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Why Do Diesels Have Two Thermostats?

Most, if not all, large diesel engines found in trucks and farm equipment employ a dual thermostat system. For the large part, the dual thermostat has replaced grille shutters from old days. Though a very effective system, unlike the grille shutters, you need to understand why dual thermostats are used and how to diagnose the cooling system.

how it all works

When the thermostat is closed (single or dual system), the coolant stays in the engine block and does not go to the radiator to be reduced in temperature. Most engines have a radiator bypass circuit that allows coolant movement in the engine when the thermostat is closed (engine cold) to hasten warm-up and to eliminate localized hot spots. Once the thermostat opens, coolant flows from the engine to the radiator, where heat is rejected into the air.

The radiator needs to be sized to dissipate the Btu of heat under engine load and the air throughput it will see. A semitruck going to town hauling a load of grain will have more natural airflow across the radiator than a combine harvesting wheat on a hot Kansas day. Due to this, the radiator is oversized for light engine load when the Btu of heat, which is intrinsically linked to the amount of fuel consumed, is low. This now causes a paradox. The radiator is designed for maximum engine load, but in most applications, the load is transient – it goes up and down. There has to be a method to control how much heat is rejected from the coolant by the radiator. If not, the engine would run too cold under light load.

In the old days, grille shutters would be used. The theory was simple. If flow across the air side of the radiator was controlled, then the radiator effectively becomes flexible – not in its size, but in heat rejection. This was founded on the need to bring the engine to operating temperature and to keep it in that region regardless of load. The shutters would close under light load and be fully open under high thermal conditions. The system worked, but early designs required driver involvement and also had the potential to fail in the wrong position if the mechanism and actuator became damaged, frozen, or stuck. 

Later shutter systems were activated independent of the driver. As an aside, many new passenger vehicles and pickup trucks employ a variation on the old system called active grille shutters. Their purpose is more for aerodynamics than engine cooling.

The dual thermostat system was designed to accomplish the same temperature control under different load conditions. Instead of manipulation of the air side of the radiator, the coolant flow to the radiator is varied.

Under light engine load, the primary thermostat of the dual system opens as would any, but the secondary stays closed. This limits the amount of heated coolant to be dropped in temperature. If the engine is put under load, the secondary thermostat, which has a higher open temperature rating, now allows full flow to the radiator. If the coolant temperature drops too much, then the secondary thermostat begins to close, limiting flow.

Keep in mind that the liquid temperature needs to reach a preset value for the secondary thermostat to open. This can be an issue during cooling system service when bleeding the air out. It may be necessary to block the air side of the radiator to allow the liquid temperature to get high enough to evoke full coolant flow. 

It is important to understand this system since an engine that is equipped with dual thermostats and runs hot only under load may have a stuck secondary thermostat. Always look there first before thinking the worst or condemning the radiator. 

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