No heater controls

[Jackrabbit41]

Ok this will be a long one. Tried doing all I could before I made a post. My heater is stuck on defrost. Cannot change it. I see the two lines coming out of firewall. I traced them down. One goes to a diaphragm on the coolant line. Looks fine. Connections seem fine as I can visually and feel. The other line goes to vaccum ball. Ball seems fine. And the rubber line has vaccum coming from intake. And I have vaccum by the relays at the connection where the firewall line is extended to the ball. What do I look for now? From what I can tell, I have vaccum coming from the ball, to the firewall. I do not have vaccum in the red vaccum line going to that coolant line diaphragm. Don’t know $#!& about Vaccum but doing my best. Thanks. It’s a 1987 with the renix 

[AZJeff]

The doors that direct flow inside the heater box are vacuum operated, as I suspect you know.   The default setting for those doors (when no vacuum in present) is the "DEFROST" mode.

 

The fact that you cannot select any mode but "defrost" suggests that you are not getting any vacuum to the heater control assembly, which is used to port the vacuum to the various vacuum servos to move the doors to the selected positions.

 

The first thing I would do is to put a vacuum gauge on the vacuum line that goes through the firewall to see if vacuum is actually getting side the cab of the MJ.  Do this with the engine running, and then also with the engine off.  If you are getting no vacuum, then you have a big leak on the firewall side of the vacuum system that needs to be uncovered and repaired.  (The vacuum line running under the battery box is a common source of leakage.)

 

If you ARE getting vacuum to that line with the above test, then the issue is inside the passenger compartment, somewhere between the entry through the firewall and the heater control assembly.    (Another way to check this is to put a manual hand-operated vacuum pump on the vacuum line entering the firewall, and pump to create a vacuum.  Then move the heater "mode" slider to various positions, and see if the air goes out the appropriate vents in the dash.)

[AnotherOldJeepGuy]

This is the one that came apart on mine multiple times resulting in defrost air only