Have you verified that the fan IS going to high speed when it should. On many (confirm on yours with Freightliner) disconnecting the wire at the controller defaults the fan to HIGH. If that solves the problem, you need to troubleshoot the controller/signal to the controller.
There ARE other work-arounds if you have confirmed that:
Fan is going to high speed and hydraulic pressure is correct so fan speed is correct. AND water pump belt (separate V belt on your engine) is properly tensioned and in good condition. AND nothing bazar is happening like fan rotating backwards (it should suck from the outside and exhaust into the engine compartment with fan blades/direction of rotation set for this direction. AND no blockage of cooling package (CAC is outboard of the radiator).
On edit: It was pointed out to me that the radiator was new, so deleted any "radiator" comment. But you still need to verify that the CAC is clean-- it is the outboard most part of the cooling package and is the place that dirt is "filtered" out and deposited.
