help!overheating

[blkcruiser]

i have a 2002 dbl cab 4 cyl auto. while driving on the freeway i noticed my temp gauge was beyond the red line, i immediately pulled over and let it cool down. there were no leaks fan belts ok. after it cooled down it was ok for the next 15 miles until i turned on the ac-then it went close to the red line again. i replaced the thermostat-no change...
anybody got any sugestions?
p.s.
ac still functions very good

[2XOverhead]

The obvious question would be are you sure there's enough coolant. I know you said you found no leaks, but did you pressure test it?

[JRE56]

Check your fan.

[The Phoenix]

So as of now, it seems your only get your overheating when you turn the A/C on?

Originally Posted by JRE56

Check your fan.

Believe it or not the fan doesn't do much for you on the freeway.

[BamZipPow]

What was the approximate temp outside? Going up a hill? Did you have any heavy loads in the back? Any passengers? When was the last time you had yer AC checked? When was the last time you had yer radiator checked and changed/flushed? Did you check yer radiator fins? How about them radiator hoses? Lot of different things to check...

Is yer truck an auto? Do you have an auxillary cooler on yer tranny? Is the transmission fluid good? Is it at the proper fill level?

When you pulled over, was the overflow bottle bubbling? If not...maybe yer sender/gauge might be wrong. You could test yer temp sender to make sure it's correct.

Does it redline when yer sitting still and with the AC on? Do you have a clutch fan or electric fan?

[Mike Murrell]

Plugged radiator, debris on radiator fins; weak water pump.

Any water in your oil?