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Old 07-29-2009, 12:37 PM   #1
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Cold Air From A/C Stops After 10 Min.

Hey, everyone. I've had my four banger TJ for about 13 years, but just now stumbled upon this site. I'm glad to have found it, and have enjoyed a lot of the posts I've read so far.

I've now got a question about my A/C. I had a leak in my A/C last year and had it repaired. The A/C ran great for about a year, but it's now starting to give me fits. When I start the engine, the A/C will blow cold and act normally (air blows out the right vents and I can properly vary air strength). However, after about 7-10 minutes, the air starts to warm up and will eventually blow hot. It's like clockwork each time I run the A/C. I first thought that my coolant was low, but this has been going on for months, and I would think that by now it wouldn't blow cold at all if my coolant was low.

Someone suggested that there might be a problem with the ventilation to the compressor and that once pressure builds up inside, the compressor stopps blowing cold air.

Does anybody have any suggestions or experience with this sort of problem?

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Old 07-29-2009, 12:48 PM   #2
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I just posted this for someone else too. Could be this.

Check the High and Low pressure switches. I had the same problem on my Mustang. I was able to apply pressure on the electrical switch that plugs into the system. When playing around with it i could get the compressor clutch to kick on and off. This might be happening for you too.

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Old 07-29-2009, 03:22 PM   #3
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Thanks for the tip. Sorry, I didn't see that thread earlier. I'll try that and see if it works.
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Old 07-29-2009, 05:12 PM   #4
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When the freon gets low - 'cause of a small leak somewhere, it can do that.

What happens if it's got a low charge of freon there is enough liquid freon to compress and circulate - get cold, but as it circulates it's spread throughout the entire system, condenser, evaporator and everything else. The compressor starts running out of liquid freon to pump. Pressure drops, then the low pressure switch senses it's low on liquid, and cuts off the clutch.

Once it's shuts off, the freon liquefies again, pressure rises enough to satisfy the switch, compressor restarts - to do it all over again. It cycles like that.

So the effect is it's cool for a few moments, then stops working.

Icing can be another cause, but unless someone's had the system open it's not likely that.

Try adding 1 can - no more. Put a thermometer in a vent and watch the temp. Doors and windows closed. You won't be able to get enough in unless you have the engine running. Run at idle, with the freon can right side up - valve at the top - shake it every minute or so. The can will feel cold if it's feeding.

Take your time - the aftermarket do-it-yourself kits have a very small orifice - for your protection. With a larger orifice turning the can upside down could hydraulic the compressor, ruining it.

Always add it to the suction side - never the high side. If you are using one of those "do it yourself" kits it will only attach to the low side anyway.

You should see the temp drop down. Watch carefully - the temp will drop as you add - but when it stops going down and starts back up STOP ADDING.
It's full.

It may take a second can to hit that lowest spot.

You may have a small leak and have to add a little every so often.

Harbor Freight - and Ebay, has nice little electronic detectors for finding freon leaks - about $30.

Using a pressure gauge with the engine off tells very little, -- it just says there is something in it - enough? Who knows.
The engine must be running and the compressor clutch engaged to mean much.
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Old 07-29-2009, 10:57 PM   #5
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Thanks for the help. If checking the pressure switches doesn't work, I'll have to dig deeper with what you've told me and see if that will work.
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Old 07-29-2009, 11:39 PM   #6
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I believe you have only ONE switch - it is a combination Lo & Hi switch.

The only thing you can check is the connections to it.
You'd have to apply various pressures to see when it switches. You'd need a special test set-up to do that.

They rarely go bad anyway.

You can jumper across the 2 wires to activate the clutch - it will stay on constant. But that won't tell you anything other than the clutch works. If it was totally empty doing that helps you fill it by having the compressor sucking.

Add one can and see if it starts blowing cool and stops the cycling - or most of it.

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