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Diagnose Faulty Expansion Valve

Posted: Mon Jun 03, 2019 8:57 pm
by Wren
I'm hoping that someone can help me diagnose what appears to be a faulty expansion valve in a 1990 Volvo 240.
The car was converted over to r-134a about 8 years ago and worked great until it developed a hole in the condenser. I replaced the condenser as well as the drier and vacuumed the system down for an hour and then verified that it held vacuum for a couple of hours. I added the recommended 2.2 lbs. of r-134a and got the gauge readings in the photo.
The inlet line to the expansion valve is getting frosty while the outlet of the evaporator is staying at near-ambient temperatures. It was 90° in the shop today.
From everything that I've read, it sounds like the expansion valve isn't allowing refrigerant to enter the evaporator at the correct rate.

Any advice would be appreciated.

Thanks very much.


Re: Diagnose Faulty Expansion Valve

Posted: Tue Jun 04, 2019 10:51 am
by JohnHere
Hello, Wren. Usually, a vacuum reading on the low-pressure side does indeed indicate that the TXV is defective (not opening enough, if at all) or there is some kind of restriction in the high-pressure line feeding refrigerant to the TXV, such as the old receiver/dryer whose desiccant bag has broken open and sent silica-gel beads downstream, blocking the line just before the TXV, which subsequently went unnoticed.

Also of concern is the HP reading of only 130 PSI, suggesting a compressor that might not be pumping up to snuff. With a 90-degree ambient temperature, you should be seeing an HP reading of 225 psi or so.

To troubleshoot this, I'd recover the charge and check that the HP line to the TXV is clear. Remove the line, if necessary, and run an old-style flexible speedometer cable through it to ensure there's nothing blocking it. Next, flush out the line with the proper solvent to ensure that it's completely clean of any residue inside. At the same time, I'd replace the TXV. Then reassemble, evacuate, recharge, and performance-check the system. If the same problems persist, I'd say that the compressor is probably defective and needs replacement.

You said that you recharged the system with 2.2 pounds (35 ounces) of R-134a, which is about 75 percent of the original R-12 charge of 46 ounces, which sounds about right. Although you could have charged it to about 37 ounces of R-134a for an 80-percent charge, I don't think the slightly lower charge that you gave it would cause the problems you're experiencing.

Re: Diagnose Faulty Expansion Valve

Posted: Tue Jun 04, 2019 11:06 am
by Wren
Thanks, John. I'll follow your suggestions. I replaced the drier, but as you mentioned, there could be some blockage coming from the desiccant in the old drier.

The 2.2 lb. charge was my best charge guesstimate (and what the Volvo conversion data sheet specifies) as I fitted a parallel-flow condenser to help temps with the r-134a conversion, so I wonder if the system capacity might have increased and needs a larger charge.

Re: Diagnose Faulty Expansion Valve

Posted: Wed Jun 05, 2019 1:24 pm
by JohnHere
It's possible that it might need a little more refrigerant, but with these conversions, it's hard to tell what the exact charge should be. Still, I don't think the refrigerant amount is your problem unless by some fluke the charge you put in it has leaked out. Even if the charge was a little low, your pressures would at least be in the ballpark. Right now, they're far from what they should be. It's possible that you have multiple problems: A blockage, a malfunctioning TXV, and a compressor that has failed internally. The suction line from the evaporator staying at ambient temperature with the system running tells us that refrigerant isn't making it through the evaporator, as you suggested. The low high-side pressure points to a compressor problem. Of course, you won't get any cooling, either, with any or all of this going on.