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Thermal Expansion Valve testing

Karl on Sat September 12, 2009 10:52 AM User is offline

Year: 1973
Make: Pontiac
Model: Formula
Engine Size: 400
Refrigerant Type: R12
Country of Origin: United States

Hello everyone, I was wondering, I was testing my expansion valve out of my 73 firebird, and according to the book, when I have 70 PSI going into the valve, and I immerse the thermal bulb into 125' F water I should get a reading coming out of 43-55 PSI. I get about 64 PSI. When I immerse it into 32' F I should get 20-24 PSI, and I get 20 PSI.

So my question is the fact that I am getting more out of the high side when it is heated, is that okay? Obviously the valve isn't binding or sticking, and hasn't lost its charge.

Edited: Sat September 12, 2009 at 11:06 AM by Karl

Karl on Sun September 13, 2009 6:25 PM User is offline

Anyone have any thoughts about this TVX? If I understand it's operation correctly, if the evap outlet is too cold, freezing up, the TVX limits the gas going into the evap, to help raise the temp, and if it's too warm it allows more refrigerant in to lower the temp right? So if it is letting more in at 125'F it should be working better?

Thanks for any help or clarification!!

mk378 on Sun September 13, 2009 11:31 PM User is offline

A TXV does not prevent evaporator freeze-up. It does not attempt to regulate the evaporator to any specific temperature.

It senses the pressure and temperature at the outlet of the evaporator to get the evaporator as cold as practical, but also be sure that complete evaporation has occurred. If un-evaporated liquid starts leaving the evaporator, the compressor could be damaged. When evaporation is complete, the temperature measured by the bulb would be higher than the boiling point of refrigerant at the evaporator outlet pressure. This is called superheat. A small amount of superheat is necessary to protect the compressor, but excessive means the evaporator has too little refrigerant flowing to it and BTU capacity will be decreased.

At 125F the saturated pressure is much higher than 70 psi, so the valve should be wide open. The exact pressure attained may vary depending on the test setup and restrictions in it. I think the valve is working well enough to consider trying it in the system.

Karl on Sat September 19, 2009 3:48 PM User is offline

I did try it, after talking with two other people, one being from a company that restores these valves, and they all agreed that it sounds like the valve is working fine, just letting more pressure through than what the book specified. Which is what I thought, but didn't want to waste R-12 to just try it and find out that that is bad for it to let more through.

So like I said, I just put it in Thursday night, and replaced the POA Valve, and Receiver/Drier, and then pulled a vacuum for 10 minutes, shut down for 15 minutes to check for leakage and it held, so then I pulled vacuum again for 1 1/2 hours to make sure everything was good. Let it sit over night and still had 30" vacuum. !! Put five cans of R-12 in and it works great.

Thanks for your input on it also!!

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