Low side too high

[Jima]

Greetings,

I have very little experience with ac systems and am being helped by a buddy who has some. We're working on my '78 Lincoln Continental system with POA valve. System was converted to R134a by a previoius owner. Suspicious that that POA valve had not been adjusted for R134a, I contacted Classic Auto Air to get them to rebuild and calibrate it for R134a. They advised me to replace the POA valve with a POA Update Kit from Old Air Products. Did that and now when trying to recharge the system the refrigerant is only going in extremely slowly, not even cooling the can. Low side pressure is around 130 and high side around 150 PSI. System was vacuumed for an hour before the attempt to recharge.

Did lose an ounce or two of foamy oil while installing the POA Update Kit. Plan was to add some back in, along with leak detector, after the first can of R134a but didn't get that far.

What should I do next? Maybe the orifice is clogged? Thanks for any help.

Jim

[JohnHere]

Before delving into it more deeply, check the simple stuff first. Ensure that your quick-connect port adapters are opening the vehicle's Shrader valves fully. If you have the type of adapters with the red and blue thumbwheels, turn both thumbwheels fully CLOCKWISE to open the Shrader valves.

Next, with the engine running at fast idle, blower on high, windows open, and the refrigerant source hooked up with the container's valve open, proceed to open only the blue low-side handwheel on your manifold gauge set (the red high-side handwheel must remain closed) and the refrigerant should then begin to flow in easily unless there's something else going on. It's easy to forget that rotating the thumbwheels counterclockwise will close the Shrader valves.

In addition, always use an accurate scale to dispense the amount of refrigerant the specs call for...in your case, charging with R-134a will require about 80 to 85 percent of the original R-12 charge.

[Jima]

Before delving into it more deeply, check the simple stuff first. Ensure that your quick-connect port adapters are opening the vehicle's Shrader valves fully. If you have the type of adapters with the red and blue thumbwheels, turn both thumbwheels fully CLOCKWISE to open the Shrader valves.

Next, with the engine running at fast idle, blower on high, windows open, and the refrigerant source hooked up with the container's valve open, proceed to open only the blue low-side handwheel on your manifold gauge set (the red high-side handwheel must remain closed) and the refrigerant should then begin to flow in easily unless there's something else going on. It's easy to forget that rotating the thumbwheels counterclockwise will close the Shrader valves.

In addition, always use an accurate scale to dispense the amount of refrigerant the specs call for...in your case, charging with R-134a will require about 80 to 85 percent of the original R-12 charge.

Thanks for the reply.

The gauges have quick connects -- interesting question on Shrader valves. Not sure how to know if the Shrader valves are open all the way.

The procedure for adding refrigerant you describe is what we did.

We're using 12 oz cans of R134a. We've added 1 and 1/2 cans so far.

The high pressure line coming off the compressor is hot so I guess the compressor is working. High and low side pressures are pretty close to R134a vapor pressure for temps of the lines at the connectors and there's frost on the expansion valve. Could all this be caused by insufficient refrigerant?

[JohnHere]

The pressures you originally posted--about 130 PSI on the low side and 150 PSI on the high side--are they static pressures (system turned off and compressor not running) or are they the pressures you get with the system and compressor running?

[Jima]

The pressures you originally posted--about 130 PSI on the low side and 150 PSI on the high side--are they static pressures (system turned off and compressor not running) or are they the pressures you get with the system and compressor running?

That was with the engine at 1500 RPM, windows open, AC on, compressor running, fan on high. .

I got a recommendation for a tech that understands these old systems. Have an appointment with him on Wednesday of next week. Too much I don't understand about what's going on. Only guess I have is the whole system is still hot enough so it contains mostly vapor and some vapor is blowing past the expansion valve, cooling the expansion valve but not much more of the system. The low pressure is about right for the vapor pressure at the temp of that line. Don't see how the refrigerant is going to go in when the temp of the suction line is at least as high as the temp of the supply can.

I'll let ya'll know what I find out.

Jim

[JohnHere]

The POA Update Kit replaces the POA valve with what amounts to a straight-through tube and a pressure switch, which is calibrated to turn the compressor clutch on and off to prevent evaporator icing. The original POA valve regulates evaporator pressure by means of internal valving to prevent evaporator icing while the compressor runs continuously. Different methods to achieve the same result.

At this point, I suggest double-checking to ensure that you installed the POA Update Kit correctly. If so, I think, then, that the TXV must be malfunctioning. That is, the valve's orifice is staying wide open (or nearly so), causing the low-side pressure to skyrocket. Since you weren't able to get the full charge in, it's still a bit undercharged as well given the rather low high-pressure reading.

You're right, of course...no more refrigerant will flow in when the system's low pressure equals the pressure in the can.

I think it's good that you found a pro to take a look at it. Let's wait and see what he finds.