Chrysler Repair: Air Vent stuck in same position, jeep grand cherokee, infinite ohms

Question
Roland,
Via internet search, found your recommendation of Jan 23, 2005, regarding a '95 Cirrus with stuck air vent.  I have a '99 Jeep Grand Cherokee that just developed same situation.  Vent control switch does nothing to move vent flow from windshield (unless you turn to off, at which time it kills the fan).  I suspect a fuse, but am unfamiliar with the "black box" fuses under the hood.  Are these easy to diagnose a blown fuse and find replacements?

Answer
Hi Michael,
I'm pleased that you checked the earlier messages and got a start on the problem (that is why I really want people to give a brief idea of what their problem is in the "subject" box). What I am uncertain about is exactly which way the Jeep vehicles handle the air distribution door movements. Up to the mid 90's Chrysler traditionally ran these by means of vacuum provided by a line that came from the intake manifold to the power brake unit and then a small line branched off from there thru the fire wall to the heater/a.c. unit. The vacuum distribution switch (labelled for various settings) then directed the vacuum to various mechanical transponders that moved the doors. Then I noticed in '96 that they changed over to an electrically operated system with motors on the doors as I described in my 1/23 response. So I can't be sure which way yours operates. But you could look for a vacuum line as I described, and finding none assume it is electrical.
Fuses are generally transparent and you can see the fuse wire and examine it for frank blowout but also look very closely because sometimes the fuse wire will separate very subtly due to repeated flexing when it is being heated and then cooled by the current that flows thru it. If you could buy a volt-ohm meter that is the best way to check a fuse. It should read near 0 ohms if the fuse is good, and infinite ohms if it is burned out or "open". Ideally the fuses will be labelled in the fuse box in the cabin as well as at the power distributtion center under the hood. But I would doubt it will say 'heater doors'. If there is a 'body computer' on the vehicle, then fuses labels might say that term, or 'heater/a.c.'.  Short of going to a library with a good collection of repair manuals, or buying such a manual from Chrysler or an aftermarket manual at an autoparts store, or a used Chrysler/Jeep manual from an eBay seller I can't give you reliable specific circuit information about your car. I have access to a library in the next town over from where I live that has such a collection so if you are unable to make progress I would be able to go look for the info and pass it on to you.
So let me know what you learn and if you are stumped.
Roland

P.S.  If it is a vacuum operated system then you should look for a loose hose fitting or a crack in the line between the power brake unit and the cabin's heater control unit. You might also hear a whistling sound if the break were in the cabin, either along the line or at the control unit. Sometimes when there is a leak, the air vent distribution direction will vary when you step on the accelerator which cause a loss of vacuum in the system, for example.