Yup. R22 is running about $30/lb so it's not the end of the world but it's days are numbered. Going the same path as R12.

And R-134

I've got about 10 lbs or R22 left over from years ago. I called the guy who did repairs about 10 years ago and he doesn't work on heat pumps anymore. He suggested that I may end up replacing it for lack of parts and lack of someone willing to work on it. If I have to, I have to, but not without giving my best effort to fix this one. This old Econar has a lot of advantages over the units that are available today. I hate to scrap it and take a step backwards over 1 little part. I'll be making a bunch of calls on monday.

Does your “parts unit”
have that same valve?

If so
measure the ohms of resistance between the two electrical spade lugs,
and from each of the 2 lugs to the metal body of the two valves.

The web link TJP helpfully gave you has the Troubleshooting on page 7.

From what you have written,
and what is on Page 7,
there is still a good chance the root of the problem is in the control board electronics or bad wire insulation.

Maybe do an internet search to see if any Forum somewhere contains Econar geo heat pump owners.

the resistance may vary between a charged system and one that is not
I would also think if the problem was on the board the fuse would blow whether the switch/valve was hooked up or not.
I do like the idea of checking the resistance between the each of lugs and the switch body.
If they are both open to the switch body, the check between the two lugs to see if they are open or shorted to each other.

One question that is important. If the lugs are not grounded to the switch body, can you hook up the wire FROM the control board while leaving the other disconnected and not blow the fuse?

I am thinking that is a "switch" that goes open or closed depending on the pressure it senses. By doing so it then turns on or off a relay or two that controls other components. 2 amps is not a lot so whatever is connected is not drawing a lot of current.
BTW I am not an A/C guy so i am flying by the seat of my pants on the above suggestions

i flew one time by the seat of my pants, by standing on the top of a step ladder where it says : "DO NOT STEP HERE"
my old man told me what would happen.
i couldn't flap my arms fast enough to have a three point landing................

I did a little more testing. The valve has 4.8 ohms of resistance across the terminals. There is no continuity from either terminal to ground.
I checked out the parts machine and it does not have an electric valve. It has one with a small tube and a bulb
But I actually have two of these heat pumps in my house, one for each end. So I shut off the operating pump and checked the resistance between terminals on that one and found 72 ohms. So at 24 volts, how many amps would flow through 4.8 ohms of resistance? I'm going to guess more than 2 amps.

Edit: I just found an online calculator. 5 amps. That's enough to blow the 2 amp fuse on the circuit board. I also checked the amps on the wire going to the valve on the unit that is running. That was .24 amps

About 5 amps

I'm going to do a test and see what happens if I run the unit with that valve disconnected. Since the thermistor has been bypassed and that has been running with that valve wide open, I don't know what the position of that valve is without any power to it and how the heat pump will run. I have nothing to lose by trying it. Also I'm going to fiddle around with that valve a little bit and see if maybe the insulation around one of the terminals is worn away or something.

Okay I ran it with the electric expansion valve disconnected. When the compressor started, it drew about 16 amps which sounds about right. But it only ran for about 30 seconds and then it shut off and the freeze protection light on the circuit board came on. I ran it through this sequence twice and it did the same thing. I'm not really sure how this freeze protection thing works. There is a T fitting on the suction line that has both the low pressure sensor and the freeze protection sensor on it. Each one is connected to a separate terminal on the circuit board. Attached is a picture of the freeze protection sensor.
I want to check the water flow, but my flow meter is unreadable because of a rust buildup on the inside of it. I'm out of time for now, but tonight I'll take that apart and clean it out and see how the flow is doing.
Does anybody know how this freeze protection sensor operates?

Sadly, sounds like the little heater element inside the expansion valve is malfunctioning.

The specs in the technical info web link list 70 ohms like your good valve has.

It might be low on refrigerant. Lines will freeze up when low. Low charge might be why the resistance is too high on valve and it's meant to blow fuse to protect system.

How would I check the refrigerant level?

You need someone with a set of gauges to check the pressure and know how to get it charged enough not to trip. I remember the old GM cars would have a fuse under the hood that would blow and keep the compressor clutch from working when they were low. With a 12oz can it was impossible to get it charged enough without the compressor running to keep the fuse from blowing. Our fix was to jumper out the fuse just long enough to fill system. I don't believe I would trust that on my home unit but a repair man might???

Just a few weeks ago I had to have my 40 year AC charged because it was freezing up. It had worked fine all summer but the last hot wave overworked it. It took 2.5 #'s of 438A which works with the existing R22 and old oil. They used pressure and temp on lines to verify a full charge. $150 service call and $100 for 438A. Plus $37.50 to replace schrader valve (tire stem) lol.that was probably the problem from slowly leaking for years.

I have a set of gages and some r22. But a long time ago an HVAC Tech told me that the gages don't really tell you how much refrigerant is in the system. That the temperatures in the system system effect the pressure. Is there a way that I can interpret the pressure to determine if it needs refrigerant? I can bypass the freeze protection to run it enough to pull some more refrigerant in.

maybe there is a chart somewhere in the interwebs similar to the temperature/pressure chart used in the FSM's to verify the R12/R134 charge in a vehicle's ac system ?

Since the 2 spade lugs power a very small electric heater strip inside the valve,
with the heater causing a metal strip to expand and “pull” a needle valve more open
I have wondered if you could use a variable temp heat gun
aimed at the top of the valve between the spade lugs
to warm it enough to let R-22 flow some
and keep the “freeze protection circuit” from triggering from too little freon flow.

But it would be better to just change the valve.

If the valve were open too much liquid droplets of R-22 might get into the compressor,
and do damage like liquid coolant can do to an IC engine piston top.

On the other “working unit” you have
you might measure the temperature of the metal below the spade lugs
when working
and then “dead cold” after the working unit has been off one hour.

You are going to need a properly functioning TXV in order to run the system long enough to check for the proper charge. Your best bet is to pull the charge R22, replace the TXV then leak test the system with dry nitrogen. If no leaks are found then pull a deep vacuum on the system. After you pulled a vacuum you can add the R22, there should be nameplate on the unit showing the lbs and oz the system holds.

You can weigh it in ( special tools required) or slowly add it in through the gauge set. If you are unfamiliar with this you can contaminate the system with non condensable gas(air), and then you need to start all over again.

You need to make sure your pressure temperature relationship is correct and not over charge the system. Overcharged = slugged compressor = game over broken compressor!

I don't think so. The tech had a clamp on temp meter similar to a clamp on amp gauge. Might be time to call a tech. Might save you time and money. No need to replace a valve if it's doing it's job.