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Occasionally6 · 02-16-2015, 06:41 AM

You can measure the voltage from the CTS that the ECU reads. It will be a nominally 0-5V signal wrt ground/earth.

If you want, you can calibrate sensor voltage against an actual temperature with the sensor removed from the engine or a junkyard sensor. Put it in water brought to the boil or cooling from boiling, with a thermometer to read the actual temperature. Then plot the temps with the corresponding voltage.

The sensor will need the ~5V reference, a resistor or two (see below) and ground to complete the circuit, but it doesn't have to be connected to the car. A USB from "12V" adaptor should work to provide the reference 5V voltage. I have bought cheap Chinese USB cables for $4 that are suitable to cut and make the connections to the sensor. I would not use a USB voltage sourced from a PC or similar device (in the interests of the device).

The caveat is that GM (usually) includes two pull up resistors inside the ECU in series with the CTS resistance. Two resistances - rather than one - are used to increase the resolution of the sensor by creating two V vs T curves/ramps. Above a particular temperature - about 50C - one of the resistors in the ECU is bypassed (a transistor is switched in the ECU to short around it).

When the sensor voltage is read with the sensor in the car, the two voltage vs temp. ramps/curves are: one for low temps, with the additional resistance, and one for higher temps, without the additional resistance. You should only be interested in the higher temps so don't really need the additional resistance other than to be aware of its existence.

In the GM ECUs for which I have info. the always-in-series resistance is 350 Ohms with the sometimes-in-series resistance being 4k Ohms. Those values may differ in other ECUs with other CTS. Measure the voltage between the "350" Ohm and CTS sensor resistances and ground. (You are doing this anyway if you measure the voltage signal in the car.)

If you do want to measure the voltage as seen in the car, you can use jumper wires to extend the wires at the CTS connector to reach the sensor that you are placing in the heating or cooling water. With the ignition keyed on the CTS voltage will be present and reflect the temp. of the sensor.

If the aim is to provide a visual indication of when the cooling fan or fans is or are active then the LED (OP) can be made to work.

It can be in parallel with either the switched or switching side of the relay that switches the fan you wish to monitor.

It does need a current limiting resistor in series with the LED. The value of the resistor depends on the particular LED you are using and its characteristics. A typical red LED might drop 2V across it and need to be limited to 30 milliAmps max. continuous current. Assuming that it will be fed from 14V (running car - charging alternator).

The resistor needs to drop:

14V-2V = 12V

Using V = I x R:

12 = 0.030 x R

Solve for R:

R = 400 Ohms.

So use 430 or 470 Ohms.

02-16-2015, 06:41 AM	#11 (permalink)
Occasionally6 Master EcoModder Join Date: Apr 2013 Location: World Posts: 385 Thanks: 82 Thanked 82 Times in 67 Posts	You can measure the voltage from the CTS that the ECU reads. It will be a nominally 0-5V signal wrt ground/earth. If you want, you can calibrate sensor voltage against an actual temperature with the sensor removed from the engine or a junkyard sensor. Put it in water brought to the boil or cooling from boiling, with a thermometer to read the actual temperature. Then plot the temps with the corresponding voltage. The sensor will need the ~5V reference, a resistor or two (see below) and ground to complete the circuit, but it doesn't have to be connected to the car. A USB from "12V" adaptor should work to provide the reference 5V voltage. I have bought cheap Chinese USB cables for $4 that are suitable to cut and make the connections to the sensor. I would not use a USB voltage sourced from a PC or similar device (in the interests of the device). The caveat is that GM (usually) includes two pull up resistors inside the ECU in series with the CTS resistance. Two resistances - rather than one - are used to increase the resolution of the sensor by creating two V vs T curves/ramps. Above a particular temperature - about 50C - one of the resistors in the ECU is bypassed (a transistor is switched in the ECU to short around it). When the sensor voltage is read with the sensor in the car, the two voltage vs temp. ramps/curves are: one for low temps, with the additional resistance, and one for higher temps, without the additional resistance. You should only be interested in the higher temps so don't really need the additional resistance other than to be aware of its existence. In the GM ECUs for which I have info. the always-in-series resistance is 350 Ohms with the sometimes-in-series resistance being 4k Ohms. Those values may differ in other ECUs with other CTS. Measure the voltage between the "350" Ohm and CTS sensor resistances and ground. (You are doing this anyway if you measure the voltage signal in the car.) If you do want to measure the voltage as seen in the car, you can use jumper wires to extend the wires at the CTS connector to reach the sensor that you are placing in the heating or cooling water. With the ignition keyed on the CTS voltage will be present and reflect the temp. of the sensor. If the aim is to provide a visual indication of when the cooling fan or fans is or are active then the LED (OP) can be made to work. It can be in parallel with either the switched or switching side of the relay that switches the fan you wish to monitor. It does need a current limiting resistor in series with the LED. The value of the resistor depends on the particular LED you are using and its characteristics. A typical red LED might drop 2V across it and need to be limited to 30 milliAmps max. continuous current. Assuming that it will be fed from 14V (running car - charging alternator). The resistor needs to drop: 14V-2V = 12V Using V = I x R: 12 = 0.030 x R Solve for R: R = 400 Ohms. So use 430 or 470 Ohms.