Working on fire engine AC (again)

[bohica2xo]

Is this a block type TXV with both the liquid & suction line running through it?

[Dgreen]

Yes, it is.

[bohica2xo]

Some block type units are adjustable. Since this is a fairly large system (TM31 compressor) It may be.

Adjustable or not, your low side pressures are controlled by the TXV.

[Dgreen]

I recently replaced the txv. Still have the old one. Will look for an adjustment. If it isn’t adjustable, could the txv be insulated from the evaporator to get it to supply more refrigerant?

[bohica2xo]

Block valves sense temperature directly from the suction line, so insulating it will have no effect.

The adjustment will be under the cover on the end opposite the sensing diaphragm. In theory they are all adjustable valves, but some are easier than others to access.

What sort of evaporator design is this unit?

[Dgreen]

It’s mounted in the cab on the inside of the roof. It is in a plastic case. There are flexible vent hoses in the case that direct the cool air to the front and rear of the cab. It is a crew cab and it’s a pretty large area.

[Dgreen]

Does this look like the adjustable variety? If so, would I screw it in or out to decrease the superheat?

[Dgreen]

This is a pic of where the expansion valve goes. When all four hoses are connected, it is held up close to the fins of the evaporator. This is the side where the warm air is pulled in.

[bohica2xo]

Wow. I would have expected a larger valve with that large of a system. That is a replacement valve for a 924 Porsche.

Backing it out reduces seat pressure, and increases refrigerant flow.

The diaphragm motor only has so much stroke. You could put that disassembled valve body in the freezer to see it in action. At room temperature it is stroked down as far as it will go to open the valve. If you freeze it the stem will retract to the valve closed position.

I wonder if that valve has enough capacity even wide open to serve a large evaporator.

[Dgreen]

Any idea how far it will need to be backed out?

I got the expansion block at a local parts store. After the guy at the store saw the condenser and block, he said it looked like R-12 technology to him.