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  #1  
Old 05-17-2022, 05:08 PM
blaplante blaplante is offline
 
Join Date: Jan 2010
Location: Southern California
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Default Oil Temp Thermocouple Fault - reading high?

I'll fess up, I'm a EEng, and I'm not sure how this could happen, but maybe someone here knows.

Is it possible for a thermocouple sensor to fail / intermittent in a way where it reads HIGH? I know a thermocouple produces a tiny voltage, and so the typical failure mode is an open, and it should read low. Or it shorts to ground, I'd expect the same.

Scenario was - older JPI monitor. Oil Temp reads sometimes absurdly high - like over 400F. Obviously not possible without smoking the oil, and the oil has no burnt smell. Yes plane is currently grounded. JPI uses thermocouples for all temp sensing.

the EGT/CHT is all reading normally.
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  #2  
Old 05-18-2022, 05:20 PM
RV10Pilot RV10Pilot is offline
 
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When my VDO oil temperature probe was failing, it spiked high into the very high 200 degrees, then read erratic for about 30 seconds to a few minutes, and then would stabilize. It was obvious that the sensor was bad since it would be virtually impossible for the temperature of the oil to change that rapidly.
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  #3  
Old 05-22-2022, 01:36 PM
blaplante blaplante is offline
 
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I'm pretty sure the VDO probes aren't thermocouples.
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  #4  
Old 05-22-2022, 02:31 PM
Ralph Inkster Ralph Inkster is offline
 
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Tried the old test in boiling water trick?
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  #5  
Old 05-22-2022, 11:58 PM
scsmith scsmith is offline
 
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I've worked around thermocouples in aero research my whole career, and I never saw an occasion where a TC would fail high. But the JPI engine monitor on our C-182B had a bad EGT probe last year and it failed high, and intermittent for awhile. I can not explain it. But replacing the probe restored proper readings.
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  #6  
Old 05-23-2022, 06:23 PM
blaplante blaplante is offline
 
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Very interesting Steve, thanks for sharing.
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  #7  
Old 05-24-2022, 10:50 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by blaplante View Post
I'm pretty sure the VDO probes aren't thermocouples.
Agreed. I believe it is a thermister potted into a brass housing. I made my own and this is how I did it. Just matched the thermister with the VDO characteristics. Thermocouples produce a micro voltage and thermisters provide a variable resistance. Very different. Thermocouples require special, polarity sensitive wire matched to the thermocouple type, therefore easy to spot.
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Last edited by lr172 : 05-24-2022 at 11:02 AM.
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