Vent selector for heater

I have an 88 XJ and I'm having some difficulty with the heater controls. The slider that controls the vents doesn't seem to be working properly. I only get air blowing out of the front defroster vent no matter what I have the slider set on.

If I switch the slider to OFF it will shut the fan off but all the remaining selections just blow air through the defroster.

Any ideas?

I may be way off, but my 98 will blow through the defrost vent under heavy load. I've been told it is due to the vacumn going low and the vent switching to the default position which is the defrost vent.

I have heard it is due to a vacumn leak, and I have heard it is due to worn engine parts causing the vacumn to go low under heavy load.

Take a look at where the slider cable attaches to the flap under the glove box. There is a press on, round locking clip that sometimes falls off. The anchor for the cable also sometimes comes loose.
If they are in good shape you will probably have to go on a vacuum leak hunt. The vacuum canister behind the front bumper (passengers side) is a good place to look, the mounts for the canister break and the hoses come loose. Also the main vacuum hose from the intake to the canister has a tendency to rot under or near the battery.
I've had all three go wrong (the canister, the vacuum hoses and the slider cable) at one time or the other, it may be one or maybe a combination of faults.

No vent selection at all means there's no vacuum going to the control panel. If your vents are defaulting to defrost on hard acceleration there is a break in the line going to the plastic vacuum canister.

The only thing controlled by a cable is the blend door which closes off outside air (the blue/red sliding bar below the different vent positions)

My vents are on defrost even at idle........the jeep is a project and it isn't registered and hasn't been out of the driveway since I got it.

Still sounds like it could be caused by a bad vac. line though.

I had the exact same problem on my '89. I had recently pulled the front bumper off and so had to disconnect the vac-lines to the cannister. I must not have re-connected them well enough because the larger line was disconnected when i was trouble shooting my vent issue.

you have a vac leak somewhere. search, its well covered.

a mouse chewed the vac lines under my dash somewhere. also was nice enough to pack my blower full of crap so it went bad