Coolant Temperature Sensor

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Coolant Temperature Sensor

Postby Peter704070 » Sun Jun 13, 2010 8:53 pm


Just a quick question, why does a coolant temperature sensor have a second resistor within the Engine ECU which isn't a variable resistor. I know the sensor is supplied with 5 volts and the other wire is earth back to the engine ECU so that will make a complete circuit.

As the sensor begins begins to get hotter due to the water temperature increasing the resistance within it changes, isn't that enough for the ECU to calculate the temperature.

I only ask as planning to make a training model to teach students. Basically Im making a frame which will hold a coolant temp sensor and use a blow torch to heat the sensor, I have a 6 volt battery but just wanted to know why I need to put in an additional resistor within the circuit wiring. Also been told to put a 350 ohm resistor within it, is that correct?

Any tips on making it better for students who are 16 years old.

Thanks Pete.
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Re: Coolant Temperature Sensor

Postby PeterF » Wed Jun 16, 2010 3:27 pm

The combination of the fixed resistor & the variable coolant sensor thermistor forms a variable potentiometer. +5V is applied to one end of the fixed resistor in the ECU. The other end of the resistor connects to the coolant sensor. The other end of the coolant sensor connects to ground. The voltage at the junction of the sensor and resistor will be somewhere between +5V and 0V, dependant on the actual coolant temperature. This voltage is measured by the ECU and converted into a temperature reading.
Be careful with the blow-torch, it will be way too hot!
I hope that explains.
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Re: Coolant Temperature Sensor

Postby Carl Grotti » Fri Jun 18, 2010 12:10 am

Just adding a little to what Peter said. You want to be careful to promote that there is only one pull up resistor in series with the ECT sensor. Some ECUs will actually switch over to another resistor at midstream temperature monitoring.

You may see approximately 4.8V at -30*C and it will go down from there as the coolant temperature warms up. You'll see about 1.00 volt when a temperature of 50*C is reached. When you reach a temperature such as 50*C, then the voltage jumps up again to say about 3.5 volts. It will start going down again as the coolant temperature warms up even hotter.

It is my understanding that this is engineered this way for better resolution.
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Carl Grotti
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Re: Coolant Temperature Sensor

Postby Alan » Fri Jun 18, 2010 8:09 am

Carl, thats a good point. The switching type of circuit is described / has a reference waveform in PicoScope Automotive software.

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