A/C issue

[Motorhead350]

A/C will go into windshield defrost mode with half throttle or greater. The only way to get it back out of the vents is to turn off the climate control and turn it back on with feet off of all petals.

Already changed the box in the firewall behind the passenger side head. A/C is cool, but not cold... probably needs a recharge.

Any idea on what is causing this defrost mode with throttle usage?

Thanks!

[fastblackmerc]

A/C will go into windshield defrost mode with half throttle or greater. The only way to get it back out of the vents is to turn off the climate control and turn it back on with feet off of all petals.

Already changed the box in the firewall behind the passenger side head. A/C is cool, but not cold... probably needs a recharge.

Any idea on what is causing this defrost mode with throttle usage?

Thanks!

Sounds like a vacuum leak. As the engine speeds up you get less vacuum.

Low vacuum makes the system go into fail-safe mode - heat & defrost.

If the o-rings (might be the source of the vacuum leak) in the EATC haven't been replaced, I can do that for you.

[justbob]

+1 vacuum leak.


Builder Of Badassery

[fastblackmerc]

Check all the vacuum hoses in the engine compartment. As these cars get older the hose get brittle and break off. Also check the vacuum tank on the top of the drivers side fender (black square box under the hood).

[Fastbob]

+ 2, vacuum is leaking somewhere as engine has less vacuum when throttle is open versus throttle closed. It is likely the usual spot, the EATC unit.

[Motorhead350]

Is the EATC unit the box by the passenger side head in the firewall?

I can give the O-rings another try.

My van goes unto defrost mode on and off the throttle, but it doesn't need the A/C system shut off to go back out the vents.

I will hunt under the hood for something disconnected or broken, but those O-rings might be the issue again hua?

[fastblackmerc]

Is the EATC unit the box by the passenger side head in the firewall?

I can give the O-rings another try.

My van goes unto defrost mode on and off the throttle, but it doesn't need the A/C system shut off to go back out the vents.

I will hunt under the hood for something disconnected or broken, but those O-rings might be the issue again hua?

It depends on what o-rings you used, was silicone dielectric grease used, who did it, etc......

EATC - Electronic Automatic Temperature Control - it's the unit in the dash used to adjust the fan and HVAC settings.

The box on the firewall is the BCM... whole different problem.

[Motorhead350]

I used I rings sold at a ford dealership. It didn't quite fix the issue last year. I am thinking it's not the o rings and is another issue. I will search under the hood later as advised.

[fastblackmerc]

I used I rings sold at a ford dealership. It didn't quite fix the issue last year. I am thinking it's not the o rings and is another issue. I will search under the hood later as advised.

So I gather they were not silicone o-rings......

[Motorhead350]

So I gather they were not silicone o-rings......

Pretty sure they were rubber. Which I am now understanding is the incorrect material. Yes?

[fastblackmerc]

Pretty sure they were rubber. Which I am now understanding is the incorrect material. Yes?

Yes, I can fix it right for you...............

[Motorhead350]

Yes, I can fix it right for you...............

Does napa or any local auto store sell the proper material? I have done it before, but let's make sure the material is right this time.

BTW PM me your address, it's about time I send you that Phil Spector DVD.

[Motorhead350]

I put in Napa rings and nothing really changed.

Upon starting the car, air came out the vents, but it was warm and I could not adjust where the air flow was coming from.

I decided to drive the car.... like I do and things went back to the way they started.

Cold air out the vents until half throttle or greater, then air goes out the defrost and will not come out the vents until I turn everything off and on again.

I will check for vacuum leaks.... or does Napa not sell the correct O-rings?

Thanks so far guys.

[ChiTownMaraud3r]

The EATC might be bad.

I had my main vacuum line that runs from the check valve under the hood, and goes into the cabin by the gas pedal, literally melted shut and in two pieces. My car would just do defrost/floor period. So definitely sounds like a vacuum problem. As far as how cold the air is, do you know if the blend door is working fine? Do a self test in case there are some codes present.

[Motorhead350]

No codes on the car, but I will check out what you told me.

Thank you.

[RF Overlord]

I will check for vacuum leaks.... or does Napa not sell the correct O-rings?Dom the correct size is -007.

If you had no vacuum at all, it would never work on the vents, it would be stuck on defrost all the time. It sounds like you have an issue with the vacuum reservoir, which is there specifically for this issue. It's the square black box on top of the driver's fender liner. There should be a vacuum hose running from there to behind the driver's cam cover, where you should find a tee and a white check valve. One side of the check valve goes to the vacuum supply from the manifold, the other side goes to one leg of the tee. The other two legs of the tee go to the reservoir and to the EATC.

[burt ragio]

Often thought what purpose the black box had located on the drivers side inner fender. Great explanation.

[O's Fan Rich]

I put in Napa rings and nothing really changed.

Upon starting the car, air came out the vents, but it was warm and I could not adjust where the air flow was coming from.

I decided to drive the car.... like I do and things went back to the way they started.

Cold air out the vents until half throttle or greater, then air goes out the defrost and will not come out the vents until I turn everything off and on again.

I will check for vacuum leaks.... or does Napa not sell the correct O-rings?

Thanks so far guys.

That's how mine acted until it finally went full defrost ( must be the failsafe mode). The EATC ring replacement was the cure.

[Motorhead350]

I had o rings go out before and the issue was not quite the same.

I will check out the lines by the box in the firewall. I already replaced it before but a bad or pinched line sounds like the issue.

Thanks everyone.

[vegasmarauder]

Make sure the vacuum check valve (on the firewall connection) is still a check vavle. Air should only go towards the engine, not back in the car when you blow through it. Had one go out on a Lincoln and it would do the switch to defrost/floor when slowly adding more throttle while climbing a hill. Close the throttle and 10 seconds later the vacuum built up enough to work correctly. Thats the purpose of the check valce and vacuum supply tank to even out the vacuum in low vacuum engine situations. Been that way on Fords since the early 50's vacuum wipers.

[Motorhead350]

Where is this check valve?

I thought I found the leak, but I was wrong.

[RF Overlord]

Right where I said it is.

Behind the driver's cam cover.

[Motorhead350]

I have getting ZERO vacuum to the EATC, so the O-rings are out of the picture.

I checked that vacuum line by the drivers head and it had zero air flow.

Does anyone have a chart for this setup?

It's a leak, but I need to find out were to look and trace it back.

Thank you all. You have been great.

[fastblackmerc]

It's a simple schematic.

Engine vacuum to check valve to vacuum reservoir tank to EATC connector (black hose).

You need to trace the hoses.

[Motorhead350]

Thanks Jim.

I'll be busy with this tomorrow.

[Fastbob]

http://www.justanswer.com/mercury/5671n-mercury-grand-marquis-a-c-vent-does-not-blow-vents.html

Try this for a diagram of the vacuum system on these cars.

[Motorhead350]

Thanks man!

About to throw in the towel. :flamer: