coolant temp sensor resistance curve

[jakeb]

Does anyone know what the resistance curve is for the temp sensor used for the gauge? I am looking for a sensor that has the same curve but an m12x1.5 thread size rather than the m14x1.5.

TIA

[b*saint]

I think you could do it when its still in the car. Take a reading at ambient and note. Take a reading at 1/4 on temp gauge, then at normal operating temp and make a chart.

[jakeb]

I will do just about that if I can't find it out there already. The sensor is actually in an e28 (same part as e30) but the sensor will go into the head of the LS1 that I am installing. The ls1 thread size is m12x1.5 and the sensor is m14.... I could drill and retap the head. Or file down the sensor and retap that.... But if I can find an off the shelf sensor that would be best. even something in 1/8" NPT would be ok

[jlevie]

A good approximation of the curve for the sensor is

R = .00134876 * e raised to the power 3842.49 / T

T is in degrees Kelvin. From that you can generate a curve.

[b*saint]

what does "e" stand for?

[iansane]

what does "e" stand for?

I would presume...

http://en.wikipedia.org/wiki/E_(mathematical_constant)

[b*saint]

Whoa where does jlevie find this info out!? Genius.

[jlevie]

Whoa where does jlevie find this info out!? Genius.

I can explain...

The temperature sensors are Negative Coefficient Thermistors. Almost all of those have the essentially the same mathematical relationship between resistance and temperature. The constants vary by material characteristics, but the curves are the same. For a given sensor you can determine the constants by getting the resistance at two temperatures.

The curve of an NTC sensor is exponential and I needed that curve to program my IQ3 dash. Knowing the equation I measured the resistance at 0 & 100C and backed out the constants. Using that data I generated a set of R vs T values that I could plug into the IQ3.

Simple...

For the curious, the equation for the ECT sensor is:

R = .01800038 * e raised to the power 3466.70 / T

Where T is temperature in degrees Kelvin

[b*saint]

So should basically all sensors work the same, you just have to calibrate the reader?

[Jaxx_]

let me know if the M10 one works. I have my values for megasquirt from 22C, 0C and 100C. (it's around here somewhere...)

[nando]

megasquirt uses the DME sensor, not the cluster sensor. they have different curves.

[Jaxx_]

megasquirt uses the DME sensor, not the cluster sensor. they have different curves.

i did miss that.

[jlevie]

megasquirt uses the DME sensor, not the cluster sensor. they have different curves.

The equation for the ECT sensor is above.

[sludge1300]

formula for dash water temp sensor

A good approximation of the curve for the sensor is

R = .00134876 * e raised to the power 3842.49 / T

T is in degrees Kelvin. From that you can generate a curve.

Bringing this back to life to report some good news. This equation works!

I accidentally sold my M52 with my Race Technology water temp sensor still installed. So, that left me with an S52 with a stock e30 temp sensor that needed to be calibrated to my RT Dash2. I don't feel like buying another sensor, so I gave this a go.

I converted Kelvin to Fahrenheit and converted the sensor range to volts by calculating for a multitude of pull up resistors and 5v input voltage.

I had to wire in a new pull up resistor because the RT one is 1k. This is too much because not only does it decrease the voltage swing, but you can see below that it's hard to fit a curve to it for some reason:



Here are a few more iterations (I went with 220 ohm), note how the trend line fits the data better as the pull up resister gets 'smaller'. I put 100 ohm in there for grins, but the current that it draws is a bit much for the small input voltage wire.







Anyway, I reflashed my dash today. Idling it leveled off at about 190 f. So, definitely within 10 deg of what the temperature actually is. Almost the whole way through the temp range the gauge read 10-15 degrees hotter than an infra thermometer I was shooting at a black painted spot on my thermostat housing.

Thanks Jim!

[chrisbmx68]

I hate to bump this thread but I am in need of exactly the same information looking for a sensor in the m12x1.25 range with a much smaller probe stickout with similar resistance to the stock brown 1 wire cluster coolant gauge sensor. I attempted to modify a stock sensor and it went poorly :(