Trane Rtua100 Problem

[shipwreck]

I'm having a problem with 1 circuit of the RTUA locking out all the time. The circuit lockout is low superheat. When the circuit is off the Evaporator pressure on the display reads 427.5 psi and the Condensing pressure reads 79.5 psi, this of course is way off the actuals. I performed the EXV test and it seemed ok. I understand that Trane does a calculation with the sensors to read pressures. There must be something wrong with the 1U3 board, sesnors are reading the right temperatures. Any one have this problem before? How do you test this board?

[jayguy]

it looks like the sensors are switched around although the 400+ sensor would require the temp sensor to be at 160F...probably a faulty sensor. verify the sensors are wired correctly and that all wire connections are tight. the sensors measure refrigerant temps and convert it to pressure. this may not be accurate if there are some non-condensibles in the system. give us some actual measured temps and pressures (as well as what the Human Interface says)

[jimbob73]

Sounds like a bad saturated evap temp sensor. I have seen this many times and every time it was a bad sensor. With compreesor off, You can verify by switching 2 sensors(suction/evap) at the module to see if problem follows the change or you can check resistance and compare to actual temp if you got the book with temp/resistance conversion charts. These thermistors are 10K ohm at 77 degrees F. Suction and saturated evap temp must be replaced at same time. They come as a matched set.

[shipwreck]

Thanks for all your help, I'm checking into it a bit more today.

[Healey Nut]

You have got a bad sensor , change them as a pair and you are good to go.
the calculation is very simple saturation versus presssure .

[vbhvac]

I'll still check connections on the plugs before changing the sensors

[shipwreck]

I checked the connections and performed the DC voltage and resistance checks. I also moved the sensors for circuit #2 over to circuit#1 which gave me a sensor error for the SST sensor. I didn't receive an error originally and the problems transferred over to the 1st circuit. I am replacing the SST and suction line sensors tomorrow. Hopefully the valves hold for the pumpdown test.

[jayguy]

the pumpdown test is a poorly worded action. the pumpdown action is intended to help remove the refrigerant out of the evaporator and liquid line. to perform the test, you will need to shut the liquid line isolation valve. during the test, the EXV will open up all of the way and stay there (that's why you need to close the liquid line valve first). the compressor will run for only 1 minute and then shut off. you will need to cycle power to perform the test more that once.

do not, under any circumstance, allow the compressor to pump down below 20 psig.

if you do, there will not be enough refrigerant to cool the motor and more importantly, the rotors will become overheated. the tolerances between the rotor and casing are so small that the rotors can actually "grow" into the casing when overheated. you will then have bought yourself a beautiful Trane manufactured boat anchor:mad: to shut the compressor off, before the 1 minute timer times out or before you hit 20 psig, you will need to kill power at the disconnect...telling the micro to stop the pumpdown or hitting the stop button will not work.

good luck

[Healey Nut]

Dont pull the dissconnect to stop the pumpdown .Pumpdown should only be performed once.!!!!! If you insist on doing it more make sure you read actual pressure at the suction port of the compressor and unscrew the control fuse when the pressure reaches 20psi. Pulling dissconnects to stop equipment is not the safest of options.

[jayguy]

Dont pull the dissconnect to stop the pumpdown .Pumpdown should only be performed once.!!!!! If you insist on doing it more make sure you read actual pressure at the suction port of the compressor and unscrew the control fuse when the pressure reaches 20psi. Pulling dissconnects to stop equipment is not the safest of options.

no...its not the best way, but it is the fastest:D