What Does the Thermal Switch Do on My Yamaha Outboard?

The thermal switch found on 150- and 200-horsepower 2.5 L V-6 Yamaha outboards controls the air valve in the thermal air valve circuit. The thermal air valve circuit works automatically during the outboard's initial start-up. Together with the electric fuel primer circuit, it forms the choke circuit and minimizes the impact of below-normal powerhead temperatures.

  1. Its Function

    • Until the engine reaches its normal temperature, the thermal switch keeps the thermal air valve that restricts the air flow to the carburetors closed. This enriches the fuel-air mixture flowing into the carburetors. The richer the fuel-air mixture -- less air and more fuel -- the faster the engine runs. As it does so, it heats to its normal temperature and the thermal switch allows the thermal air valve to open.

    Its Method

    • The thermal switch is located at the end of the thermal air valve, found beneath the spark plug on the No. 3 cylinder, threaded into the side of the cylinder head. When the engine's temperature is below 100 degrees F, the switch keeps the valve closed. This makes the air-fuel mixture richer than normal. As the engine's temperature rises into the normal operating temperature range, the switch opens, allowing the thermal air valve to open. When the valve's passages open, more air enters the carburetor and balances the mixture.

    The Enrichment Circuit

    • Higher-horsepower V-6 Yamaha outboards are equipped with this thermal air valve and an electric primer assembly that form an "enrichener circuit." Yamaha uses an enrichener circuit on the 150- and 200-horsepower in lieu of a conventional choke. When the ignition key switch is turned to the "on" position and pushed inward on a cold engine, the enrichener circuit engages. The electric primer assembly pipes fuel directly from the carburetor fuel bowl to the carburetors and the thermal switch that controls the thermal air valve.

    Small Switch, Big Job

    • The engine's water pump, located in the "foot" or lower unit of the outboard, draws water from the water in which the outboard is immersed. The water pump is directly driven by the engine's driveshaft and begins to operate as soon as the engine starts. While the water pump operates while the engine is running, the engine's thermostat won't allow the water to enter the engine's cooling passages until the engine reaches its operating temperature. Until that time, the richer fuel-air mixture, controlled by the thermal switch, ensures your Yamaha outboard warms properly, giving you better performance and economy.