problem with a/c

I just bought another XJ94 from the old owner,who put 75K on it. While driving around during past two weeks in the middle of outside temp of mid 90s day time with a/c is on, I noticed my compressor and aux fan both constantly engaging and either seems to be come disengage under a/c normal cycle regarless of where I move the position of the gauge on the control temp dashboard. Although, I get some cool air coming out, but still not offseting the uncomforatble outside temp. Even driving at night, when outside temp is cooler, the result of compressor and aux fan do not become engage is still there. I am afraid continue driving on a long distance will put my compressor into disable

Took it to Jeep shop to look overall a/c system and found there is no leakage in the system, output pressure reading is well above average. Tried replacing a new thermostat with old one and the result is the same. Can any expert help me with this.

I am now little confuse as having reading through previous posted column related to a/c issue, which said aux fan will engage or disengage base on the engine or radiator temp, while other colleague tell me that aux fan is direct hooked up to the compressor and will operate coincide with it.

I am now little confuse as having reading through previous posted column related to a/c issue, which said aux fan will engage or disengage base on the engine or radiator temp, while other colleague tell me that aux fan is direct hooked up to the compressor and will operate coincide with it.

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It's both the A/C turns the fan on and so does engine temp when needed.
Two pressures are important, the high pressure and the low.

As stated above, the eelctric fan is supposed to come on every time the A/C compressor cycles.

Is your compressor cycling alot? if yes, you could just be low on freon, which would explain the poor cooling performance.

According to my 98's Factory Service Manual if the A/C compressor cycles more then 5 times in a minute the freon R134A is low and the system needs to be recharged.

Thank you for the useful feedback. Sorry forget to mention that freon level is O.K. I took a look at my friend XJ94 today and found the position of his thermostat where its metal rod end is inserted into blower housing. I assume the rod is the sensor reading the temp and regulate the compressor to be off or on depend on the the temp. Problem is I have a same hole location where thermostat' rod end is to be inserted into my blower housing, but seems there is something inside the hole, afraid to use force as I might break the themostat rod end. Any idea of why the hole is blocked? Can I drill the new hole into blower housing (afraid might damage something inside)?

Do you think my problem will go away if follow the steps above?.

The compressor cycling is controlled by a low pressure switch, not by temperature. How did you determine the system was properly charged?

I'm not sure how you deduced that the system is fully charged, but if so and still no cooling you MAY have a bad expansion valve. How often does the compressor cycle on and off? You need to find out if the refrigerant is moving through the system. You can do this by peeking into the sight glass on the drier (if there is a sight glass on it) or feeling one of the rubber lines..but be careful they may be hot. The low pressure side line coming out of the evaporator should be very cold. It may be to your advantage to find someone close with some HVAC experience, A/C systems are kind of tricky to work on, especially if you don't have any experience. That being said, It will also do you a great deal of benefit to pick up an automotive air conditioning manual. I think Haynes offers one, I've seen them at the local chain auto parts stores with the other auto manuals. Start there and read that book, it will help you tremendously!

Read these links:
http://www.familycar.com/Classroom/ac1.htm
http://www.meineke.com/stuff_about_cars/how_work_ac.asp

The AC system all works off the pressure-temp relationship of the refrigerant, think back to your High School Physics and Boiles Law and expanding gases absorbing energy.

The Compressor turns off before the evaporator reaches 32°F, so that you don't ice over the evaporator and block air flow. Since its a sealed system, the refrigerant won't reach 32°F until it drops to a certain pressure, ~25 PSI, so the low pressure switch in the system turns off the compresser before hitting that pressure. After the compressor has been off for a few seconds, the pressure will raise high enough that the switch turns the compressor back on.

At the highest setting, the Compressor should not be turning on and off, unless its cold outside. If the compressor is turning on and off, then you could be low on refrigerant.

The only way to truly tell whats going on inside an AC system is too measure the pressures, High and Low, while its operating on FULL.

Thanks guys, especially Jim and Pressurerat. I could hardly tell how many times my compressor clcle within the specific time. but noticed it's always on when I am driving.
The way I checked the level of freon is by looking into window glass of the drier. Mine seems crystal clear. Also, checked both of low and high end pressure hose running into expansion valve, one seems cold and hot as supposed to. I guess I will do a test on exxpansion valve, but how could you tell exp valve gone bad?.
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Also, notice a slight different when compare to my friend XJ94 and that is when my friend starts his rig in the morning, both clutch fan and aux fan come on automatically, but mine does not. Not sure this would relate to my a/c issue or not. The old owner also installed a theft remote system, could the guy hooked up the wire that may interfere or interrupt any a/c delay or fan clutch switch?