This is for the E-series Vans that have this problem of the A/C blowers in the dash defaulting to the defrosters while accelerating, then coming back while going back to normal speed. There is not a lot of information as to what causes this or how to fix it. This is what I did after a lot of research and asking questions. I don't know when Ford did this. But they buried a vacuum tank and a small check valve that controls or feeds the heater/A/C control unit. My van is a 1997. The problem is from the mid 90's to around 2003 or 2004. The check valve and the vacuum tank location is between the inside heater core case and the evaporator case which is under the hood on the right side of the vehicle. According to Ford to replace these two units, you must remove both cases from the vehicle. To remove the inside heater core case and box unit, you have to remove the dash. The evaporator case can be removed after you discharge the A/C, the battery and battery tray and just about everything around the A/C evaporator case. Ford dealers gets about 15 to 17 hours labor to replace the HVAC check valve and the vacuum tank. Crazy, right? !!!!!. The other fix is bypass that whole system with another external check valve and vacuum tank. The problem here is getting the control vacuum lines up to the dash control unit and finding a place to put the vacuum tank
This was my fix after looking at my options. First I popped the cover on top of the dash which uncovered some of the HVAC vacuum lines. The black vacuum line at this location is the one that feeds the HVAC control head in the dash. I disconnected the line and capped it. I then went under the hood and found the black line that gets its vacuum from the engine. I disconnected that line and connected it to a hand held vacuum pump. I gave it a few pumps and could only draw a little vacuum that quickly disappeared. Vacuum Leak !!!??. I removed the lower right side dash panel and the steel reinforcement behind it to get a better look at the heater box that would have to come out if you were to follow Ford procedures. I thought to myself, there's no way I'm going to remove the dash to get this heater box out. Looking at the lower far right corner of the heater box you can see the black vacuum line going thru the firewall. But wait, it goes thru the plastic heater box case.
I then went under the hood to see if I could find the other end of this line coming into the engine compartment. At this time I took the Battery, the battery tray, the blower fan and the front right headlight unit out. To get the blower out, you have to remove the dryer/receiver bolts and pull the unit toward the front as far as possible. I then pulled away the wire harness from the evaporator case as much as possible. I took a mirror with a bright light and tried to get a look at under the evap case. Trying real hard you can see the check valve at this point, but there is no way you can get to it.
Here's the fix. Taking some careful measurement from a given point under the hood on top of the fender well and under the heater. Take the right front tire off and transfer those measurement to the underside of the wheel well. This will put you just about at the 12 O'clock position. I measured out a 5" long (front to back) and a 3" side to side rectangle. I took my wizzor and cut the panel out. Make sure you cut just as deep as to cut thru. You don't want to cut anything on the other side of the cut. Remove the panel and there they are !!!!! the check valve and the flat vacuum tank. The check valve is just push rubber connectors, but the vacuum tank is held in by a small screw.. I checked my vacuum tank and it held vacuum, so I didn't have to take it out. I checked the check valve and it didn't work. I took the old one to Autozone and matched it up with a Dorman check valve for $3.98. I connected the proper vacuum lines to the check valve and installed it back in it's hole.
I have duel batteries, so I switched over to battery 2 and started the vehicle. It worked as it should.
OK !! Now put the panel back in. can't weld it, too much heat. Take the little panel you cut out and weld two small flat flanges on the short sides. Drill some holes in the two flanges. Clean, prime and paint the flange, so it doesn't rust. Put a bead of body panel poly seal around the flange and the hole in the inner fender well. Then stick the panel up into place and using short self tapping screws, screw the panel in place. I then put another thin coating of body seal around the edge and smoothed it out with my finger tip. Final step was 3 or 4 coats of undercoating
Then I put everything back together. If for any reason I have problems with this again, I have a panel that I can open up and gain access. I hope this helps anyone that might have this problem. My 2 cents !! Good Luck