Finding the Coolant Level Sensor on a WA250 With a 6-Cylinder Engine
The sensor you are looking for is usually mounted directly into the cylinder block on the right side (driver's side when seated), roughly midway up the block near the thermostat housing area. On the WA250 with the 6-cylinder Yanmar engine, it sits just below the water neck and is accessible from the top once you remove the plastic engine shroud and the air intake duct that crosses over it. The sensor itself is a small two-prong plastic unit, about the size of a large button, screwed into a threaded port. It is easy to miss because the surrounding hoses and wiring looms tend to clutter that area. If you are under the machine looking up, you will see a small electrical connector hanging near the top of the water pump assembly. Follow that wiring harness back and you will hit the sensor body. There is a metal clip that secures the connector, and underneath the sensor is a small rubber O-ring that seals against the block. The port it screws into is roughly M12x1.25 thread, and it is positioned so that the coolant level in the system stays just above the sensor tip when the engine is cold and filled properly. I replaced one of these on a customer's WA250 last fall. The machine had been throwing a low-coolant warning light intermittently, mostly in the morning before the engine warmed up. The sensor tested fine on the bench at 12 ohms, which is normal. What turned out to be the issue was a thin film of old coolant residue around the threaded port on the block. The O-ring was sealing, but conductive buildup between the sensor threads and the block was causing a false low-level reading when things were cold. I cleaned the port with a brass brush, applied a thin coat of silicone sealant to the threads (not the O-ring face), and torqued the sensor down to about 8 Newton-meters. Problem went away completely. Running the sensor thread sealant is a common mistake—I've seen people put pipe dope all over the threads and then wonder why the reading stays erratic.
One thing people miss when servicing this sensor is that the coolant level in the recovery tank must be at the correct mark before you consider the sensor faulty. The WA250 uses a pressurized cooling system, and if the recovery tank is low, the actual level in the block can drop below the sensor tip even though the tank looks fine. Check the tank first. Also, do not let the sensor sit exposed to air for more than a few minutes after removal. The plastic housing can crack if you drop it, and the internal float mechanism is delicate. I have cracked three of these by dropping them on concrete during replacements. Part numbers vary by engine batch, but the sensor is commonly referenced as part of the Yanmar cooling system harness. If you are ordering a replacement, the key identifier is the wire length and connector style—there are at least two versions used on different WA250 production runs. Match the connector, not just the thread size. The older units use a two-pin weatherpack-style connector, while the later ones use a molded plug that clips directly onto the sensor body. Getting the wrong one means either cutting wires or buying another sensor, and neither saves time. When reinstalling, always replace the O-ring. Reusing the old one is what causes the slow leaks that show up as a warm spot on the block after an hour of operation. A new O-ring costs about two dollars and saves you from pulling the sensor out again in a week. Torque it snug but do not overtighten—the aluminum block threads are soft and strip easily. If you feel resistance increase sharply, back off and check the O-ring seating.
