. Each one seemed to hold air, and the actuators sprang to life once again. For the first time in a year, I had cold air blowing from the vents. The problem couldn't be in the lines. I pulled the controller head for a closer look.

The head itself is a bunch of electronics, a control panel, and one removable plate with four solenoids. The vacuum hoses come into this through a manifold, and the head controls trigger the solenoids to route vacuum from the black hose to the others. This triggers different actuators under the dash. Something was amiss in the manifold.

I returned to YouTube looking for rebuild instructions. I found this extremely helpful video from a Chicago mechanic. The solenoids contain an o-ring that dries out, wears out, and loses the ability to hold vacuum. I obtained close to the recommended o-rings from Lowes (#36, 5/16 OD, 3/16 ID, 1/16 thickness) as I was not willing to wait for Amazon. A little Oatey silicone lubricant made the tight squeeze work a little better. I found I had to seat the solenoid heads at least once before total reassembly. It was too difficult to do so at the end and fight with the other small parts at the same time. 45 minutes later, I had full control of my AC restored.

I can't believe it was this simple to fix the controller. I think I was intimidated by the AC (having spent $1500 last year to have the dealer redo the whole system from seals to refrigerant). I didn't want to break anything. A few targeted troubleshooting steps helped assuage any fears of irreparable harm, and now I have a comfortable cabin once again.