Heat/AC only blowing through defrost: running out of ideas!!

[DoctorHugocat]

I have a '00 Sport, and have been trying to trace the common problem of heat/AC only coming from the defrost vents. I spent a week reading every forum post and watching every video I could find, and I was optimistic; but now I’m running out of ideas and I’m getting frustrated! Can anyone offer advice please?

I’ve looked at:

• All the vacuum lines in the engine bay: there are a couple of rough spots, but no holes/breaks. T-pieces look fine, as well as the line into the firewall.
• Vacuum canister under the bumper: all well.
• Control panel: all seems fine behind the controls; no hissing when the motor’s running.
• Passenger lines/actuator: I removed the glove box and checked the actuator/lines behind; all in good shape, and the actuator holds vacuum. No vacuum detectable from the line when the engine’s running.
• The cruise works!

So I’m a bit stumped; I guess the next stage could be trying to get access to the driver’s side actuators to see if a line fell off or the door mechanism is broken? This is not easy, so if anyone can offer advice I’d love to hear!! Thanks for looking.

 

[75SV1]

'00 and '01 are the electric actuator. I can't remember if I had that exact problem or not. There is the blend door linkage. It goes bad. I should have replaced it when I had the HVAC box out. I think Dorman or Crown make an upgraded replacement. I used an aftermarket steel one. Let me see if I have a link for it.
OK:

Jeep Cherokee '97-'06

Failure of your heat and/or A/C in your Jeep Cherokee is due to the failure of your blend door. Heater Treater replaces the cheap, plastic blend door with steel. Replacing the blend door with the same plastic part from the manufacturer WILL result in continued problems — the same system design...

heatertreater.net

Don't know or remember if this would cure what you describe. Sounds vaguely familiar though. I did install and use this. It worked for me. A bit of slop in the linkage though. Not much. Kind of a pain cutting out the panel to install it. Also, being upside down.

 

[DoctorHugocat]

'00 and '01 are the electric actuator. I can't remember if I had that exact problem or not. There is the blend door linkage. It goes bad. I should have replaced it when I had the HVAC box out. I think Dorman or Crown make an upgraded replacement. I used an aftermarket steel one. Let me see if I have a link for it.
OK:

Jeep Cherokee '97-'06

Failure of your heat and/or A/C in your Jeep Cherokee is due to the failure of your blend door. Heater Treater replaces the cheap, plastic blend door with steel. Replacing the blend door with the same plastic part from the manufacturer WILL result in continued problems — the same system design...

heatertreater.net

Don't know or remember if this would cure what you describe. Sounds vaguely familiar though. I did install and use this. It worked for me. A bit of slop in the linkage though. Not much. Kind of a pain cutting out the panel to install it. Also, being upside down.

Thank you! I'll do some research on this. I do vaguely remember reading something about the company, or sayw a YT video. I'll go investigate. Much appreciated!!