Overheating AC?

Whenever I park my truck and have the ac on, after awhile the temp gauge starts climbing...but only when the ac is on and it's parked. I put in coolant but seems it didn't help much. Maybe the ac just needs to be charged up? Any suggestions??

 

By "coolant", are you talking about antifreeze/water mixture, or refrigerant?. A/C systems will always add a heat load via the condenser as long as the condenser is in front of the radiator, versus being separate like some FWD cars and some trucks. Most cooling system are heavy duty enough to handle to additional load without problems, but must be kept in good shape to do so. So, make sure cooling system has clean coolant, no mud, insects, excessive number of bent fins, the shroud, and all foam and rubber seals to direct air through the condenser and then the radiator. The same for the bugs and mud go for the condenser also. Your problem sounds like the fan clutch though; since at happens at idle. Most A/C equipped vehicles with rear wheel drive/4WD use a high pitch fan blade that spins relatively slow most of the time due to the shear effect of the oil in the clutch, but when hot should lock up and move a lot more air when temp increase. When you idle to the point temps go above normal, a simple rev of the engine should allow you to hear the fan, and opening the hood and holding your hand behind the fan should yield a lot of air movement. Check the fan clutch, and the rest of the cooling system. Another minor detail is; if refrigerant has been added to the A/C system over the years without purging the air out of the gauge set lines, the head pressure could be high; not high enough to cause the problem, but enough to aggravate it some (excessive head pressure would cause a pressure switch to open and stop compressor feed).