I have a 99 xj classic with an issue(s) as to which vents are moving air. The air conditioning and heat work fine, but if I want to have it blowing out of the front with any force, I have to switch it to defrost. The it blasts out of the two middle front vents just fine. I have been putting off trying to repair that because it works fine like that. But winter is coming and I cannot get anything from the defrost vents.
I don't want to just tear into things blindly so I hope someone here may have an idea as to where I should start looking.

First thing to check is the vacuum line that is attached to the intake manifold. Make sure it is attached and not cracked

The switch does change, I can hear it change. when I change selections, so I "assume" the vacuum line is doing it's thing, but somewhere in there things are not switching to the correct vent. I have mostly read about things sticking in defrost only, but have not seen anything about any module that prevents any defrost airflow. The temperature **** works fine to adjust cool to hot.

There is a little electric valve under the glove box (close to the center console) that controls where the air comes out. Its pretty easy to get to from under. Pop it off, turn the key on and see if it moves when you select different things.

Thank you.

I have a similar issue. The vent your referring to is behind your stereo head unit. It switches flow from face to defrost. If you pull the stereo you’ll be able to get a visual on the actuator. Where the foam strip is behind your stereo there is a gap you can get your fingers thru to actually feel if the actuator is moving the door at all. My actuator works fine but will not operate the door. I can manipulate the vent with my hand to get full defrost. The fan now moves the door around on its own. Fortunately I can get some defrost on bi-level up to fan speed 2. Anything more than fan speed 2, the door slams shut and all air flow goes to side window defrost. Not a problem in the summer though. I’ve had the issue for a few months, but I looked into it about a week ago and found the problem. I need mine working for winter also.

i just thought of something else. In VA your defrost has to work to pass state inspection. Just letting you know so you don’t get caught off guard.

I just bought a '99 XJ today and have the same problem, you can hear it try to change, but no airflow out of the defrost. You have any luck?