Pressure switch issue?

[aribert]

61 Ford Falcon
64/64 Mustang underdash unit
11x20 parrallel flow cond
Sanden 508 comp
Parker filter w/ U.A.C. GY209 press switch (see images)
R12

First post (for the last decade or so - my 4 digit password no longer worked).

I installed this system in '13 though I started gathering some A/C parts as early '83. After buying an older F150 that had suffered Black Death and working thru repairing that, with much information gleaned from this forum, I finally did the A/C in my Falcon. Only 18,000 miles since the A/C was installed - probably about 25 to 30% of the driving with the A/C on. Suddenly the A/C does not work unless I jumper the pressure switch. Filter is a Parker and I presume the switch was preinstalled. Current switch is for 134A not R12. I am guessing the switch has both a high and low limit based on the numbers. There appears to be continuity across the switch - using a multimeter I read about 45 ohm - guessing its not passing current? How do I diagnose the problem? If the switch is bad, I presume the system must be evacuated to replace - correct? I've never had a system evacuated - will the evacuation also remove the oil in the system? I have a virtually empty 20lb R12 jug, would it be common for the evacuated R12 to be put into my container? If the switch is bad - how do I go about sourcing a replacement - what specs / criteria? I would presume this is something I could source from the forum sponsor.

As background: R12 system. Gages connected to ports on the comp. 85F amb about 60% RH. 1500 rpm gages read: 10 to 10.5 psi suction port, 270 to 275 pressure port. 38F with thermometer in the fins 1.5 hrs later, 88 F amb, engine compartment still warmer than amb, opening the gages to equalize pressure = 95 psi. Driving, about 88F amb, max A/C, surface streets, 30F in fins with just a light frost sheen on the copper tubes in the evap.

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[JohnHere]

First, I presume that you're referring to R-12 and not R-22—correct?

[aribert]

YES - thank you for the correction. Now to see if there is an edit function. Now the OP has been corrected to R12. I have both obsolete refrigerants on hand and am bad with nomenclature.

[JohnHere]

YES - thank you for the correction. Now to see if there is an edit function. Now the OP has been corrected to R12.

Ahhhh...okay

Only 18,000 miles since the A/C was installed - probably about 25 to 30% of the driving with the A/C on. Suddenly the A/C does not work unless I jumper the pressure switch.

IMHO, I recommend not jumping anything because if you force the compressor to run, it might starve for oil and ruin it.

I am guessing the switch has both a high and low limit based on the numbers.

That's correct. It's called a binary switch.

There appears to be continuity across the switch - using a multimeter I read about 45 ohm - guessing its not passing current? How do I diagnose the problem?

If the system has a low charge, which is what I suspect, the switch is doing its job. With the ignition and the A/C on, do you have 12-13 volts on one of the wires leading to the switch?

I've never had a system evacuated - will the evacuation also remove the oil in the system? I have a virtually empty 20lb R12 jug, would it be common for the evacuated R12 to be put into my container?

I think what you mean is "recovered," which is different than "evacuated." Regardless, I believe you'll be hard pressed to find someone who can recover the R-12 because most shops have phased out their old R-12 machines in favor of those that can handle R-134a, and more recently, R-1234yf. Your empty 20-pound R-12 cylinder isn't really designed for recovery, but it could be done with a little ingenuity and the right set-up.

Recovery will remove some oil while evacuation will not. A typical RRR machine has a graduated cylinder that measures how much oil comes out so that the same amount of oil can be put back in.

If the switch is bad - how do I go about sourcing a replacement - what specs / criteria?

Rather than searching for a new switch, which it might not need, first ensure that you have the correct charge in the system. I suspect that it's just low on refrigerant due to a leak. With a car this old, it has a sight glass (S/G) on the Parker receiver/dryer. Add some R-12 to the low side via an R-12 Manifold Gauge Set until the compressor engages and you start to see some bubbles and/or foam in the S/G. Then proceed to "sniff" the system with an electronic refrigerant detector to find the leak(s).

After the leaks are fixed, re-charge the system by monitoring the S/G as before. Continue slowly adding refrigerant a little at a time until the S/G clears, then add an ounce or two more. That will result in a full charge, which should restore good cooling as the temperature measured at the center vent.

[aribert]

JohnHere:

Thank you for your prompt response. I also at first presumed that the system was low.

Yesterday I slowly added refrigerant until the low side was reading 14 psi and the temperature at the evap coil got progressively higher until the temp in between the fins was in the very high 40s F. I kept observing the sight glass and never saw any bubbles or foaming as I added refrigerant. At that point I saw no reason to continue and I bled the system back down to about 10.5 psi on the suction side and evap fin temps were back into the mid/upper 30s F. I'm pleased with the cooling ability of the system currently (if the switch is jumpered).

The low pressure value seems way low to me but that might be because I am thinking of more recent experience with 134a where I am typically in the 25 to 35 psi range on the suction side. Might there be a different problem that where the suction side pressure is low but the system still cools well?

