Have a 2003 gmc truck 1500hd. When i come to a stop or at idle , my ac blows warm air out. At cursing speed it blows cold air
Have the refrigerant charge checked. More than likely, you just need a refrigerant recharge.
Depending on mileage and particular maintenance the cooling fins on the condenser in front of the radiator could use a cleaning or maybe the fan clutch (if it has one) could be weak and not drawing enough air at idle.
Even simpler: seized motor, no voltage to fan, or bad ground wire to condenser fan can preclude it from running at all. On the road, plenty of air through the condenser – cold air; at idle, none; result – just what she experienced.
Always start with the simplest – and cheapest (KISS) diagnosis.
No sure if the GM HD1500 trucks used an electric fan for the radiator or not here.
the chevy and gmc trucks have a mech clutch fan but inside the fan shroud there is a elec. fan mounted aganst the rad it is very hard to see but its there i would check that and make sure the cond. behind the grill is not full of bugs and a plastic bag good idea to wash it out once a month with a good hose like at a car wash works great