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I have a pressure/temperature sensor for a vehicle, which I decided to tear apart to see exactly how the sensor works.

This sensor as designed by the manufacturer it is meant to be immersed in oil to detect fluid temperature. The pressure sensor side of it seems pretty straight forward in that there is a hole in the sensor physical body and a PCB.

I am stuck on how the temperature sensor works in this specific sensor, if you look at the picture I attached the entire PCB is covered in a glue like substance I am guessing to insulate it from the oil. The temperature sensor is covered in some sort of cement like substance and is bonded only to a metal rod.

I am a little loss as to how the temperature sensor would be able to detect change in the fluid temperature if there is all the insulation glue on the board isolating the sensor from any fluid. My only guess is the metal rod acts to conduct heat towards the temperature sensor on the PCB. So to prove this I have tried measuring the temperature signal via a pin on the sensor and oscilloscope. I've immersed the sensor in oil, raised the heat of the oil and applied the proper 5 V to the sensor. For some reason the temperature signal will stay only at 1.3 V regardless of oil temperature.

I am beginning to think maybe the sensor can only read the temperature with a pressurized fluid, and not just sitting in fluid. Any input is appreciated thanks enter image description here

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  • $\begingroup$ two common types of temperature sensors are thermocouples and RTDs. since there is a pressure sensor involved, there's probably already a system for handling sensors where the resistance changing indicates the value- so leaning toward RTD. you may want to figure out whether you are powering and reading it correctly. $\endgroup$
    – Abel
    Oct 9, 2021 at 2:32

2 Answers 2

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The sensing element of the transmission pressure sensor consists of a membrane that is deformed by the applied pressure.

The deformation is converted into a voltage proportional to the pressure. An electronic evaluation circuit then amplifies and digitizes the voltage.

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  • $\begingroup$ Its a transmission pressure/temperature sensor $\endgroup$
    – Gh88
    Apr 25, 2020 at 1:43
  • $\begingroup$ I see now, thanks, Here is the explanation of the sensor from Bosch. $\endgroup$
    – Ryan Griffith
    Apr 25, 2020 at 14:10
  • $\begingroup$ @RyanGriffith Did you mean to provide a link? $\endgroup$
    – Eric S
    Oct 8, 2021 at 18:34
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There are two issues at play:

  1. Why does the sensor work when it is covered in glue?

  2. Why doesn't it work in your tests?

There are two perfectly good explanations.

  1. The glue is thermally conductive. These sorts of sensors are often encased in a glue which is chemically and electrically insulation but thermally conductive to protect the electronics.

  2. The sensor is a thermocouple, which means it responds to changes in temperature by producing voltage, not by changing resistance. Applying 5 volts to it does no good, you need to measure the current it generates.

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