Chasing a weird vacuum/AC issue

In the XJ I just purchased, my air/AC only blows from the defrost position. I noticed that the kid I bought it from had installed a bumper and failed to replace the vacuum canister after. Everything I've read pointed to that being my issue. No problem. Got a new one, relocated it to a new spot and reconnected. Checked for leaks, all seemed good.


Got back inside to check what had changed, and no joy, but I did notice some new things. If I run the jeep with the blower in the defrost position for a little while, and then turn it to the heater position, I can hear a decompression noise somewhere. I can't find it by myself, but I can hear it better on the driver's versus the passenger side. I also noticed that my AC seems to be engaged even when I'm not using the AC selections in the switch. It blows all the time. Lastly, I checked that little hose that Ts off the vacuum lines under the hood to see if it was damaged. It seems fine, if not a little pinched. While I was doing that, I could feel cool air blowing back out into the engine bay from the other side of the firewall through a hole in the grommet around where the heater core pipes pass through.


I've spent a fair bit of time trying to figure this out, but being that its fall now its a lower priority than some other things I need to look at. Still, I'm guessing this will effect my heater too. If someone has an idea, I'll try it.

It sounds like damper issues to me and yes the AC compressor runs in the defrost mode.

RCP Phx said:

It sounds like damper issues to me and yes the AC compressor runs in the defrost mode.

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I'm not exactly sure what you're referring to when you say "damper". My searches are talking about the blend door. and that isn't it.


I did some more testing to see if I could figure anything else out. I disconnected the little vacuum hose at the T in the engine bay just to see if it was holding vacuum. When I disconnect, I can hear the vacuum decompress, and I can do this repeatedly. That tells me that the vacuum canister is doing its job and I don't think I have a vacuum leak anywhere. I did test my blend door, and it works fine. I can control hot to cold no issue. The only thing I can't do is select where the air comes from (and choose air versus AC, because it seems the AC is always engaged no matter what I choose. I also took off the glove compartment to look behind it, and I don't see anything in there either except some unused electrical connections, which is probably to be expected for my model.



Behind the glove compartment on the far right, there is what looks like an actuator that doesn't seem to do anything based on my controls. I can move it with my hand easily enough, and I can hear that it changes the air flow, but moving it manually still doesn't change the vents that the air comes out of. At least not that I can tell.


Is it possible that its the vacuum switch in the dash, or is somthing connected to that actuator bad?

Look up under the passenger side dash. You can see the blend door there. Move the controls from defrost to any other position and you should see it move. You might have rodent nests in there preventing the door from moving.

cruiser54 said:

Look up under the passenger side dash. You can see the blend door there. Move the controls from defrost to any other position and you should see it move. You might have rodent nests in there preventing the door from moving.

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Not sure which model but the earlier models having the cable pop off of the actuator arm is fairly common. The round locking clip falls off.

A loss of vacuum typically causes the air, both heat and cold to come out the defrost vents.

A typical point of failure is a busted vacuum hose under the battery. You can verify this with a hand vacuum pump to actuate the vacuum diaphragm
on the blend door. The other things are a bad heater/AC control switch or leaking/unplugged vacuum harness in the dash.

Your profile says 2000. Is that the year of your XJ?

Do you have Cruise Control?

old_man said:

A loss of vacuum typically causes the air, both heat and cold to come out the defrost vents.

A typical point of failure is a busted vacuum hose under the battery. You can verify this with a hand vacuum pump to actuate the vacuum diaphragm
on the blend door. The other things are a bad heater/AC control switch or leaking/unplugged vacuum harness in the dash.

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I believe this to be the issue. I do have some vacuum in the system, but I don't think its enough. I read somewhere that even if there was a problem with the diverter vent or blend door, the max AC setting should actuate the fan door under the dash if there's enough pressure. Mine doesn't do that. I'll try and spend some more time looking for a leak. What is your advice for the easiest way to identify those (aside from visually) if you're not a mechanic with specialized tools?

Saudade said:

Your profile says 2000. Is that the year of your XJ?

Do you have Cruise Control?

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So the 2000 in my profile is my son's Cherokee, and the only one I had when I joined the forum. I now have 3, a 97, 98, and his 2k.


This thread is about the 97 manual. It does not have cruise control. I'd like to add the factory cruise in the future, but that's another thread...

Before you get too far down the rabbit hole make sure this is hooked up and secure.

8Mud said:

Before you get too far down the rabbit hole make sure this is hooked up and secure.

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I think that does not apply to a 97, the blend door motor is directly connected to the damper as well as the other 3 vacuum actuators.

SurplusFan98 said:

So the 2000 in my profile is my son's Cherokee, and the only one I had when I joined the forum. I now have 3, a 97, 98, and his 2k.


This thread is about the 97 manual. It does not have cruise control. I'd like to add the factory cruise in the future, but that's another thread...

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Thanks, that helps. The blend door on a 2000 is electrically driver. Yiour 97 is cable operated like 8Mud posted.