Defrost Only - Don't think its O-Rings

[khamilton1]

The other day I started having the A/C shift to defrost whenever I gave it throttle. Now it's full defrost all the time, I can't get it to adjust to anything else. Reading on here I decided to check the O-rings. The previous owner just got a Dorman reman EATC from Autozone before I bought the car. I still took that apart and found nice shiny orange silicone o-rings. Looks like Dorman rebuilt them correctly.

Now, I was reading on this forum trying to determine if there is a blend door associated with the defrost thing and I got a little confused as everything I read was about the heat/cool blend door. Since the #1 reason for this tends to be vacuum I decided to try a smoke test. I only found one small leak on the EGR housing (not sure if that would be the cause of the defrost only thing).

I looked under the dash and found what looks like a white vacuum hose just hanging out not plugged into anything. I have attached a picture of this hose. I doubt this is my issue as it just started happening about a week ago and this hose appears to be taped up.

Besides tearing into the dash, is there something else I should look into?

51901

[offroadkarter]

That vac hose you found is for the auto release E-brake that 300A's had. The later cars that lacked this feature just had that hose chilling.

I can't say I know the exact problem, I had an issue with my 03 where it was basically stuck on dash vents only and in order to get air to the feet, I had to select floor then shut all the dash vents. I'd found some rumblings on CVN of people who had the same problem and it was a door in the dash that was stuck (not the blend door which is for temp). You might have a similar issue, just spit balling ideas here.

[khamilton1]

Thanks for the help. I did notice just now that it was blowing floor and defrost when I had the fans on high. Hitting any other option on the EATC, i can hear a noise like something is attempting to happen. I did do an EATC self test and it returned all 8s so it thinks everything is good. The part that gets me is it started only going defrost when I hit the gas, letting off the gas it returned to dash vents. In my logic, this still points to vacuum related. I wonder if one of the lines going to the EATC has a leak in it....

[offroadkarter]

Thanks for the help. I did notice just now that it was blowing floor and defrost when I had the fans on high. Hitting any other option on the EATC, i can hear a noise like something is attempting to happen. I did do an EATC self test and it returned all 8s so it thinks everything is good. The part that gets me is it started only going defrost when I hit the gas, letting off the gas it returned to dash vents. In my logic, this still points to vacuum related. I wonder if one of the lines going to the EATC has a leak in it....

Sounds like your problem is certainly vacuum related still. I'd get a mityvac and test each line, I would imagine you could just pull the vacuum plug out of the eatc and test each individually from there.

[88LTDCV351]

I know someone told me, and it was the case for me also, but the O-rings can look good but not be. Just saying.

[khamilton1]

I may just order a set of the 007 o-rings just to make sure. I'll pick up a mityvac today and see if I can test those lines this weekend.

[ARodmarauder]

I had a similar situation happen to me on my 03 F150 only had defrost. Took it apart replaced the o rings and fixed the issue


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[fastblackmerc]

The other day I started having the A/C shift to defrost whenever I gave it throttle. Now it's full defrost all the time, I can't get it to adjust to anything else. Reading on here I decided to check the O-rings. The previous owner just got a Dorman reman EATC from Autozone before I bought the car. I still took that apart and found nice shiny orange silicone o-rings. Looks like Dorman rebuilt them correctly.

Now, I was reading on this forum trying to determine if there is a blend door associated with the defrost thing and I got a little confused as everything I read was about the heat/cool blend door. Since the #1 reason for this tends to be vacuum I decided to try a smoke test. I only found one small leak on the EGR housing (not sure if that would be the cause of the defrost only thing).

I looked under the dash and found what looks like a white vacuum hose just hanging out not plugged into anything. I have attached a picture of this hose. I doubt this is my issue as it just started happening about a week ago and this hose appears to be taped up.

Besides tearing into the dash, is there something else I should look into?

51901

See here:

https://www.mercurymarauder.net/forums/showthread.php?99619-FOUR-MOST-COMMON-MARAUDER-HVAC-PROBLEMS

BTW...

I can repair your EATC, included is an upgrade to the button and fan wheel lights to LEDs.

PM me for details.

[jgc61sr2002]

Mine had the same condition.
It was a vacuum hose made of white plastic running along the firewall in the engine compartment that was cracked.

