Finding Resistance Curves for IAT & CLT Sensors

When setting up a MegaSquirt-II or III ECU, you’ll often use pre-defined sensor options in TunerStudio for Intake Air Temperature (IAT) and Coolant Temperature (CLT). But sometimes you’ll run into sensors that aren’t in the dropdown list. In that case, you’ll need to generate your own resistance curve to ensure proper calibration.

This guide walks you through the process of measuring resistance at three temperature points so you can enter accurate data into TunerStudio.

Tools You’ll Need

  • IAT or CLT sensor (removed from your car)

  • Digital multimeter

  • Cooking thermometer

  • Pan & stove

  • Ice

  • Metal coat hanger (or similar suspension method)

  • Tape or clamps to secure the sensor

  • Notepad for recording temperature and resistance values

Step 1: Ice Bath (~33°F / 0.5°C)

  1. Fill a pan with ice, then add water to the top.

  2. Insert the cooking thermometer and allow the temperature to stabilize (around 33°F).

  3. Suspend the sensor tip in the water without submerging the connector.

  4. Measure resistance with the multimeter.

  5. Record both the water temperature and resistance.

Step 2: Warm Water (~100°F / 38°C)

  1. Refill the pan with fresh water and heat it to about 100°F.

  2. Remove from heat to stabilize the temperature.

  3. Suspend the sensor in the water.

  4. Record the temperature and resistance.

Step 3: Boiling Water (~212°F / 100°C)

  1. Bring the water to a rolling boil, then remove from heat.

  2. Allow it to stabilize briefly.

  3. Suspend the sensor tip in the hot water (be careful of steam).

  4. Record the resistance and temperature.

Entering Data in TunerStudio

With three resistance points recorded:

  1. Open TunerStudio.

  2. Go to Tools → Calibrate Thermistor Tables.

  3. Select either CLT or IAT.

  4. Enter your recorded temperatures and resistance values.

  5. Save the calibration.

Important: Ensure you choose the correct unit (Celsius or Fahrenheit) to match your measurements.

Why This Matters

Properly calibrating sensors ensures:

  • Accurate fueling calculations

  • Correct warmup enrichment

  • Reliable operation across all temperature ranges

If your sensor is not in the TunerStudio preset list, following this process gives you the data you need to dial in your ECU for precise performance.