Honda 37870-PK2-015 Sensor Assembly, Water Temperature (Hadsys)

First of all, note that the sensor uses a thermistor which is a temperature dependent diode that detects changes in coolant temperature. Also, the basic firing duration is calculated from these signals that are transmitted via this sensor to the ECU. Moreover, one should note that increase in coolant temperature brings down resistance of the thermistor.

The coolant temperature information is conveyed to the instrument panel through the PCM from the Engine Coolant Temperature (ECT) gauge sending unit. To test the gauge sending unit, first ensure the gauge and its wiring are functioning properly. Begin by checking the fuses, then with the ignition switch OFF, disconnect the wire from the ECT gauge sending unit and ground it to a known good point on the engine block using a jumper wire. Turn the ignition switch to the ON position and observe the ECT gauge; it should move towards the hot ("H") mark. If the pointer does not move or moves erratically, check for an open circuit or high resistance in the wire, and if the wire is fine, replace the gauge. If the issue persists, disconnect the yellow/green wire from the ECT gauge sending unit, connect an ohmmeter to the terminal where the wire was attached and a good ground on the engine block. The resistance should be 142 ohms at 133°F (56°C) and 32-49 ohms at temperatures between 185-212°F (85-100°C). When removing the ECT gauge sending unit, allow the engine to cool below 100°F (37°C) to prevent burns from spraying coolant. Note the anti-theft code and radio presets, then locate and disconnect the negative battery cable and the ECT gauge sending unit electrical connection. Drain the coolant into a sealable container and remove the sending unit using a pressure switch socket or box-end wrench. For installation, apply a sensor-safe liquid thread sealant to the threads, install the new sensor, and tighten to 7 ft. lbs. (9 Nm). Refill the engine with a 50/50 solution of water and Honda approved or equivalent coolant, reconnect the negative battery cable, enter the anti-theft code and radio presets, start the engine, allow it to reach operating temperature, check for leaks, and bleed the cooling system to remove air as necessary.

The Engine Coolant Temperature (ECT) sensor works as a thermistor whose resistance varies with coolant temperature. Hence ECT determines how much fuel is going to be mixed with air into the engine. The engine will lose its power and stop if it is unplugged. So you should measure the ECT sensor resistance while the engine is cold and hot to perform this test. In other words, it should read 5000 ohms once it is cold then 100-400 ohms in its hot state. This could mean that the sensor may not be working properly. Disconnecting the battery, draining the coolant, removing the old sensor, and installing a new one with Teflon sealant will facilitate replacement. Top off the coolant, start the engine, and check for leaks afterwards.

The final step is to install the new sensor by reversing the removal process and ensuring it is tightened securely.

The ECT sensor can be unscrewed using a wrench. Most deep sockets won't fit.