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Old 09-06-2011, 11:53 PM   #1
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'94 YJ AC Problem

Hi Folks,

I have a 4.0l '94 YJ with the afterthought AC fitted. It bolts to the bottom of the heater assembly as if Jeep added it as an extra!

Anyway, it works fine. The air is cold, the fan is quiet...but......it cycles on and off too often.
As I am driving the air is cold coming out of the vents, then the compressor cycles off and the air gets warm. Then the compressor cycles back on and the air gets cold, then it cycles off etc.
It's as if there is a thermostat somewhere telling the AC that it has cooled the car down to the temp set and now it is keeping it there. My problem is that it is still 100degF outside and it ain't cold enough in the car!

So, is there some sort of temperature sensor that thinks the car is cold when it isn't?

Any ideas gratefully received

Neil
ps. Had the gas topped up a while ago and it made no difference.

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Old 09-07-2011, 01:00 PM   #2
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Not too familiar with this particular AC system but in general there is a temp sensor on the evaporator core that cuts off the compressor if it gets too cold to prevent it from icing up. They do not commonly fail though. Also there is a high and low pressure switch (either separate or one unit) that will cut off the compressor if pressure climbs too high or low. High pressure can be cause by over charge, or blockage of the airflow through the condenser, commonly from a piece of newspaper or something stuck to it. Or a clogged orifice or expansion valve. Low pressure could be caused by under charge. I would suspect high pressure rather than low since it works momentarily then cuts off. The only reliable way to know is to put a gauge on it and watch the pressures as it's running.

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Old 09-07-2011, 01:01 PM   #3
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I'll bet there is a leak somewhere. Compressor should run if on. It is short cycling. A few causes of this could be low pressure in system. Bad pressure switch. Bad compressor. Bad or clogged receiver dryer. I would have a a/c specialist troubleshoot it to get it diagnosed properly.
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Old 09-07-2011, 05:13 PM   #4
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Mine does the same thing. I had mine vacuumed down, leak tested and charged and it still does the same thing. It blows ice cubes for 7 minutes, turns off with warm air for 45 seconds and back to cold again. Mine seems to cool longer on cool mornings and cyles on and off more as the outside temp rises. Any suggestions would really help.
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Old 09-08-2011, 08:28 AM   #5
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Quote:
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i'll bet there is a leak somewhere. Compressor should run if on. It is short cycling. A few causes of this could be low pressure in system. Bad pressure switch. Bad compressor. Bad or clogged receiver dryer. I would have a a/c specialist troubleshoot it to get it diagnosed properly.
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Old 09-08-2011, 09:03 PM   #6
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My 94 does the exact same thing.....i am suspecting its a little low on freon... and thats it...the high low pressure switch is sensing low freon pressure and it cuts off till the freon builds pressure back up.... i am betting on this being the problem....ya'll trying to make this sound a little more difficult that it really is.....just my two cents....will have mine recharged to prove this theory and let everyone know....personally i could care less about a/c in a jeep, i drive mine open 90 percent of the time with sides off and just top on in the summer....
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Old 09-10-2011, 12:12 AM   #7
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Driving with the top off may be fine in Irving, Texas. In Oman in the Middle East the summers are over 130F in the shade. The winters are high 80s and that is when the top comes off. For the other 8 months of the year the AC is on.

The pressure is correct as I have just had it refilled and it had not lost any pressure worth talking about since it's last refill. So no leaks.

It is as if the AC thinks we have reached the temp set and is now cycling the compressor to keep the temp at my setting. I do have a potentiometer next to the fan switch that is ment to set temperature. Perhaps that has failed?

Looks like I need to do more digging.....
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Old 09-10-2011, 08:57 AM   #8
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for the most part, you'll only find thermostats in newer vehicles where you can manually set the temp.. Vehicle a/c's will run until you manually turn them off. I'd look elsewhere.

Mgosset1..if you look on an earlier post he said he had the refrigerant checked out.
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Old 09-10-2011, 12:08 PM   #9
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ps. Had the gas topped up a while ago and it made no difference.
I bet your elither still low, or there is a restriction somewhere after the low/high pressure switch, and its fooling the system into under/overpressure mode shutting off the compressor.

I bet your still low, just because you had the refrigerant checked doesnt mean they did it right.
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Old 09-10-2011, 12:48 PM   #10
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i'd replace that sensor switch, not uncommon for them to go out, especially if your doing a pressure check and it checks out fine.....could even be the wiring going to the sensor switch may be corroded or something
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Old 09-11-2011, 01:46 AM   #11
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When I turn the temperature potentiometer to the left the off cycles get more often. If I turn it to the right the off cycles are less.

This would sort of indicate that I do not have a fault in the cooling system just that the car thinks it is cool enough - to the left is warmer, to the right is cooler.

If the compressor just came on when you turned on the AC and never cycled off why would I have a temperature potentiometer?
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Old 09-11-2011, 02:54 AM   #12
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hot cold modes.... all vehicles have this, back to the 60's

Used to be a lever that controlled a mix door.

You sure its a potentiometer?
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Old 09-11-2011, 12:25 PM   #13
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If you do have a temp. control pot you may have answered your own question. However, if the pot only controls the damper system, it's not the problem.
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Old 09-30-2011, 04:43 AM   #14
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Fixed!

So the fix was partly me being stupid (it was like that when I bought it, honest!) and partly the way it has been fixed in the past.

I have a sliding lever on the heater system that says 'off, screen, vent.' In the off position it lets air in from outside. So I was trying to cool 100F air everytime the compressor came on and letting 100F air in when it was not. No wonder it had a hard time cooling. so I moved to lever to vent and now the air inside is recirculated. The compressor is cooling air that it has already cooled when it comes on.

The other problem was the temp probe. Behind the right hand vent I could see a copper tube and the heat exchanger. The copper tube is some sort of temp probe and it was pushed two inches into the heat exchanger. I removed the vent and pulled the probe out, laying it infront and touching the heat exchanger like it is in my house AC. On the short drive home the compressor did not cycle off and I got constant cold air.

If things start to ice up I can always put the probe back in but not quite so deep!

Cold air just in time for winter and the roof coming off......oh well, ready for next summer.

Neil
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Old 07-12-2014, 08:16 AM   #15
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I have a similar problem. I've tried the switching to vent thing with minimal success. I've also messed with the copper probe but never pulled it all the way out like you did. I'm going to give it a shot today and see what happens.

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