Stopped by the local napa and bought 12ft worth of vacuum hose so I can get started on replacing each line. Idk if it will help any but it only seems to switch to defrost when the tranny is in overdrive. Everytime it switches I can hit the od button off and then ac blows out vents perfectly. I'm assuming that there is little or no engine vacuum in overdrive? I'm about to be fed up with this vacuum system. Why didn't they just go with electric. You'd figure a break in a wire would be much easier than vacuum leaks. Given the information, would the leak be obvious or could it even possibly just be the smallest pinhole that is causing this?

 

You've tapped me out on ideas but did you fix what may be a leak that you mentioned in the first post?

 

The hole is in a soft rubbery-type hose in the Crankshaft Air Intake. There is no vacuum on that hose whatsoever so that is not my problem.

I think my problem lies with yet another bad/wrong check valve. I bought a universal check valve from autozone - Search: check valve | Check Valve. Universal Type | AutoZone.com in which replaced a check valve identical to this one - 1986-1991 Ford Aerostar Vacuum Check Valve - 4-Seasons.

Sorry that the links take up alot of space, but with the universal check valve, I can blow through both sides. If you blow through the bottom, it should instantaneously be closed off, but its not. If I plug one of the 2 smaller nipples on the other side of the check valve and tap the other nipple with my finger, I can hear the diaphram plop. It should be to where I cannot blow through it at all whether I have my finger plugging the other holes or not.

Same goes with if I suck on one of the top 2 nipples with the other plugged, it should be instantly closed off. As stated previously, I can suck through it until I tap a finger on the opposite side of the valve to hear it "plop" closed.

So I think I am gonna head to the salvage yard tomorrow morning and pick up a few of these and try them out and see how well it goes. The universal check valve that is on there now obviously has a different size "valve space" I like to call it to where it might only work on systems that demand less/more vacuum to close off the valve.

 

Looks like the check-valve that I picked up from the salvage yard fixed my issue. Great to finally feel the A/C through the vents consistently.

 

Blue side goes to intake manifold. There is a second check valve inside vehicle behind the ac control box. This is on a 2x4 not 4x4. Was this the check valve issue?

 

Had similar situation. Tested all vacuum lines/connections etc. Made a homemade smoke machine. In the end, the damper door that "switches" the air movement (the one dead center of dash) was failing, and breaking away (dry-rotted plastic) from the rod that turns it , powered by the vacuum actuator, (which worked fine). Instead of removing the entire dash to get to that damper door, I removed the center bezel over the radio and vents, reached down the duct work, crunched and crumbled the remains of the damper door with my hand and removed it. Now, to get more DEFROST, I'll have to close off my panel vents with the dials.