I have an odd problem with my heat-a/c control, this is not the normal vacuum leak I encountered with my old jeep. All modes except the defrost/floor mode work. When I place the vent control on the floor/defrost my heat will only come out defrost, I can switch between all other vent modes and it works fine. I'm not sure where to start with this issue if anyone has any ideas or a fix I would greatly appreciate it.

 

I have this same problem after replacing my heater core. Mine also won't switch from heat to cold either

 

Research vacuum lines.

 

What's the problem. I don't understand. Also your year and if you have AC would help.

 

its a 99 limited. the problem is when i switch it to floor/defrost heat only comes out the defrost. All other settings work, if i switch it to just the floor it works fine

 

Ok I see, the defrost and floor work separately but not together. If the other settings all work then a vacuum issue isn't your problem.

 

thats what i figured, do you have any idea what to look for to fix this problem?

 

It could be either a issue with the control in the dash or the actuators. This is a total guess however.

 

The Defrost Door Actuator has two positions; one is full defrost and the other is Bi-Level. In the pic below you can see two vacuum tubes going to the Defrost Actuator. When you select Defrost vacuum is ported to the middle (Yellow) tube and the actuator moves the Defrost Door to the open position. When Bi-Level is selected, vacuum is ported to the bottom (Blue) position tube on the actuator and the Defrost Door moves to a mid position, and at the same time vacuum is ported to the Floor Actuator to open the Floor door.


My guess is that the Defrost Door actuator is defective or the Bi-Level vacuum tube is not connected (not a reasonable assumption).


Can you feel air coming out of the floor console rear vents when Bi-Level is selected? Sometimes air coming out of the front footwell vents is hard to feel.


The vacuum schematic below shows the vacuum routing.

 

I can feel very little (almost nothing) coming out of the floor on the bi-level. before it stopped working I could feel a lot more air movement. With it on the bi-level selection you can hear it whistling around the floor vent door.

 

I must ask: Is this an issue you're having that you can't live with?


The reason I ask is that if the Defrost Door Actuator is defective you'll have to pull the dash apart to get at the actuator.

 

I would like to fix it, it gets really cold up here in Vermont. Its the only setting that i really use as it clears the windshield and keeps my feet warm

 

Did you say air flow is acceptable out the front floor vents and at the rear of the floor console when Floor only is selected?

 

yes it has perfect air flow when the just the floor only is selected

 

To get a (limited) look at the defrost door actuator linkage you can remove the trim panel and knee blocker from under the steering column, and using a flashlight, look up to the right of the steering column and you can see the door actuator bell crank and arm where it attaches. See pic below.


You need vacuum in the system to do this:


While looking up there, operate the mode selector from any selection to Defrost. You should be able to see the actuator operate the bell crank, then select Bi-Level, the actuator lever should move part way.


Deplete all the vacuum in the system by making multiple selections until no more door movement is heard. At this point the Defrost Door actuator should have opened the Defrost Door.


If the Defrost Door actuator does not move midway, I would think the actuator is defective.


By looking at the pic below you can see to what extent the work would be to get at the actuator.