What to do if the thermister is built into a PCB?

On some AHU units you might encounter that the thermister (temperature sensor) is built into a sub-board. If this is the case, we can still control the AC but need to identify the correct wires to tap into.

Below is a real case example we experienced:

In this example, after looking at the Diagram on the AC and referring to the manual we found R1T was the ambient sense and R2T was the coil sensor (physically touching the metal coil body upon inspcetion).

R1T is actually built into a sub PCB which contains also a reset button, LED indication and buzzer for the user.

We referred to the manual but it did not have detailed information on the PCB harness wiring (8 wires) indication.

We know thermisters are essentially variable resistors that change their value based on temperature, we can then test this on the board to be sure.

We measured the resistance the wiring harness disconnected and applied heat to the sensor to check the resistance changes (put a finger on the sensor for instance). Happy that we are certain we got the correct component for the thermister we traced the PCB tracks to the 2 connecting wire points. We then repeated the resistance check on the 2 pins to ensure we got the right points.

Now we know which two wires are related to the thermister we can tap into those 2 wires (leaving the other 6 wires intact and untouched), remember to cut into the 2 wires in a place where you can easily connect the ACES2 cable or reconnect original sensor (such as when someone comes to the site to service the unit).