i have a 2004 s-10 blazer with a 4.3 ltr. it started overheating so i had the raidator flushed and replaced the thermastat. that fixed the problem so i thought, now it runs at 210 degrees untill i turn on the a/c.after about 20 minutes of running the a/c the temp will go from 210 to 260. if any one has had this problem help would be much appreciated.

 

To get cold air inside, heat has to be generated somewhere else.

When was the last time you cleaned the exterior of the radiator and A/C condenser?

 

check the fan clutch, mine was doing that same thing,

 

Swart and twisted are correct mine did the same thing and the fan clutch fixed it but it is also good to check your condenser and radiator for blockage.

 

how can check to see if my fan clutch is bad?

 

I've tried testing mine two different ways,
1) stick a long piece of heavy cardboard. That should stop the fan.
2) use a water hose, its messy but the pressure should also stop the fan.