Inside A Yanmar Thermostart
What is a Thermostart?
A Thermostart is a very efficient glow plug used in most of the Yanmar tractors. Instead of relying on just a heating coil like other glow plugs- this unit also drips a small amount of diesel fuel onto the red hot coil. This produces thick smoke, lots of heat, and occasionally small flames. All of these things help the engine to start in the coldest weather.
So how does it work?
The fuel line is connected to the thermostart through the fitting (A). The fuel is not allowed to leak out because the set screw (C) is tightened up against the check ball (B) - forcing it against the ball seat inside fitting (A).
When you are ready to start the tractor the glow plug gets energized through the terminal (D) and heats up the element (F). This causes the hollow tube at the bottom of (A) to expand. When the tube expands it pulls the set screw away from the check ball (B). This allows a very small amount of fuel to drip down onto the heating element which starts to smoke and burn.
Why do they go bad?
Over time the thermostarts, just like any glow plug can go bad. This is typically because the heating element, which is sort of like a very heavy duty light bulb, burns out. The check valve can also start to leak over time.
Where can I get a new one?
Special thanks to TractorByNet member "California" for the great pictures of the thermostart in action.