1997 jeep grand cherokee--no defrost

the damper or flapper ibside the heater box is not moving. If they are vaccum actuated could be lack of vaccum or bad diafram in the actuator...replace

sounds like the blend door motro may be bad, this motor is screwed to heater box, there are a few of these motors one for pass/side for hot/cold air then one to control position of air like defrost or vent or floor, then the driverside cold/hot air, i think this motor is on driverside part of heater box, to make sure locate each motor while moving all controls until you spot the one that doesnt move, then remove it and move door manualy to see if you get heat, then replace that motor, hope this helps,

bad switch the cheapest solution or the blend door is not responding to the switch you can locate the blend door under the dash

Sounds like you need a blower door acuator motor, which moves to direct the heat to where you want it to be

The HVAC blend door is stuck common problem in jeeps.

passenger side lower firewall ,look for rubber hose with a split across the end of it ,squeeze it to get the muck out of it

There is a vacuum leak under the hood somewhere. Check the vacuum at the speed control servo. If there is no vacuum on the line with the engine running, follow the line to find the leak. The vacuum reservoir is mounted under the battery tray. I would look in that area after confirming no vacuum at the servo.

Check the control cable that operates the diverter door in the heater box at both ends, to make sure that it operates the door when you move the control lever.

Its a vaccumm line (and possibly routing switch (the rotary one)) problem.

Sounds like you have a vacuum leak, those controls are are accuated by vacuum.