I have just put a completed temperature sensor sheet into the dropbox.
It has M20, M50 and S50B32 Coolant sensor data and M50 and M52 air temp sensor data on it.
I ordered a couple of different ones and oddly the only one that didn't measure right is the Febi Bilstein sensor. It claims to be an M50 part number but the resistance matches the Siemens ECU on the S50B32
General observations from this are:
- M20 and M50 Coolant sensor for the ECU is nearly identical.
- Cheap Intermotor or FAI sensors are no problem, Febi Bilstein ones might be a problem? (Ironic?)
- The thermistor used for the air temp is the same as the coolant temp, in a pinch they could be interchanged and the car would barely notice.
- The ECU does not actually measure the resistance/voltage of the sensor but there is actually a voltage divider arrangement of resistors with the temp sensor forming one leg. The ECU has the voltage on the non sensor leg fed into the A2D converter. You should only ever see ~4-0.5v with a working connected circuit, if you unplug the sensor then you will see 5v due to an open circuit.
I took a look at the High Res photos I have of the ECU boards and identified the resistor that forms the voltage divider with the two sensors.
This meant I could calculate the expected voltages the ECU will see from the sensor at various temperatures.
The formula for that is on the spread sheet.
A mathematical function for the curve of the sensors resistance is on the sheet as well.
These results should be about as accurate as any out there on the internet, the sensors were held down to the heat bed of the 3D printer and the temp was set, once the printer read the correct temperature the heat was held until the sensors stabilized at each point, these measurements were repeatable within 5 Ohms.
This will form the basis of the sensor readings on the Arduino so I had to get it right or it would have been a waste of time.
https://www.dropbox.com/s/xjf35hb1blznb ... .xlsx?dl=0