Novak Conversions Jeep Wrangler TJ engine mounts

HVAC stopped blowing around 55mph

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Daughter reported a new issue on her 97 where the A/C was blowing kinda warm (I think this is an a/c charge issue) but when she got up to around 55 it stopped blowing altogether. And then if she slowed down it started blowing again.

Other known issues include the fact that it only wants to blow on her feet or defrost; never at the "face vents."

Any ideas? It being speed dependent tells me that something along the lines of forced air into the cowl is pushing one of the vent doors shut? But I'm pretty ignorant in this area. More ignorant than usual, even.
 
Most likely a bad blend door i.e the linakage being broken. Before anything id check the small vacuum lines 1st in the engine bay and closely inspect for any cracks on the lines and rubber elbows etc. if anything take out the glovebox and you'll see a plastic manifold with varied colored vacuum lines and check if they're fully inserted ,those go to the selector,to the plastic manifold out to blend doors and what not!
I've had that problem before and it turns out that 1st the elbow on the intake manifold was old, 2nd the line was cracked right before it goes in the firewall !
 
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The defrost/feet thing I figured was vacuum, having fixed a similar issue on my 02 with new lines in the engine bay and new check valves. I'm pretty sure I checked that stuff when we got it and I found it to be ok. But I haven't looked at the lines behind the dash or the actuators. I'm not even sure what I'm looking for with the linkages/actuators.
 
There's a bundle of colored vacuum lines on the back of the control unit to check, also the vacuum reservoir under the battery and the two check valves, one by the firewall and one at the intake manifold. You can check a lot of the lines and the reservoir with a mighty vac, and the check valves by just making sure they only pass air in one direction. Also a switch terminal on the control unit touches one of the colored vacuum lines and that terminal likes to get really hot when the blower is on high, mine melted a hole in the yellow vacuum line.
 
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BTW to check the control vacuum lines you don't need to remove the control unit, just pull the supply by the fire wall and pull a vacuum and see if it holds. Do it at each setting of the mode switch. It will leak down by design between modes as you rotate the switch but should not leak when firmly clicked into a mode.

Do this for the vacuum reservoir too so you don't have to remove the battery tray to get to it. (You can get to the line from underneath the Jeep, you'll see it going under the battery tray)
 
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B1Toad said:
BTW to check the control vacuum lines you don't need to remove the control unit, just pull the supply by the fire wall and pull a vacuum and see if it holds. Do it at each setting of the mode switch. It will leak down by design between modes as you rotate the switch but should not leak when firmly clicked into a mode.

Do this for the vacuum reservoir too so you don't have to remove the battery tray to get to it. (You can get to the line from underneath the Jeep, you'll see it going under the battery tray)
Click to expand...

Yep,a hand held mityvac should tell the story.

It kinda reminds me of my old 66 rambler with vacuum wipers. Once at speed on hills in heavy rain i needed to choose between the throttle or wipers,lol. The ol 232 didn't have the manifold vacuum once the pedal was down
 
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Rickyd said:
Yep,a hand held mityvac should tell the story.

It kinda reminds me of my old 66 rambler with vacuum wipers. Once at speed on hills in heavy rain i needed to choose between the throttle or wipers,lol. The ol 232 didn't have the manifold vacuum once the pedal was down
Click to expand...

I remember those vacuum wipers! My brother's 64ish? Rambler with the clothes iron looking tail lights had them
 
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Any vacuum leak anywhere can cause this. My 99 did it and had a crack in the line just inches off the intake manifold.
 
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hear said:
Daughter reported a new issue on her 97 where the A/C was blowing kinda warm (I think this is an a/c charge issue) but when she got up to around 55 it stopped blowing altogether. And then if she slowed down it started blowing again.

Other known issues include the fact that it only wants to blow on her feet or defrost; never at the "face vents."

Any ideas? It being speed dependent tells me that something along the lines of forced air into the cowl is pushing one of the vent doors shut? But I'm pretty ignorant in this area. More ignorant than usual, even.
Click to expand...

Check the vacuum hoses, check valve and vacuum reservoir in the engine compartment, the check valve should pass air one way and not the other. The plastic vacuum lines crack and break. The reservoir in under the battery tray you can access it from under the vehicle. Check for damage and one hose should be connected to it.
 
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hear said:
Right, which is why that she has it blowing at the feet has me a little surprised.
Click to expand...

My 97 has a hard time switching the HVAC output but will instantly snap to feet, it seems to want to default there as well.

I'm gonna be trying some of these suggestions out myself, seems like a fairly easy fix to an annoying problem.
 
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Looks like on our Jeeps, according to the icon which points at the floor and defrost at the same time, defrost includes the floor so that's probably the default.

jeep defrost.jpg
 
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lBasket said:
My 97 has a hard time switching the HVAC output but will instantly snap to feet, it seems to want to default there as well.

I'm gonna be trying some of these suggestions out myself, seems like a fairly easy fix to an annoying problem.
Click to expand...

Does the switch move freely? Does it seem to go to the right position when you are decelerating, you may have a small leak that is over come by high vacuum. Also check the door actuators and see if the doors move freely. The one actuator is a dual diaphragm which can act up. They seem to leak more often from my experience. Here is screen shots for what to check.
IMG_0682.png
IMG_0681.jpeg
I use a hand vacuum pump the test the doors. I remove the control head and apply vacuum to each actuator and watch it operate and check for binding. If you don’t have a hand pump you can use a vacuum hose with a vacuum connector on each side and jump from the supply hose (black hose) to each actuator.
 
Hope to get to this soon. But also, 97's gonna 97 sometimes. Check this out.

1722746437034.png
 
Went to tackle the A/C problems tonight, only to find that the dang thing blows plenty cold. I tried to explain this to my wife, but if there's a topic, she knows about it. Turns out what is happening is that the face vent blows cold until (presumably?) there's not enough vacuum to hold things steady because then it starts blowing cold on my feet instead. Then if you slow down, it moves back to the upper vents.

Interestingly, I swear when I looked at this a few months ago it would only blow on feet/defrost, but now the sliders control things 100% correctly, outside of this one vacuum issue. I don't claim to know how this system works, but it would appear that I probably don't have a vacuum leak, since most of the time it's perfect. Or at least not a leak in the typical way. I'm guessing it's a check valve or maybe the vacuum box under the battery or the hose leading to it are the issue. It's not able to store (or deliver) the vacuum the HVAC needs when the engine vac is at a minimum.

Am I high, or does this sound plausible?
 
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