A/C BLOWING THRU DEFROSTER UNDER LOAD

[Firedawgy]

OK here's a weird one for you Explorer fanatics. I have a 97 EB with the computerized climate control. When the A/C is on, as it usually is in South Carolina, and I put the engine under load, accelerating or whatever, the A/C cuts off from the vents and blows out the defroster until I let off the gas. Any ideas about whats going on here or what to do to fix it? Thanks for the great inputs in the past

 

[mrboyle]

It sounds like there is a problem with loosing vacuum to the blend air door under acceleration. I'm not sure where to look, but that sounds like what is happening.

 

[Adam Berger]

I think your right. All new cars have an inbuilt safety feature of defaulting to the demister when there is loss of vacuum to the A/C controls. This is to make sure you can always see out of the windscreen. And since an engine doesn't produce vacuum under load, i'd say you have a leak.

 

[kandtholman]

Your vehicle is supposed to go to defrost mode when under hard acceleration or a heavy load. Every vehicle I have ever driven that was made after 94 does that.
Kurt

 

[addkev]

Even older cars did, that always runs off of vacuum. Under hard acceleration you will always encounter it.

 

[kandtholman]

That is what I thought but I could not remember if my older cars did that or not.
Kurt

 

[Robert]

There is something wrong. My Wife's '93 Mercury Sable does not do it, my '94 Ranger, '95 T-bird, '94 Caravan, '95 Astro and '97 Caravan did not do it, and my current '97 Explorer does not do it. I wouldn't say that every car built since '94 does it. It might default to defrost mode when it looses vacuum, but by default it shoudn't loose enough vacuum to cause it to go to defrost mode under heavy acceleration unless there is a problem. When I floor my '97 Sport with the A/C on, it continues to blow out the vents as has every car I have ever driven with the exception of one that did have a known vacuum problem.

 

[kandtholman]

My 95TBird did it, my 86 Cougar did it, my 85 300ZX did it, my wife 96 Cirrus did it, my 92 X does it, my moms 94 sable did it, my moms 96 accord does it, and my dads 99 J**p limited does it. The only ones I cant remember is if my 82 S-10, 82 Jimmy Full Size, and both my 89 Honda civics did it.
Kurt

 

[Firedawgy]

Well I found it. I turned down the stereo, you know how loud kids listen to it, and heard the vacuum leak under the passenger side of the dash. It was just a line that slipped off. Now its blowing cold and hard. Thanks everybody for your input, you came thu for me again.

 

[Robert]

kandtholman - Maybe all of the cars you have ridden in had a similar vacuum leak as Firedawgy found? The only vehicles we had in common was the '95 T-bird which mine did not have that problem. Having just looked at the '97 Explorer A/C drawing, the defroster/vent blend door is controlled by a solenoid that enables/disables vacuum to the door. When no vacuum is being applied the door defaults to the defroster mode. If everything is hooked up properly, putting the engine under load will not affect the vacuum to the solenoid. If there is a leak behind the dash, then the engine load can cause the vacuum to bleed off the solenoid which will close the door. Once the load is diminished, enough vacuum gets applied to the solenoid to open the door back up.

 

[Robert]

Just checked the '93 Sable A/C vacuum distribution. It is almost identical to the Explorer except that it uses a seperate vacuum tank for reserve vacuum after the check valve. The Explorer uses a combination check/valve-vacuum tank. Both systems are completely controlled by one vacuum line from the vacuum distribution manifold. The line from the vacuum distribution manifold goes through a check valve to maintain vacuum in the A/C system. If there is a leak between the check valve and any of the three vacuum motors, the motor will release the door that it is controlling when the engine is under load. If there isn't a leak in the system, the check valve will maintain the vacuum in the system which will keep the door in their selected position. A faulty check valve will also cause this problem except that all doors will go to their default position when the engine is under load. When you put an engine under load, the vacuum decreases. If the A/C vacuum system has no leaks then the doors will not be affected by engine load. At least that is the way it is on a '93 Sable and '97 Explorer.

 

[kandtholman]

I am listening to what you are saying Robert and I understand. Here is the thing, it is an automatic thing that cars and trucks do for a short time. We are talking like a few seconds. Now if the system stays blowing in defrost mode then yes there is a vacuum leak somewhere. I know this because my 300zx would only blow out the defrost. I had to replace the entire vacuum system so I have become familiar with the most pain in the butt system out there. After I redid the entire system it blew fine but would still default to defrost mode under acceleration and load fo ra few seconds and then stop. These posts are putting a lot of usefull information on vacuum systems.
Kurt

 

[ChrisCam02]

Robert- Its funny that your '94 caravan doesn't have the A/C-defrost bug... We have a '92 caravan that blows A/C from the defrost even under normal accel... It's done this since new.- Then again its got a whimpy 2 liter 4 cyl... Which in my opinion is too small for a van. We haul a 25 foot sailboat with it, and I'm telling you- it struggles. I also have an '89 Saab 9000 Turbo w/ Auto climate and a 98 Saab 900S with manual climate control... neither do that. Now- more on topic- my X's A/C doesn't work at all :-(... Its been pretty hot here in maine lately- I'd like for it to work. I think I'm going to try bridging the pressure switch and test the compressor harness for power- as suggested in other posts.

Do you ever notice how people from maine will tell you that they're "telling you"??? "Its cold out there today- I'm telling you!" -Bob Marley

-Chris

 

[Robert]

The 4-cylinder probably can't maintain enough vacuum to keep the A/C vent solenoid closed. When you get on the gas, the vacuum decreases to the lesser items (such as A/C). If the fail safe mode is the same as it is on Ford's it will divert the air to the defrost vents when the vacuum is lost. My '92 had a 3.0L and my '94 had a 3.3L and neither of them had the problem.

 

[unabonked]

Ok I got another one for ya. My 98 (auto) never blew through the vent, but my mom's (96) does when going up hills. The thing is, I'm not accelerating hard. Infact the RPM range stayed the same.

I was doing 70-80 in 5th gear. I tried keeping the RPM steady up a hill (resulting in slowing) It blew through the vent up the entire hill. I tried keeping the speed steady (increasing RPM) and it did the same thing. Now the AC needs to be charged (all i get it outside temp air), but It does seem to be a problem since my 98 never did that no matter how hard i accelerated.

 

[Robert]

It sounds like you have a vacuum leak somewhere in the system. Yours might me a little more severe since it doesn't take as much for yours to blow through the defrost vents.