1994 Jeep Cherokee Vents don't blow cold or hot

The mode selection is done through a series of air doors that are operated by vacuum. When there is either a partial or total loss of vacuum, the system will default to either defrost or on some vehicles, heat. Acceleration can also have an effect on the doors for the same reason.

Look under the hood for a small black plastic hose coming through the firewall. Follow it to the vacuum source looking for any breaks, cracks or leaks. Also follow any other lines that branch off because they may go to a vacuum reservoir (storage tank) and a leak there will cause the same problem. Using a vacuum gauge to determine if you have full vacuum at any given point is the best way to find it. You can cut into the line anywhere and splice it back together with a piece of vacuum hose. Find the broken line, you found your problem.

Given the history of this, you may want to look for pinched or disconnected hoses under the dash.

We have taken the dash off and checked every hose and flapper plus the vacunm hose to no avail could it be the control box? When we have dash off u CAN feel it on the defrost and at u're feet thanks Linnea

What is the vacuum reading on the supply hose?

I'll have to get a gage can I get one at Auto Zone etc thank u very much

Yes, most any parts store will have them. Some rent or loan them.