temp gauge

I have a 94' chevy c/k 1500 and i can be riding around for hours w/o the a/c on and as soon as i turn the a/c on the temp gauge starts to rise but as soon as i turn it off the temp gauge goes back to normal does anyone have any ideas on what this could be.

Since turning on the a/c puts an added demand on the cooling system, it could be a bad fan clutch, bad radiator or bad water pump. Also make sure coolant levels are correct.

When the a/c is on, the condensor (in front of rad) will get warm and if the rad is not working well, the truck can overheat.

Good luck,

//2000CAYukon