Replacing a UMA Oil Temp Sender

[Janekom]

We have a problem with a very erratic oil temp reading on a Garmn G3X system and the the sender unit is a UMA. I see that the Uma unit is a 5K thermistor unit. On the bench it measures 110 Ohm while the temp is about 20 degrees C.

First of all - is this reading right?

Secondly - I have tried to install the unit that Dynon sells. We have selected "Thermistor" in setup. At ambient it seems to be OK but seems to run away as the oil temp goes up. Will this work?

Thanking you in advance.

[g3xpert]

Originally Posted by Janekom

We have a problem with a very erratic oil temp reading on a Garmn G3X system and the the sender unit is a UMA. I see that the Uma unit is a 5K thermistor unit. On the bench it measures 110 Ohm while the temp is about 20 degrees C.

First of all - is this reading right?

Secondly - I have tried to install the unit that Dynon sells. We have selected "Thermistor" in setup. At ambient it seems to be OK but seems to run away as the oil temp goes up. Will this work?

Thanking you in advance.

Hello Jan,

Sorry to hear you are having troubles with your oil temp measurement.

The UMA 1B3-2.5R oil temperature sensor that we provide is not a thermistor. It is a precision platinum RTD sensor for which the resistance varies very predictably with temperature. It reads 100 ohms at 0 deg. C with an additional 0.38505 ohms for each deg. C above 0 degs.

So, if the temperature in your example is 20 deg. C, the resistance across the two black leads of the probe should be 100+(20 x 0.38505) or 107.7 ohms.

We recommend you carefully check all wiring and connections to make sure this isn't causing the erratic readings. Also check the wires going into the sensor to see if there is damage there. It is also possible that a problem has developed inside the sensor leading to the erratic readings.

While we don't generally recommend thermistor based sensors for important measurements like oil temperature, you are right that you can select a probe type of thermistor and use one. Unlike PT100 RTD sensors which are all alike in response, there are a large number of thermistor temperature response curves.

If you look at Characteristic Curve Table 2 in this document, you will see the temperature versus resistance response curve that is supported by the G3X oil temperature sensor type of "Thermistor".

Let us know if you have additional questions.

Thanks,
Steve

[Janekom]

Steve, I was looking for a Garmin support forum and could not find one. Thank you very much for your reply. So then it seems that my static reading on the bench is OK. I have "inspected" the two black wires and could not see a problem like chafing through or shorting out.

It is late evening here but I will surely try and fault find further in the morning. I have downloaded the install manual today and will use that as guidance as well.
Thank you very much.

[gfb]

Sorry to revive the thread. Is this the correct probe for oil temp?

https://www.aircraftspruce.com/catal...clickkey=8052#

[g3xpert]

Originally Posted by gfb

Sorry to revive the thread. Is this the correct probe for oil temp?

https://www.aircraftspruce.com/catal...clickkey=8052#

Hello gfb,

The Garmin part number for the oil temperature probe for a Lycoming/Continental engine is 494-70004-00 (see page 21-4 of the Rev. AA G3X Installation Manual) and that part number is shown on that ACS page, so it appears to be the correct probe.

Let us know if you have further questions.

Thanks,
Steve

[Plummit]

Will this same probe work for an I0-540 with the G-900X system?

-Marc

Originally Posted by g3xpert

Hello gfb,

The Garmin part number for the oil temperature probe for a Lycoming/Continental engine is 494-70004-00 (see page 21-4 of the Rev. AA G3X Installation Manual) and that part number is shown on that ACS page, so it appears to be the correct probe.

Let us know if you have further questions.

Thanks,
Steve