I HAVE A MONTERO SPORT WITH A 3.5 ENGINE. The vehicle overheat when idling at a light or in traffic. It cools down to normal temp once you back rolling. Coolant resevor and radiator are full. Theres no leaks. It still overheats when condenser fan is on. I also do notice the lights dim just a bit when idling and go back to normal once rolling. Once the car start overheating,and when the temp get bout 3/4 on the temp gauge you can hear a lite squealing come from the front of the car. The squealing goes away once temp goes back to norm. I notice the engine's fan spins when engine on but stays a consistent speed. What can be the problem here? What should I check first in this scenario? What are all the possible causes?

 

Can you hook up a ScanTool to your vehicle to see if the temperature reading on the instrument panel is confirmed by the reading ECU gets (uses different sensors).
You can also try one of those laser temperature readers on the block to confirm the coolant temp increase at idle.
Possible culprits - radiator with restricted flow, radiator with plugged up air passages and bent foils, failing water pump.
The dimming of the lights is not likely to be related to overheating. it is just that because alternator has a reduced output at idle.
Squealing belt - which belt squeals? Is it serpentine belt or the timing belt? If it's serpentine belt, it could indicate that its old and stretched too far or that the tensioner is not holding tension properly. There are marks on the serpentine belt tensioner that show when the belt is too overstretched. If it's the timing belt squealing, then it could be slipping on the water pump and producing the squeal. That would indicate that water pump is hard to turn (failing) and might just be your cause of overheating. You can take off the serpentine belt and drive a little without it to see if the squealing will occur. That will confirm that its coming form the timing belt. Make sure you have battery fully charged before doing this, since you'll be drawing all power needed from the battery for the duration of your drive.