[justbob]

If you have high boost levels you’ll blow off the white vacuum line under the dash. I had to superglue mine.


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[offroadkarter]

I feel like there should be a one way check valve installed to prevent that, my old BMW 745i which was factory turbocharged had such a thing in order to keep any positive pressure out of the entirely vacuum controlled HVAC system.

[khamilton1]

I think the Dorman unit is built slightly different. The 007 o-rings don't fit. They are quite a bit bigger than the opening it needs to slide into. I've attached a picture of the Dorman o-ring vs the 007 o-rings I got. The 007 o-ring is the blue one. The red Dorman o-ring fit snug. I'm going to hit up a junk yard and just get a replacement Ford unit and hope for the best. I did use the Mityvac and tested vacuum on each line. All of them held vacuum except the black one. I'm pretty sure the black one is the purge or whatever you want to call it, though.

51905

[fastblackmerc]

I think the Dorman unit is built slightly different. The 007 o-rings don't fit. They are quite a bit bigger than the opening it needs to slide into. I've attached a picture of the Dorman o-ring vs the 007 o-rings I got. The 007 o-ring is the blue one. The red Dorman o-ring fit snug. I'm going to hit up a junk yard and just get a replacement Ford unit and hope for the best. I did use the Mityvac and tested vacuum on each line. All of them held vacuum except the black one. I'm pretty sure the black one is the purge or whatever you want to call it, though.

51905
The blue o-ring is the wrong size.

The black line is the supply. There is no purge line.

The one you get from the junk yard one will probably need the same repair.

I can repair your EATC, included is an upgrade to the button and fan wheel lights to LEDs.

I have repaired a couple of hundreds of these.

PM me for details.

[khamilton1]

PM has been sent!

[fastblackmerc]

PM has been sent!

PM replied to!

[khamilton1]

So I got a EATC from a friend, that is confirmed working. With it installed, I still only have defrost.

I did find the vacuum feed line that runs up by the cowl and no leaks in it that I can see. I unhooked it from the coupler right near the suspension air pump and hooked the mityvac to the line going towards the interior. I was unable to hold a vacuum at all and heard a hiss from around where the line goes into the cowl. My question is, with the car off, should this line hold vacuum?

[fastblackmerc]

So I got a EATC from a friend, that is confirmed working. With it installed, I still only have defrost.

I did find the vacuum feed line that runs up by the cowl and no leaks in it that I can see. I unhooked it from the coupler right near the suspension air pump and hooked the mityvac to the line going towards the interior. I was unable to hold a vacuum at all and heard a hiss from around where the line goes into the cowl. My question is, with the car off, should this line hold vacuum?

You have a vaccum leak. The Ford HVAC system is a "fail-safe" system. A no vaccum condition results in heat and defrost so the windshield stays clear.

Check the vaccum tree behind the engine, mounted to the firewall and below the cowl.

Inside, there should be vaccum at the black line going into the EATC.

Remember these cars are now over 18 years old. With the hot and cold temps under the hood the vaccum lines will start to fail.

[khamilton1]

Welp. After taking apart the entire vacuum tree at the firewall and testing the check valve in the A/C line, line that goes to the driver side vacuum reservoir, and the black line that goes to the EATC, I confirmed no vacuum leaks. I put it all back together and still defrost only. I decided to unhook the vacuum lines from the back of the EATC and crank it up. I hooked a vacuum gauge to the black line and confirmed it was pulling 20-25... uh... units when the car was running. I jumped this to the blue vacuum line and VOILA, I have air blowing out the dash vents and not defrost. Thank god I don't have to pull the dash for anything.

and now for the part that i feel dumb for. I hooked it back up to the test EATC i borrowed that i know works.... and everything is good now. I have full vent control again!

So, this confirms the EATC I had in the car has bad o-rings (surprise, surprise) and I have no idea why it didn't work the first time when I used the test EATC from another car. Unless the check valve was stuck and my vacuum testing on it knocked it loose. Who knows.

Thanks to everyone that assisted. I'm not a great mechanic, but I'll try anything once lol.

[Turbov6Bryan]

I’ve had some not work once replaced. Not sure why. Then swapping to another it too doesn’t work. Giving it a few minutes plugged in key off… then works forever.