Overheat alarm but not overheating!

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   This is a curious one that currently has me scratching my head. Although posted on the 415 panel I believe the engine instrumentation is the same on all the current D2 engines.
My D2-40 overheating alarm is triggering when the temperature at the sensor is only 85C (checked with a cheap IR thermometer but feels about right). Also water flow (seawater and coolant) is OK.

My first thought was a faulty sensor but this appears to match the resistance curve given in the MDI testing manual (snippit attached below; you don't need a breakout box - just remove the wire and measure to earth) so I now suspect the voltage that is used in the sensing circuit (nominal 5V open circuit but falls as the sensor resistance falls), or the alarm trigger circuit.

When the alarm triggers I get the following readings  at the sensor

Sensor resistance  65 ohms
Sensor temperature 83C (as measured with the IR thermometer and correlates with the resistance)
Sensor voltage 0.62V.

My questions -
Does anyone know at what voltage the alarm should trigger?
Assuming the sensor is functioning correctly (which I'm convinced it is) is it the voltage that is too low or the alarm trigger is too high? 
Any other thoughts gratefully received. Although I'm sure the engine is not overheating I don't like disabling the alarm so need to fix this.

Thanks

Roger

PS The engine control panel is the simple one - alarms are indicated in the tacho window; no gauges.

Edited by High Time - 12 February 2016 at 17:40

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High Time Members Profile Send Private Message Find Members Posts Add to Buddy List Vice Admiral Joined: 04 September 2012 Location: Portsmouth UK Status: Offline Points: 819	Post Options Post Reply Quote High Time Report Post Thanks(0) Quote Reply Topic: Overheat alarm but not overheating! Posted: 12 February 2016 at 16:48
	This is a curious one that currently has me scratching my head. Although posted on the 415 panel I believe the engine instrumentation is the same on all the current D2 engines. My D2-40 overheating alarm is triggering when the temperature at the sensor is only 85C (checked with a cheap IR thermometer but feels about right). Also water flow (seawater and coolant) is OK. My first thought was a faulty sensor but this appears to match the resistance curve given in the MDI testing manual (snippit attached below; you don't need a breakout box - just remove the wire and measure to earth) so I now suspect the voltage that is used in the sensing circuit (nominal 5V open circuit but falls as the sensor resistance falls), or the alarm trigger circuit. When the alarm triggers I get the following readings at the sensor Sensor resistance 65 ohms Sensor temperature 83C (as measured with the IR thermometer and correlates with the resistance) Sensor voltage 0.62V. My questions - Does anyone know at what voltage the alarm should trigger? Assuming the sensor is functioning correctly (which I'm convinced it is) is it the voltage that is too low or the alarm trigger is too high? Any other thoughts gratefully received. Although I'm sure the engine is not overheating I don't like disabling the alarm so need to fix this. Thanks Roger PS The engine control panel is the simple one - alarms are indicated in the tacho window; no gauges. Edited by High Time - 12 February 2016 at 17:40
	Roger High Time (415 #038)



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