Chrysler Repair: overheating when using the AC, upper radiator hose, coolant temp

Question
1995 chrysler la baron. 3.1 v6 . it runs fine until i switch on the a/c. once it starts up, gradually the temp guage increases up to hot. it will even set off the check engine light.  the fan on the radiator is kicking on when the compressor comes on . i cleaned out the radiator and the fluid was clean. i changed the fluid anyway. 50/50 mix.  when i turn off the air , the temp drops back down to normal. what do i need to check now ?  we live in az. and need the a/c to be working right.  thanks  

Answer
Hi Michael,
There will be greater heat dissipation requirement when the AC is brought into play so it is not surprising that the gauge moves up toward H. The bottom line question is whether the temp is going so high as to exceed the boiling point of the coolant which is in the range of 260F. The gauge is qualitative, not quantitative. So observe whether you are getting any steam blow out from the radiator cap or overflow bottle as a sign of that. Otherwise without an electronic readout thermometer to measure the coolant temp at the neck of the thermostat housing/upper radiator hose area we are unsure whether the system is really overheating. Being near H or briefly in the red is acceptable, but stop if you see steam and let things cool off.  
My initial suggestion would be to put a light bulb behind the radiator and look in thru the grill to see if there is much blockage of the fins that dissipate the air. You might have a build up of leaves or dirt that is blocking the ambient air from flowing thru the radiator. I would also try to readout the engine controller for what is causing the check engine light to come on, which unless the engine has died should be accompanied by a fault code that identifies the reason for the light occuring. Just turn the ignition key: "on-off-on-off-on and leave on" doing this within an elapsed time of 5 seconds or less. Then observe the check engine light which will be "on" to start to flash, pause, flash, pause, etc. Count the number of flashes before each pause. Then repeat to make sure that you have the count accurately. Group those numbers into pairs to form two digit fault code numbers. The last such number will always be 55 (two groups of 5 flashes each) which is the code for 'end of readout'. Then write back with the other code numbers, or go to www.allpar.com/fix/codes.html for a translation. I can give you some specific suggestion once you have the code numbers.
The other possibility is that the water pump is not working efficiently, but let us leave that to later when we know there is a problem and the other corrections haven't worked.
Roland