Fixing Incorrect Water Temperature from Thru-Hull Speed & Temperature Sensor (Raymarine i40)

 

   

I recently noticed that the temperature reading on my Raymarine i40 wind/speed/temperature display was reporting values over 100°F, even though the actual water temperature should have been in the 70°F range.

This kind of error is a common symptom of a failing thermistor inside the thru-hull speed and temperature transducer. The typical solution is to replace the entire transducer. However, if the thermistor has not completely failed, it may still be possible to recover accurate readings by recalibrating the effective resistance seen by the instrument.

Following guidance from the Raymarine support site, I was able to restore meaningful temperature readings without replacing the transducer:

Reference:
https://support.raymarine.com/s/article/Speed-is-not-working-on-my-instrument--and-or-the-water-temperature-is-wrong?language=en_US

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Understanding the Problem

The thermistor in the speed transducer is measured between the brown and white wires.

In my case:

• Measured resistance (white–brown): 4.58 kΩ
• Displayed temperature: 104°F (clearly incorrect)

This indicated that the thermistor’s resistance curve had drifted, causing the instrument to interpret the water temperature as much hotter than it actually was.

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The Fix: Adding Series Resistance

To correct the reading, I inserted additional resistance in series with the thermistor:

1. Cut the white wire.
2. Spliced in a 10 kΩ potentiometer.
3. Adjusted the potentiometer while monitoring the displayed temperature.

Increasing the series resistance lowers the temperature reading, effectively compensating for the thermistor drift.

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Calibration Results

After adjustment:

• Target water temperature: 66°F
• Additional resistance required: 8.54 kΩ

Once tuned, the instrument displayed accurate and stable temperature readings.

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Final Thoughts

This approach provides a practical workaround for a drifting thermistor and can extend the life of an otherwise functional transducer. While not a permanent factory solution, it’s an effective field fix—especially useful when hauling the boat to replace the thru-hull sensor is inconvenient.

If the thermistor continues to degrade, full transducer replacement will eventually be necessary. But in the meantime, this simple modification can restore reliable performance.