Abstract: A system for controlling the heating of diesel fuel in a fuel tank of a diesel engine system to prevent diesel engine control electronics damage. The system is operable to bypass heat exchange medium flow at predetermined temperature ranges to prevent further heating of diesel fuel in a diesel fuel reservoir tank by the heat exchange medium. More particularly, this mechanically operated system permits identical temperature control means for controlling various temperatures of hydraulic oil in hydraulic oil reservoir tanks. The system may also be utilized to facilitate flow of a heat exchange medium which cools fluid in either diesel fuel or hydraulic oil reservoir tanks.