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  #1  
Old 04-01-2019, 07:34 AM
rvdave rvdave is offline
 
Join Date: Jun 2005
Posts: 459
Default Temperature probe fails

I?ve had problems with cht probes coincidentally with parking the plane outside for a week both at sun n fun last year, a Florida trip this year. In both cases after takeoff was surprised to find the readings quite low like 300 & erratic vs 1300ish. Received a tip from someone about heating up the base of the probe to near red hot three times between cooling off. It actually worked and brought the probe to accuracy after failing on its own to come back. I have drip loops at the probes so moisture would not run into the probe but somehow condensation must be entering.
Now recently my oil temp probe is erratic and wondering about trying that but hesitant since it normally doesn?t see temps that high as a torch might bring. Anyone with oil temp probe symptoms?
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  #2  
Old 04-02-2019, 12:02 PM
sblack sblack is offline
 
Join Date: Dec 2009
Location: Montreal
Posts: 1,520
Default

you don't mention what sort of engine instrumentation system you have. An efix, engine monitor, a separate gauge for each type of probe? Are these the CHTs that are on the spark plug gasket or are they the bayonet type?

If you are getting rouge instrumentation readings and it is an engine monitor with all probes going to it then the common cause is often the engine ground. Any sort of compromise of the connection will bugger up the voltage being read by the probe. You are trying to see very small differences in voltage so if the ground connection, or any other connection for that matter is not good the readings will be ****. And if it is more than 1 probe it is more likely a common cause than 2 or 3 probes failing. But to know more you need to provide more information.
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  #3  
Old 04-02-2019, 12:55 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,710
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Quote:
Originally Posted by sblack View Post
you don't mention what sort of engine instrumentation system you have. An efix, engine monitor, a separate gauge for each type of probe? Are these the CHTs that are on the spark plug gasket or are they the bayonet type?

If you are getting rouge instrumentation readings and it is an engine monitor with all probes going to it then the common cause is often the engine ground. Any sort of compromise of the connection will bugger up the voltage being read by the probe. You are trying to see very small differences in voltage so if the ground connection, or any other connection for that matter is not good the readings will be ****. And if it is more than 1 probe it is more likely a common cause than 2 or 3 probes failing. But to know more you need to provide more information.
Scott,

It was my understanding that thermocouples are two wire devices, with one being positive and the other negative. Pretty sure it is closed loop with no additional ground required at the sensor end. I do agree that any connection issue on either of these two wires will create problems, as the measurements are in millivolts.

Am I missing something here?

Larry
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  #4  
Old 04-03-2019, 08:24 AM
rvdave rvdave is offline
 
Join Date: Jun 2005
Posts: 459
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I will have to double check connections at the probe, makes sense that it could become resistive with moisture but figured a lot of time had gone by on cross country trip that would?ve dried things out but worth looking at.
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  #5  
Old 04-03-2019, 10:01 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Originally Posted by rvdave View Post
I will have to double check connections at the probe, makes sense that it could become resistive with moisture but figured a lot of time had gone by on cross country trip that would’ve dried things out but worth looking at.
Given the very low voltage and current of the thermocouples, they are sensitive to resistance in the connectors. That said, a sudden change in readings is not likely from corrossion. However, bumping or stressing a weak connector/crimp while working on the plane is very possible. I would check the wiring and connectors for the problematic thermocouple before replacing.

Larry
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  #6  
Old 05-01-2019, 02:47 PM
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walkman walkman is offline
 
Join Date: Oct 2011
Location: Atlanta
Posts: 893
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The spade connectors they all seem to come with suck. I replaced all mine with WeatherPack/Delphi automotive connectors. Works great.
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  #7  
Old 05-01-2019, 05:02 PM
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rocketbob rocketbob is offline
 
Join Date: Jun 2006
Location: 8I3
Posts: 3,763
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Quote:
Originally Posted by lr172 View Post
Scott,

It was my understanding that thermocouples are two wire devices, with one being positive and the other negative. Pretty sure it is closed loop with no additional ground required at the sensor end. I do agree that any connection issue on either of these two wires will create problems, as the measurements are in millivolts.

Am I missing something here?

Larry
Yes you are missing something. Most engine thermocouples are grounded at the tip for better response times and more accurate measurements. The drawback of this is that a ground loop can occur where voltages can get induced from things like ignitions and alternators. A K-type CHT probe at 400F will output 8.316mV so these are very small voltages and there are plenty of ways inaccuracies can be introduced.
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