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Old 09-12-2010, 09:03 PM   #1
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Air Conditioning

So I left my jeep sit for about 7 days after going on a business trip. When I returned, it started up fine, but when I turned on the air conditioning no cold air.

The compressor is running fine, and cycles properly. Low pressure line is ice cold and the air switches directions when i move the vent switch. When I switch to heat, hot air blows out, and then returns to the outside temp when I switch it back to cold.

I have been reading a bunch of other posts where people are mentioning the vacuum lines. Not sure what would cause something like this to happen just after a few days.

Any help would be greatly appreciated. Im pretty familiar with the ac system. Ive replaced the compressor and all the lines about a year ago by myself.

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Old 09-12-2010, 11:08 PM   #2
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Possibly the blend door isn't closing all the way.

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Old 09-13-2010, 12:08 AM   #3
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Anyway to check if the door is working properly? When i move the temperature from cold to hot, the air gets hot. So it would seem there is something actuating the door.

Also; should the drier be cold when the AC is running? or just the main line into the evap?
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Old 09-13-2010, 06:54 AM   #4
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Yes, the dryer will feel cold, eventually getting wet with condensation. If it's not getting cold in a couple of minutes then it's not the blend door - low on freon, restricted etc.

Since hot air is stronger than cooling air, even if the blend door is only partially stuck, you'll feel heat, not cold.

The blend door - look under the dash and actuate the controls back and forth. You should see the door moving (not the door itself, but the actuating arm.) When you switch from heat to cold the door is supposed to swing, redirecting the air. If something got in there - a stick, a paper, your spare key, etc, it may be holding it from moving all the way. (That's not unusual.)
You should be able to tell if it's moving all the way. Give the arm a push with your fingers.

On older TJ's, not sure till when, maybe 2000, that door was vacuum operated. Blowing cigarette smoke through the lines often can find the vacuum leak.

On newer TJ's, the blend door is operated by a motor - actually 2 motors to open and close the door - both mounted on a module. Find the connector, measure the voltage to it. And CHECK THE GROUND. My '03 Rubi had a ground problem on that circuit since new (never did find it.) Rather than find the bad connection, I simply ran a new ground to it.
The little motors module is replaceable - but bench test it before replacing - they are expensive - and I'd bet, dealer only.

Whatever happened to simplicity? - cables did the job reliably for many years.

The designers need to be reminded --- KISS
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Old 09-14-2010, 05:54 AM   #5
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Ok so it's definitely not the blending door. What would cause a blockage in the drier or other lines from not running the jeep for seven days?
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Old 09-14-2010, 10:42 AM   #6
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You missed the point.

If the drier and lines are getting cold, then the freon system is not blocked. The evaporator is still making cold air!

But that cold air is not reaching the cab, it's being diverted somewhere or mixed with hot air so it appears to be ambient air.

Sounds like the temperature blend door is only moving part way. There are 2 doors - one diverts the air from the defroster ducts, the other from hot to cold air. Playing with the A/C switch will show you which is which.

Look underneath the passenger side dash. Blower on High, operate the switch from cold to hot a few times - you'll see the lever move on the BLEND DOOR. Move the switch to the cold side - notice which direction the arm moves. Push it farther with your fingers. You should get cold air from the dash vents now.

As I stated before, depending on how old the TJ is, it may be vacuum operated, or it may be electric motors.

The vacuum diaphragm can fail, or the motors can fail.

Check the vacuum, or the power there, depending on what you have.

As I remember there are 3 wires to the motor harness, gnd, hot, cold. The motors run very briefly, controlled by the PCM, stopping in the position that is called for by the temp switch on the dash - that allows mixing of both hot and cold air for the desired cab temperature.

A dealer's DRB scanner, or a good aftermarket software program can operate the blend door from the PCM.
Typical failure - wherever it's getting it's ground goes bad.

A week? - dumb luck, rats, mice, or Murhy's Law.
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Old 09-19-2010, 08:25 AM   #7
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I tried operating both the doors and all functions seem fine, I.e. Switching from hot to cold and switching the vent positions. What else could be restricting the air?
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Old 09-19-2010, 11:27 AM   #8
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Need to know more information -
The dyer and lines are getting cold?
What year is it - is it vacuum controlled or electric?
WHICH DO YOU HAVE?

The vacuum diaphragms can go bad or leak too much, vacuum lines can leak - the smoke test will determine that. Did you do it?
OR:
The electric motors can fail - pushing on the arm will see they are only moving part way or not moving at all.

You said the doors open and close OK, did you actually see them move or just observe where the air is coming out?

If you saw the arms move, did you push on them to see if they'd move farther? If there's something that dropped down the dash vent it may be holding the door open.

Pushing on the arms you can usually feel there's something in there. My gate key dropped down the dash vent - held my blend door open part way - few screws and the assembly came apart enough for the key to drop out.

On customers cars I've found all sorts of things, toys, Kleenex, even unused condoms.

If the Freon system is working (getting cold up front,) and the blower is working (you can hear that,) and the blend door closes off the heater ducting and opens the cold air ducting (all the way, else it's still mixing,) then there's nothing really left. It HAS to be one of the 3.
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Old 09-20-2010, 06:03 AM   #9
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So I ended up taking it somewhere. They said I was low on r134a. Apparently although my low pressure line was getting cold the compressor was cycling too often. I replaced the lines, dryer and compressor within a year and it's been working perfect ever since. Car sits for a week then the ac doesn't work. Seems to me that maybe I had been on low on freon all along since my compressor only runs for 4-5 seconds. What are the symptoms for over pressure vs low pressure? What would cause a leak in such a short period of time? How should the compressor be cycling and at what pressures should it be at?
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Old 09-20-2010, 08:50 AM   #10
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So now the symptoms have changed?

In your first post you said """""The compressor is running fine, and cycles properly. Low pressure line is ice cold and the air switches directions when i move the vent switch. When I switch to heat, hot air blows out, and then returns to the outside temp when I switch it back to cold."""""

But you don't want to answer the questions to help you.

Now you say """""my compressor only runs for 4-5 seconds. """""

How can anybody help you when you don't answer a few simple questions, don't do as suggested, then change the symptoms?


In your first post you said: """""Im pretty familiar with the ac system."""""

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