Diff gen, but if it's similar to '97... 3/32 ID rubber tubing will help you in diagnosing, it fits over the nipples on the connector and inside the female side. On mine, behind the glove box there are the two lines black and white, there's a connector, you can get to it with no problem (or flip glove box down). Remove connector inside, attach tubing to the nipple for the black (or grey), suck on it (or use vacuum tool if you have one). Does it hold vacuum? Likewise test the white line (going to the heater valve), plug it inside (put a tube and put a golf t in it) and suck on it at the heater valve, it should also hold vacuum. On mine, the latter is leaking,
I can't find the part for it (but for yours you can). "vacuum harness".
1998-2011 Ford Ranger Vacuum Harness F87Z-19C828-DA | TascaParts.com
Disconnect line from heater valve and reservoir, pull through from inside cab, replace. If leak is there, which it probably is. At the price, I'd do that (if leak is in one of those lines) versus trying to mess with it.
I don't know how different your gen is from mine, but the first things is find where is vacuum leak, assuming it's in one of the lines. Obviously there is more to it all, but there's no reason to suspect leak in other lines or in the selector or the lines to the vent actuators or the actuators ("vacuum solenoid") themselves.
Once you have chased this and you think vacuum is ok (no leaks), run the engine, select "Off" (you should see the recirculate door be actuated, it's right there behind the glove box). Turn the engine off. Watch the door - it should hold for awhile (because vacuum is in the reservoir) - maybe 45 seconds - before it starts closing. And of course the other test is, does it work lol.
What I did for temporary fix is I attached black-black (or grey-grey) at the firewall with a short stub of tubing. Then I put a golf t in the white side of the connector. Now all is good, you do want the recirculate feature, it blows cold as ice. Of course now my heater valve doesn't shut like it's supposed to on Max or Off, but I'm kind of stuck absent good part to install. That would result in a bit colder air, but it doesn't hurt anything, pending real fix. I'm tempted to order the '98-up line, but I well might be throwing away the money. Other option is junkyard I guess. It'd be easy enough to test a j-yard line and see if it's good.
There's really not very much in terms of lines on the engine side, just, you can't get to part of it because of where it runs. Another option for me would be take out existing then try to find leak/splice in good tubing.
@mikkelstuff, you should be able to disconnect lines on engine side; look under glove box, there's a rubber grommet, the 2 lines/grommet are an assembly, you don't have to mess with it from the engine side, just pull through into the cab, likewise insert the new ones, maybe tape the ends to not get crap in them. When you push enough through you'll be able to grab them from the engine side.
Don't know if any of that helps but both poster's issues should be straightforward (if it's in the lines discussed, which it probably is).
As you know no vacuum always defaults to defrost. Mine was fine except would default only on Max a/c which knowing what I know now, would point me to the heater valve line since that's the only time it's activated (except for Off, and then you have no vents working at all).
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