The Difference in Thermal Vs. Non-Thermal Fan Clutches

Both thermal and nonthermal clutches are filled with silicone based oil. The low viscosity oil channels energy to the fan by transferring centrifugal hydraulic pressure from the clutch housing cavity to fluid coupling teeth and the clutch plate. This causes the fan to speed up or slow-down in conjunction with the engine’s temperature as hydraulic pressure increases or decreases.

Thermal clutches are equipped with a thermostatic valve that allows hydraulic fluid to bypass the clutch plate when necessary. As engine temperature varies, a temperature-sensitive bimetal coil spring expands or contracts, opening or closing non-return inlet and outlet ports between the hydraulic reservoir and the clutch cavity. When the engine is cold, the non-return drain valve opens, and draws oil back into the reservoir. This reduces hydraulic pressure on the fluid coupling and increases slippage, slowing the fan down or stopping it altogether. As the engine heats up, the drain valve closes and the inlet valve opens gradually. Once the engine reaches normal operating temperature, the cavity drains slightly, reducing fan speed. When the engine heats up beyond a certain point, the cavity fills up completely, increasing the fluid coupling’s efficiency and speeding up the fan to maximum rpm.

The nonthermal or “torque limiting” fan clutch is not equipped with a thermostatic valve. This type of clutch operates by centrifugal pressure generated between the housing and the clutch hydraulic coupling. In addition the reservoir valves are pressure sensitive and less efficient. However, the built-it torque limiting function decreases maximum fan speed to between 1200 and 2200 rpm, depending on the application, when the engine reaches cruising temperature. Note: nonthermal clutches last longer than thermal clutches and are cheaper to replace.

The 1970 Pontiac engineering brochure put it all in a nutshell with the following statement: “Fans. They cool. They also eat up horsepower. Conventional fans slurp up 20-24 horses at 6000 fan rpm. Flex fans, about 5 or 6. And clutch fans, about 4 or 5 horses. Take your choice. Why the difference? The flex fan blades flatten out at high rpm. Less resistance. Less drain. The clutch fan is thermostatically controlled. Cold, it runs no faster than 1000 rpm. Hot, it engages and picks up to 1800 rpm. All because of a little clutch.” As an added point, nonthermal fans are less efficient than thermal fans. This results in greater power loss and slightly higher fuel consumption. Please note: Flex fans have since been recalled because the blade roots are subjected to fatigue after extended use, causing blades to shatter at high speed.

Since fan clutches lose efficiency after extensive motoring and tend to slip and cause overheating, consider installing a new unit when you repair or replace your water pump. Although you’re able to improve efficiency and lower fuel consumption by upgrading from a nonthermal fan to a thermal fan, the reverse is not true; never replace a thermal fan with its nonthermal counterpart.