I will check for voltage at the switch plug tomorrow morning. I expect to find voltage there since the moment the switch socket is jumpered, the compressor clutch engages.

Looking at the switch - are the numbers on the side of the switch (2,28) the pressure set points of the switch? If so, what units, since PSI does not make any sense to me (I would expect the high limit of the switch to be greater than 28 psi).

[tbirdtbird]

Vent temps of mid thirties means your core is going to freeze solid, because the core is even colder.
This is an AC, not a freezer chest. The low side of 10.5 psi will indeed give you low vent temps per the PT tables, and lead you to believe that nice cold air you are getting is a good thing. But at a low of 10.5 psi you have precious little mass flow and equally precious little oil return to lube the comp.

What was wrong with vent temps in the mid 40s? That is about where they should be.

[aribert]

Vent temps of mid thirties means your core is going to freeze solid, because the core is even colder.
This is an AC, not a freezer chest. The low side of 10.5 psi will indeed give you low vent temps per the PT tables, and lead you to believe that nice cold air you are getting is a good thing. But at a low of 10.5 psi you have precious little mass flow and equally precious little oil return to lube the comp.

What was wrong with vent temps in the mid 40s? That is about where they should be.

I am measuring in the fins of the evap next to a copper tube. Vent temp would be somewhat warmer but did not measure. Distance between core and vent is about 2.5 inches. Car does not have solar tint glass (I think that became standard on all cars in the US in the early '70s) and I tend to be too warm even with all vents pointed at the driver if the output is not cold. I don't have my notes from yesterday with me at the moment but **guessing** core temps in the mid 40s was about 12.5 to 13 psi. At 14 psi suction pressure, the core fin temp was 49 F.

So where should the theoretical suction / discharge pressure ranges be for an 85 F day, 60% RH with R12?

[tbirdtbird]

BTW we talk in terms of vent temps, because that is what the driver feels.
The R12 PT table tells us that at a pressure of 33 psi gives a temp of 35°, just a bit above freezing, to avoid turning the evap to a block of ice.
Of course, you could adjust the freon until you are at a psi of 1, which would give you a temp of -20. This might get you cold enough, but would only last a few minutes until the compressor grenades due to lack of oil, and the evap turns into a major block of ice.

It does not seem that you understand the consequences of a lo side pressure that is too low
Perhaps it would be beneficial to have window tint installed after all.

Here is a link to the R12 PT chart, see for yourself.
https://ssl.ca/wp-content/themes/sincla ... eChart.pdf

You were better off when you were in the mid forties

[JohnHere]

I don't have my notes from yesterday with me at the moment but **guessing** core temps in the mid 40s was about 12.5 to 13 psi. At 14 psi suction pressure, the core fin temp was 49 F.

When you say "core fin temp"—the evaporator temperature, as I understand it—something must be off with your guesstimated numbers. Looking at my Manifold Gauge Set for R-12 (low-pressure gauge), 12.5-13 PSI corresponds to an evaporator temperature of about 8°F, and 14 PSI corresponds to about 9°F. I'm perplexed as to where the mid to high 40°F temps are coming from. To reach those evaporator temperatures, the low-pressure side would have to be between approximately 42-45 PSI.

So where should the theoretical suction / discharge pressure ranges be for an 85 F day, 60% RH with R12?

Although certain variables can affect this either way, the "theoretical" pressures for R-12 and R-134a at an 85°F ambient will be close, with the low pressures corresponding to an evaporator temperature of just above 32°F.
R-12: L=30-32/H=170-190
R-134a: L=28-30/H=195-215.

[aribert]

tbirdtbird & JohnHere
Thank you both for still following this thread. On Saturday I will use my newer 134a compatible gage set - since I can remove the quick release 134a end connectors and should then be able to use the gage set for R12. Maybe I have a gage issue with my R12/R22 gages. BTW, I was also seeing a much higher pressure on the discharge side - 10.5 low and 275 high. I looked at the chart tbirdtbird attached and I saw the same thing that the core temp should have been much lower for the pressure shown on the gage. I would expect the core temp to be within say 10 deg F of the refrigerant temp. I am not quite freezing the core; though while driving, at medium fan speed and max temp, thrmometer in the fins reading 30 or 31 F, I could see a shimmer of frost develop on the copper tubes at the end of the core (at high fan speed the temp rises to 37 F). I will also have a second thermocouple/reader first to confirm that both are reading the same and to be able to monitor both fin and vent temp - with an underdash unit where I have direct line of sight of the core I expect the core and vent temps to be very close.

If I get similar results (10.5 psi low), I will increase the charge until the vent temp reaches mid 40s. If the low side is still too low and high side too high relative the ball park figures JohnHere posted - what would that mean? Expansion valve issue? System has not been opened since I installed the system in '13.

tbirdtbird:
I very much appreciate you explaining why I need to see higher pressure on the suction side for system durability.