Here's a strange one for you...
my 1992 3.0 shorty's heater/air conditioning controls are doing some realy wierd things. Periodically when I have the max A/C on the air stops coming out of the vents for about 10 seconds then returns even though the fan motor continues to run. Another time, the air somehow got redirected to the defrost vents even though the control **** remained on max A/C. On that occasion, I turned the **** to defrost then back to max A/C and the air returned to the dash vents where it belonged. Sorry to go on and on, but theres more, on a few occasions, with the A/C on, I got a very brief blast of warm air (warmer than ambient) out of the vents. And finally, the air works better at idle than at highway speeds.
I think the idle / highway speed diffrence may be related to low refrigerant charge, but the rest of the issues have me baffled.
Any ideas?

 

Sure. The air is directed by a vacuum motor. When the vacuum drops (like under acceleration) the air goes to the defrost. I had a problem driving in the mountains with my old 91 3.0. Going up hill the air would divert to the defrost and the rear would drop to the floor and put out hot air. I do mean hot. Just like having the heat on. Basically it was in heat mode for the back.

If this happens under normal load you may want to look for a vacuum leak.

Also there should be a WOT switch that will stop the A/C compressor. When you floor it, the computer will shut down the A/C to give more power to the drive train for things like passing. What were you doing when you got the warm air?

 

I cant say for sure what the throttle position was when the warm air came out, but the or shutting off wouldnt cause "instant" warm air would it? I will check the vacuum line for leaks when I have a chance... Do you think poor vacuum could cause A/C performance at highway speed?

 

Most likely, that vacuum motor is toast. Try to apply vacuum to the motor and see if you have a leak. When mine acted that way, I usually have a busted vacuum diaphgram.

 

Where is the vacuum motor located?

 

Look hard at the vacuum check valve. I don't think I have a pic of where it is . . . well, I guess I can go take a shot . . .




Check to make sure that air only flows one way through this valve: pull it off and blow through it. Air should not flow freely in one direction, but should flow somewhat in the other.

If that valve leaks (or, if you have any significant leak in the HVAC vacuum system), you'll get the "air blows out of defroster on accel" symptom.

Regards,
Al S.

 

gadzooks!! those are great pictures guys!!!, my 2 cents to add is that my 91 had similiar problems till i opened up the dash and traced the vacume hoses from the control levers on the drivers control pod, over toward the passenger side. one of those thin little tubes (vacume lines) came off at an attachment to a small canister looking things almost right behind the radio. all cool after that rick

99 windstar aka "son of bean"
previous 91 aerostar aka "green bean"

 

Thanks to everyone, the problem is fixed... Cracked vacuum line right at the check valve.

 

Sorry folks, I thought we had put this one to bed, but there are still issues...
I suspect the vacuum check valve is no good, seems when I blow through it with abrupt force it works, but if I blow more lightly, it has some blow by.
the diaphram must be getting stiff??
So off to the local Ford dealer I go to buy a new one and when I heard the price, they had to pick me up off the floor!
Can you believe over 100 bucks CDN?
Short of a used one at the scrap yard, does anyone know where to buy these in the aftermarket?

 

Aerocook:

When you go to your Auto Parts store look on the wall where they have odds and ends pieces like for little things that is lost or wear out on the cars. If you don't see one there then look look at the tools and look for a Brake Bleeding tool. It's a clear piece of tubing with a vacuum check valve. It looks just like the one on your aerostar and will work. The tool is under $10.00...........

Jay

 

Thanks for the tip Jay.

 

Towing a U-Haul 6x12' enclosed trailer from the Seattle area to Denver last week (trailer only 1/3 full, but I'm sure it was still loaded somewhere north of 2500 lbs!) with my '97 4.0l AWD fully loaded, I was also having the divert-to-defrost on all hills. The Mity-Vac says I have no appreciable leakage in the HVAC system (at least, in Vent mode, which is where I was having the symptom), so I'm suspecting my check valve. Blow/suck seems OK, but the response is slow and I'm thinking that I am pushing more air with my "mouth test" than normally occurs in normal operation. I'm gonna go fetch a replacement today.

Meanwhile, as the location of the check valve is similar but not the same as the pics of my '89 I posted above in 2005, here are a couple pics of pics of the later/4.0l location.

(click on any image for larger)

 

I have the same issue, the check valve works if there is a sudden pressure change, or if you try to blow, but if the change is more subtle, then the valve fails to work correctly.

 

Hmmm, it seems that I never followed up on this thread.

Replacing the check valve shown above did fix the problem.

 

FYI

Dorman does make a universal check valve.

Dorman® Help!™ 47149 - Vacuum Check Valve | O'Reilly Auto Parts