the ECT i worked with was an NTC thermistor type and as it is heated its resistance changes, depending on manufacturer but mostly range from 24000ohms at 10 degrees to 200 ohms at 80 degrees. As the temperature raises the resistance decreases because of the internal properties of the thermistor.
5v passes through a Known pull up resistor then through the ECT variable resistor and then to ground. The ECU gets its signal off the voltage drop between the two resistors, how this voltage drop changes depends how hot the ECT is because its resistance is important. As the ECT resistance rises (getting hotter) the it consumes more voltage because of the more resistance therefore the known resistor uses less voltage making the volt drop lower. As the temperature raises the voltage drop lowers and the ECU can calculate the coolant temp by these various voltages.
I placed my ECT sensor in a bowl of warm water and slowly heated the water up and measured the resistance and compared it to the temperature.
| Water temperature (Degrees) | Resistance (k ohms) |
| 33 | 1.36 |
| 35 | 1.32 |
| 45 | 1.14 |
| 50 | 0.88 |
| 55 | 0.68 |
The line on this graph indicates that as the temperature rises the resistance lowers.
This also show that the ECT is working as it should and is within spec according to the manufacturers specifications.
ECT on-car
Engine: Toyota 1zz
First i back probed both of the wires with a pin to find the input and ground and discovered that the white one was the input and the Brown was ground. as i turned the ignition on an measured the volt drop with the red probe on the white wire and the black to a good ground i got a reading of 4.5V, this reading tells me that the engine coolant temp is cold because of the voltage being so close too 5V and this reading is correct to the manufacturers specs.
After i started the engine up and let it get up to operating temperature the reading changed to 0.8V meaning the engine is now fully warmed up.
The ECT sensor voltage signal affects the fuel injection output from the ECU because when the engine is cold it needs more fuel to warm up therefore the ECU needs to know how hot or cold the coolant temp is to determine how much fuel to inject.
If the ECT sensor has a loose plug, split wire, corroded terminals this would result in an incorect voltage to the ECU because all these things create an added resistance.
I also tested the ground wire of the ECU by correctly back probing the brown wire and started the engine, i got a reading of 0.3mV which is a good reading because it is so low it indicates that the ECT has a good earth. It is important to measure the ground wire because if the reading were to be higher that about 0.5v it would mean there is an added resistance also causing the ECU to receive incorrect voltages from the ECT. And this would cause the ECU to inject less fuel and run lean. The circuit diagram is the same as the off car one at the top of the page.
