A set of control functions have been investigated for a computer controlled diesel cooling system, using the vehicle engine cooling system code. Various engine operating conditions such as the engine load, engine speed, and ambient temperature are considered as the controlling variables in the control loops. The truck simulated in the study was an International Harvester COF-9670 cab over chassis heavy-duty vehicle equipped with a standard cab heater, a Cummins NTC-350 diesel engine with a McCord radiator and standard cooling system components and after-cooler. The vehicle also had a Kysor fan-clutch and shutter system.Comparison simulation tests between the conventional cooling system and the computer controlled cooling system using the Vehicle-Engine-Cooling Computer System model under different ambient and route conditions show that the computer controlled cooling system would offer the following benefits: 1. Increase of coolant, oil and engine wall metal temperature at low ambient temperature, and light route load conditions will be beneficial in increasing engine thermal efficiency. 2. Higher oil temperature at low ambient temperature, during cold start, benefits faster temperature rise of the engine inner parts, which should reduce the engine wear. The control of oil temperature under variable load conditions should reduce the thermostress in the engine wall metal. 3. Reduced fan engagement time at higher ambient temperature and heavier load conditions saves energy consumed by the fan. 4. Water pump speed control shows the means to save power in driving the pump.The validation of simulation results by experimenting with a computer controlled engine cooling system is presented in a companion paper.