Heater control not working

I have searched and cannot find anything. Maybe I am not using the right terms. But I have a 90 cherokee and the heater control unit on the dash is not changing the vents. I am trying to charge the AC and it will not engage anything. It is stuck in the defroster position no mater where I put the selector. Any ideas?

Thanks

If the vents are not changing and it is always blowing out the defroster, you have a vacuum leak somewhere. The A/C, however, is electrically actuated, and should run regardless of the vent setting. The low pressure switch will prevent it from running if there is low/no charge, however, so you will have to jump the switch to get the compressor to run in order to fill it.

If the system was completely discharged, it's a good idea to have a shop pull a vacuum on the system before it is filled, in order to remove any water from the system. Also, you will want to replce the receiver/drier.

so no matter where you put the vent selector it blows defroster?
Sounds to me like you've got a vaccum leak, the whole thing is vacuum operated.
Check the vacuum accumulator, its in the front bumper, on the passenger corner in front of the fender. Then trace your vaccum hoses from there. A popular place for it to crack is under the battery.

I will bet thats it. I changed out the front bumper becouse I bought the jeep aftger a wreck. That might have been when it stopped working, after I chaged the bumper. I am not driving it yet becouse it is getting new axles and a lift. I will check tomorrow night and post an update.
Thanks for the fast response, I love this Forum.

OK, I have traced the Vacuum line that comes out of the firewall to the vacuum canister and replaced it due to some cracks. NO CHANGE!!!!!!
Replaced 2 other vacuum lines that I think go to the 4x4 disconnect axle since that didnt work either, NO CHANGE!!!

I removed a cover under the dash and saw a diaphram and moved that, Now that made it change from defrost to floor. There is a vacuum line coming out of that but it goes up into the dash and I cant fint it.

I am wondering if something might be broke from somewhere in between the firewall and the unit where it is controlled?

Any more Ideas?????

it's possible. You're either looking at a vaccum leak or the vacuum motor has died. THe vaccum motor doesn't fail that often, but it is possible.

Check with a guage to see if you've got vacuum going into the dash from the line under the bumper. If you do, you're gonna have to take the dash apart and find the broken line.

might be a stupid question but, what is a vacuum tester and where can I get one?
Also, does anyone know if that vacuum connector by the firewall that goes to the front axle can be removed after I install my posi lock cable? And if it can, should I change the vacuum bubble under the bumper to a smaller one?

Autozone probably your best bet. Should be $30-35. Make sure you get the one with the gauge.