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  #1  
Old May 28th, 2019, 12:58
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Quailman Quailman is offline
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96 XJ HVAC only through defrost vents

Hey all, I have had an issue with my '96 XJ Sport for a while now where the HVAC only blows through the defrost vents regardless of what vent setting or blower speed I set the control to.
The only threads I can find suggest a problem with the vacuum canaster. I've tested all that, and from what I can tell, all vacuum lines under the hood are A-OK. I even disconnected the vacuum going to the canaster and plugged it with my finger just to be sure, still only coming through the defroster.
I did delete the Heater Control Valve, and blocked that line off with duct tape (although after checking, the HCV line doesn't have vacuum at all anyways).

I have checked the HCV line, blend door line, and foot vent line (everything I can reach without pulling the kick-plate or the dash out) and can feel NO vaccum pulling on those lines whatsoever, regardless of the vent setting.
A few months ago, I pulled a heater control unit from the junkyard and swapped it in with no change, so I swapped the original back in. I understand that the one I got from the JY could also be faulty, but unlikely.. right? Maybe it's got something to do with my vacuum line "harness" (for lack of a better term) that plugs into the control unit?

Does anyone have other suggestions on what else to check? I would really rather not start pulling the dash apart to get the heater box out unless I absolutely have to.

Thanks in advance!
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Old May 29th, 2019, 09:59
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Re: 96 XJ HVAC only through defrost vents

How did you check the lines? Visually only?

Disconnecting the canister (under the front fender) would only "break the circuit".

Look under the battery tray as the vacuum supply line runs under it.

You may need to get a hand held vacuum pump and test each circuit.
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Old May 29th, 2019, 10:40
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Re: 96 XJ HVAC only through defrost vents

Quote:
Originally Posted by Saudade View Post
How did you check the lines? Visually only?

Disconnecting the canister (under the front fender) would only "break the circuit".

Look under the battery tray as the vacuum supply line runs under it.

You may need to get a hand held vacuum pump and test each circuit.
Basically just visually only yes. I think the PO re-routed the vacuum line already because that line runs along the side of the battery rather than underneath, and has a rubber padded "sheath" where it makes contact with the battery to prevent rubbing.
I think my next step is to pull the dash controller and pop out the supply vacuum line that goes to it. If there IS vacuum on that line, then that would indicate a bad controller, right?

I just went out and checked a few things. No fuses are blown, relays are good - but the electric fan does NOT kick on when switched to A/C. I haven't probed the compresser yet, but I don't believe that is kicking on either..

Thanks!
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Old May 30th, 2019, 16:54
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Re: 96 XJ HVAC only through defrost vents

So now that I’ve been messing around with the vents lately, they randomly sometimes decide to blow from the face vents (in fact, all the vents at the same time). Then as soon as I mess with the vent settings, it defaults back to defrost only and won’t blow out any other vent again. This doesn’t seem to be affected by RPM or driving speed whatsoever, and the electric fan still isn’t kicking on.
I’m very confused now.. could it be the supply line to the controller is loose?
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Old May 30th, 2019, 18:17
trippled trippled is offline
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Re: 96 XJ HVAC only through defrost vents

The first thing I would do is check for vacuum supply at the control head, then you can start isolating circuits. A cheap vacuum gauge or hand pump would be very useful for diagnosing the problem, so you can see how much vacuum you have and leak checking individual circuits and the actuators.
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Old May 31st, 2019, 07:03
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Re: 96 XJ HVAC only through defrost vents

I will check this ASAP, thank you for the advice.
I've also noticed that when the face vents do decide to work, ALL vents blow for about 10 seconds as if the blend doors are opening and closing, but just VERY slowly..

The A/C definitely needs to be recharged due to a leak (separate issue). I know the compressor will not turn on if the system is too low, but is it the same for the electric fan? Because that won't turn on either.. Does the e-fan turning on have anything to do with vacuum? (I'm honestly not sure)

I've already checked the relays and fuses and they're fine.. I'm seriously stumped.
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Old May 31st, 2019, 11:32
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Re: 96 XJ HVAC only through defrost vents

The slow air speed when "all doors seem open" makes sense, it's that it's going everywhere instead of one place.

You're talking about the HVAC fan, right? try all speed settings - if none by max works, your fan speed resistor pack is shot. If no speed works, then go under the hood and track down the plug for the fan, and try jumping it straight to battery. If that works, it's the wiring somewhere between the switch and the fan; if not, the fan itself is dead.
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Old May 31st, 2019, 21:25
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Re: 96 XJ HVAC only through defrost vents

As regards the blower motor, you should pull the switch and take a look at the wiring behind it. Odds are good the plug body (which plugs into the back of the switch) is half melted. Or worse.

I recommend you look into a relay modification such as this: https://www.jeepforum.com/forum/f11/...re-fix-580627/

If you need to replace the plug behind the switch I suggest you look for a Renix era junkyard donor. Those ones seem to have used better wiring or shorter runs. Their plugs usually aren't so badly melted.
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Old June 4th, 2019, 09:52
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Re: 96 XJ HVAC only through defrost vents

Update:
I recharged the system and the e-fan works now. Turns out it IS connected to the failsafe that prevents the compressor from turning on if the system is too low.

Now my only problem is the vents still. To answer your question, all 4 fan speeds work fine, definitely not the blower resistor.
I also took apart the HVAC controller and cleaned all the components, lubed up all the rubber pieces that deal with the vacuum settings with dielectric grease. Still having vent switching issues. It will work on face vents only sometimes, and then when I try and switch the control to a different vent, it will refuse to switch back. I have a vacuum pump/gauge on order to check all the vacuum lines.
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Old June 4th, 2019, 10:15
RCP Phx RCP Phx is offline
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Re: 96 XJ HVAC only through defrost vents

Have you measured the vacuum at the input to the switch?
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Old June 22nd, 2019, 10:45
Sulfur Sulfur is offline
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Re: 96 XJ HVAC only through defrost vents

Quailman,
Your vent issue is definitely vacuum related, I would check the lines with a vacuum pump if that looks good then it's either the main vacuum line to the hvac controler or the controller itself. I just went through the same issue. You can hook the hand held vacuum pump strait to the line feeding the hvac controller. With the controller off you should be able to pull a vacuum and it should hold, if it bleeds down there is a leak. If that is good start her up, turn on the fan and go through the different modes. You may have to give the pump a few squeezes when you change modes to make the actuators open, but once open they should hold vacuum. This will help you figure out where the leak is.
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Old July 11th, 2019, 21:03
200k96 200k96 is offline
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Re: 96 XJ HVAC only through defrost vents

There's a vacuum fitting at the firewall that's actually a one way check valve .. Make sure you don't reinstall it backwards during your testing ...
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Old August 5th, 2019, 21:06
97xjnova 97xjnova is offline
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96 XJ HVAC only through defrost vents

Thanks guy for all the info


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