2000 Mitsubishi Montero Sport

[Mark86]

Thanks for continued interest.

The three TXV’s look identical in every manner, except for the ink stampings on the power head cap. It could be that these are all made by the same manufacturer. As mentioned I’ve run into OEM parts that are suspect to repackaged aftermarket.

The TXV must have a minimum flow. It can never just close or shut down flow, and that seems to be the issue.

I think the bulb is sending a cold signal, and lifting the needle, moving the seat up, and reducing flow, too much.

[JohnHere]

The TXV must have a minimum flow. It can never just close or shut down flow, and that seems to be the issue. I think the bulb is sending a cold signal, and lifting the needle, moving the seat up, and reducing flow, too much.

You are correct. A TXV doesn't completely shut-off the flow. It's a throttling device. Your valve might be affecting the refrigerant flow, possibly due to a false signal, as you mentioned. It could also be the bulb itself due to the gas having leaked out, which is often hard to determine. As a first step, have you taken another look at the sensing bulb, its mounting, and insulation?

[Mark86]

I don't have my reference to the OEM bulb insulation technique other then an OEM condesor assembly I viewed on EBay. There is no mistaking how the bulb mounts, as there is an indention for the bulb to seat. There is a metal clip (OEM) to attach the bulb. All parts are new so there is no residue blocking metal to metal contact.

Then I used this to insulate the bulb.
https://www.aircraftspruce.com/catalog/ ... ey=2018844

I think in this case, it would perform better with no insulation.

My next step is to remove the evap and turn the set screw CCW a full turn, reinstall and test.

[JohnHere]

TXV20230318_161510.jpg (249 KiB) Viewed 2991 times

I don't have my reference to the OEM bulb insulation technique other then an OEM condesor assembly I viewed on EBay. There is no mistaking how the bulb mounts, as there is an indention for the bulb to seat. There is a metal clip (OEM) to attach the bulb. All parts are new so there is no residue blocking metal to metal contact.

Sounds good, Mark.

Then I used this to insulate the bulb.
https://www.aircraftspruce.com/catalog/ ... ey=2018844
I think in this case, it would perform better with no insulation.

It appears the material you cited would work, but I'd try Prestite™ just to be sure. At least you wouldn't have to recover the system again. With no insulation at all, it'll get a false signal because the bulb will pick up heat from the engine compartment, throwing it off.

My next step is to remove the evap and turn the set screw CCW a full turn, reinstall and test.

The pictured TXV is one that I have on hand. If yours looks similar, the smaller male threaded end on the brass body (opposite the diaphragm) contains a female Allen-head screw inside that allows adjusting the tension on the superheat spring, which in turn, changes the operating pressures. If you do change the factory setting, ensure that you accurately count and write down the turns or partial turns—and whether you turn the screw CCW or CW—so that you can return to the factory set-point if necessary.

[Mark86]

That is in deed the valve. I disassembled the valve I first removed to examine it. I’ll post pics if I can. A seat, a spring and the Allen set screw.
Function:
Bulb Sense Warm - Bulb refrigerant expands and drives diaphragm down, which drives needle down. The needle moves to push the seat down, allowing more flow.

Bulb Sense Cool - The refrigerant liquifies, contracts, and diaphragm raises, raising needle, seat moves up and restricts flow.

Equalization tube, provides a sense pressure to diaphragm.

Spring applies pressure to seat, which is in contact with needle. More preload on spring, means more static force on seat/needle, which in turn then allows the needle and seat to rise (restricting flow) with less signal from sensing bulb. Ie. Less bulb pressure to close needs/ seat.

Reducing spring pressure will require more signal from the sensing bulb (bulb must be colder) to close the seat.

[DetroitAC]

That's a pretty close description to how they work.

I would describe it's function as a mechanical superheat calculator coupled to a spring that provides a bias, the whole contraption controlling a metering valve.

It's easiest to explain a TXV that has a refrigerant charge in the dome that is the same as the refrigerant being used, so that's what I'll explain with R134a

There is a small amount of R134a refrigerant in the dome/capillary/sensing bulb, it is gas and liquid in equilibrium (saturation), there is not enough liquid to fill the volume of these parts, with a TXV that has a MOP (Maximum Operating Pressure) there is just enough liquid so that at higher pressures/temperatures, all the refrigerant can exist as a gas. For most "normal" operating conditions, the refrigerant is a gas/liquid mixture and behaves per any R134a PT chart.

The dome contains a flexible metal diaphragm, corrugated stainless steel usually, that has a socket welded to it in it's center to accept the pin, perimeter is welded to the topside and bottom side around all three parts biggest diameter. The flexible disc allows the pressure on it's bottom side and pressure on it's top side to put a net force on the top of the pin.

The pin pushes down a ball and cone seat throttling valve (or might be a machined cone/cone for commercial TXV). The force to push down the pin and therefore the ball comes from an imbalance of pressures (and therefore forces) from each side of the diaphragm.

The top of the diaphram is subjected to the pressure equal to the saturation pressure of R134a, but for the temperature of the evaporator exit flow.
The bottom of the diaphragm is subjected to the pressure of the evaporator exit flow (in your case, because there is a sensing tube that communicates this pressure to the bottom of the diaphragm).

When a system with a TXV first starts operating, the TXV is closed. This is because there is no difference between the temperature (and therefore pressure) of the evap exit and saturation temperature/pressure at the evap exit. Since there is no difference in pressure on the diaphragm, the pin cannot overcome the preload force of the bias spring, valve stays closed.

Once the system starts up, the suction pressure starts dropping. The evap exit temperature does not drop because no refrigerant is being fed, and here a difference in pressures/forces begins to happen. The sensing bulb/dome will stay at a warm temperature/pressure as the pressure on the underside of the diaphragm drops. Since there is now a downward force on the pin, the spring is compressed, the valve is pushed open, refrigerant flows.

The TXV comes into an equilibrium quite quickly where the force that is pushing down the bias spring/metering valve is equal to the force coming from the diaphragm/pin. The TXV then operates to keep a mostly constant SUPERHEAT, i.e. a difference between the evap exit temperature and the evap exit saturation temperature. I say mostly because it is not a constant control device, there are a lot of influences, but for the most part if the bias screw is adjusted to get an operating superheat of 10 degrees F at maybe 1/2 load (like blower at 50%) then it will be in the same ballpark at full load and at 1/4 load.

TXVs are in reality way more complicated than I've described, I've tried to keep it as a TXVs 101 lecture.

I hope this explanation helps, and as a side note you can't see a TXV move on the bench unless you can put pressure in the sensing port, there is a pressure difference of ballpark 50-150 psi across the diaphragm!

[Mark86]

Spring, Seat and set screw

[Mark86]

Thanks for the detailed description Detroit.

[Mark86]

Outlet

[Mark86]

Inlet