Finding the Resistance Curves on IAT & CLT Sensors

If you are using a Megasquirt II, we will need three different resistance readings to be able to generate a resistance curve for your sensors.  If you are using a Megasquirt I, your results will need to be entered into EasyTherm.  The following instructions will give details on an easy way to produce the required information.  You will need the following items to be able to complete the below steps: Your Sensor, a Multimeter, a Pan, Cooking Thermometer (grocery store), a metal coat hanger, some Ice, and a Stove.

  1. Pull the needed sensor out of your car.  (Most manufacturers use CLT and IAT sensors with the same resistance curve… but you might test both to be sure)
  2. Fill a pan completely full of ice and then add water till it’s close to the top.  Put the cooking thermometer in the water and wait for the temperature to stabilize.  It will probably hang around 33 degrees F when it’s ready.
  3. You will need to rig up a coat hanger so that it can suspend the sensor end into the water without submerging it. Here is an example of what we did.
    1. Bend the coat hanger so that you can set your sensor between two wires and then tape your sensor to the wires.
    2. Suspend the sensor into the water and let it sit for a moment.  Make sure it is not completely submerged, you do not want the connecter leads getting wet.
    3. Use a multimeter to determine the resistance of the sensor. Record the temperature and the corresponding resistance you found.
  4. Pour out the ice water.
  5. Fill the pan with normal room temperature water and heat it up on the stove to about 100 degrees or so.
    1. Put the pan on a different burner that’s not on, this will stabilize the water temp.
    2. Suspend the sensor into the water using your coat hanger rig and let it sit for a moment.
    3. Use a multimeter to determine the resistance of the sensor. Record the temperature of the water and the corresponding resistance you found.
  6. Get the water up to a rolling boil.  Once it gets to a steady boil (around 212 degrees F).  Turn off the heat.
    1. Put the pan on a different burner that’s not on, this will stabilize the water temp.
    2. Suspend the sensor into the water using your coat hanger rig and let it sit for a moment.
    3. Be careful of the steam coming off… its really hot!  I am sure you can guess how I know this.  (it wasn’t just the thermometer that told me)
    4. Use a multimeter to determine the resistance of the sensor. Record the temperature of the water and the corresponding resistance you found.
  7. Let your sensor dry off and then put it back into your car.  Enter your readings into TunerStudio for Megasquirt II or III users (Tools > Calibrate Thermistor Tables) or use EasyTherm for Megasquirt I ECUs.  Note: Be sure that you properly choose Celsius or Fahrenheit when entering the information based on how you measured/recorded the temp.