Visit the SpitFireEFI.com Downloads Section and get the EasyTherm setup files, and install the software. Also, before going any further, please create a backup of the data that’s currently on your ECU, as we will be overwriting it later in this guide. To do this, turn on the ECU, and connect to it with MegaTune, then select File – Save As. This will create an .msq file with all of your calibration settings.

We assume here that you already have a set of corresponding temperature and resistance values for the sensor(s) you’re using. If not, some common values we’ve often used are included at the end of this walkthrough. However, the best way to obtain the values for the exact sensors you’re using is to actually take resistance measurements at room temperature, close to freezing, and in boiling water. Be sure to note the actual temperature when you take the resistance measurement. This will give you the most accurate results. Once you’ve done this, input the corresponding temperature and resistance values into the fields provided in EasyTherm. Also note that you can put in the temperature values in either Celsius or Fahrenheit. Be sure that the “Bias Resistor” value stays at the default 2490 ohms. Under “Code Version” near the bottom of the screen, select “Custom”, and be sure that “Start Address” remains at “F500”. Also, we’ve noticed on some setups folks only choose to use 1 custom sensor, and leave the other sensor as a standard GM sensor. For example, if you were to use a White VW 16v Coolant Temp Sensor, and a GM Intake Air Temp sensor, you would put the VW values into the Coolant Temp side of EasyTherm. You should then re-enter the values for the GM Intake Air sensor even though it already exists in the MSnS-e code – this helps to keep things more organized as you get to moving files around and setting everything up later.

After inputting the proper values in the previous step, use the “Write .inc and .s19 Files” button to create a new set of temperature conversion files for your sensor setup. Once you’ve hit the button, you’ll be prompted to select an .s19 file – select the one that’s inside the 029y4 folder within your main MegaTune directory (or the appropriate folder for the code version you’re using – see our guide on configuring MegaTune if you have any questions on this). Once you’ve selected this file, EasyTherm will create the appropriate .inc files and a new .s19 file for the code version you’re using.

The easiest way to accomplish this is to use the existing EasyTherm function for downloading the .s19 file. It’s important to note here that if you’re reprogramming an ECU that’s already in a car, disconnect the coil(s) from the ECU before going any further. In order to reprogram the ECU, first remove the top portion of the case. Then, find a very small paper clip or piece of wire and connect the two pins labeled “Boot”. Once these are connected, power on the ECU – you’re now in reprogramming mode. Be sure that you’ve selected the appropriate COM port for you computer (whichever MegaSquirt is connected to) and that “Custom” is still selected in the “Code Version” menu. Next, hit the “Download to MegaSquirt” button in EasyTherm. Once prompted, select the “msns-extra_mod.s19” file that EasyTherm has created, and you will see the downloading window. Once downloading is complete, power off the ECU, remove the Boot Jumper that was inserted earlier and put the top of the case back on. The ECU is now flashed with the modified sensor values. If you created a backup .msq file of the settings that were on the ECU before you reprogrammed it, now is the time to put that data back in the ECU. To do this, turn on the ECU and open MegaTune, then go to File – Open and hit OK when you’re asked whether you want to “Send and Burn Data to Controller”. Once you’ve put your .msq file back into the ECU, power off, then reconnect the coil(s) before powering on again.

Find your EasyTherm install directory, typically C:\MegaSquirt\EasyTherm, and select the three .inc files that were just created – airdenfactor.inc (IAT values), matfactor.inc (IAT values), and thermfactor.inc (CLT values). Move these three files into the mtCfg folder in your car’s project folder (if you’re unsure where or what this is, please see our Guide to Configuring MegaTune). This will allow MegaTune to properly display your altered temperature/resistance values when you’re connected to your car.