our suburban tempurture overheats when the ac is on.

Possible causes, debris in the fins of the radiator blocking air flow, run a garden hose from the engine side out with low water pressure. Another cause could be a worn out fan clutch not drawing enough air through the radiator. Bottom line is when the ac is on you need more air flowing through the radiator and condenser and it's not happening, shouldn't be terribly expensive to fix.

In addition to the above mentioned things it could also be a partially clogged radiator.