Automotive Air Conditioning Information Forum

manifold gauges are not truly dependent on the refrigerant you are using. The only thing that you need from a gauge is the psi. So, any manifold set will do, if they haven't been dropped, damaged, etc. Be sure your gauges are zeroed in, there is an adjustment for that using a screwdriver once you pop the clear cover or unscrew it

I am glad you understand what I was trying to explain

tbirdtbird wrote: ↑Wed Jul 20, 2022 10:42 am manifold gauges are not truly dependent on the refrigerant you are using. The only thing that you need from a gauge is the psi. So, any manifold set will do, if they haven't been dropped, damaged, etc. Be sure your gauges are zeroed in, there is an adjustment for that using a screwdriver once you pop the clear cover or unscrew it

This^

Another thought I would like to add is that the pressure and degree scales are calibrated with one another and will appear accordingly on the dial. With multi-refrigerant Manifold Gauge Sets, more than one degree scale will be shown.

aribert wrote: ↑Wed Jul 20, 2022 9:50 am 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.

As long as the ends of the R-134a hoses connected to the quick-couplers have female 1/4" flare fittings, they should easily connect with the male R-12 test ports. If not, you'll need adapters to make it work.

aribert wrote: ↑Wed Jul 20, 2022 9:50 am 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).

If the evaporator starts to freeze-up at pressures below about 30 PSI, the binary switch should cycle the compressor off for a short time if it's calibrated correctly. It says that the switch is designed for R-134a in one of your photos, but that's not to say it wasn't re-calibrated for R-12 at some point. If not, the compressor will cycle off too late, reducing the evaporator temperature to below 32°F and increasing the likelihood of evaporator freeze-up.

aribert wrote: ↑Wed Jul 20, 2022 9:50 am 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.

Be careful that you don't overcharge it, though. Wear good PPE while you're working on it: Shatter-proof eye-wear, long sleeves, and sturdy gloves. An unexpected, sudden release of refrigerant and oil due to over-pressurization and/or a burst hose can freeze and severely damage eyes and exposed skin.

Several issues can cause the pressures to be "off." If the low side stays abnormally low, the TXV could be suspect, or it could be partially blocked by desiccant beads. An abnormally high high-side pressure is usually the result of a condensing issue—that is, lack of airflow across the condenser.

Today I got back to A/C trouble shooting. I started off with my newer 134a gages. I was able to unscrew the quick release end details from the H & L press hoses and connect them to the R12 system. THe fill hose had an acme thread and I did not have a suitable adapter so I was unable to fill with this gage set.

Engine off, warm engine compartment, 80F ambient the gages read 100 psi. I started the engine and after about 2 to 3 min run time (I was preoccupied with getting the idle speed correct) the gages read L: 24 psi, H: 210 H fin temp 49F (I had not secured the 2nd T-couple to the vent yet). After an additional 5 min of run time the low pressure dropped. L: 15 psi, H: 215 psi with fin temp of 40F and vent temp of 44F. After and additional 5 minutes the pressures were stable at L: 14.5 psi and H: 225 psi fin 38 and vent 44. Does this imply a constriction / blockage? I had noticed a similar drop days ago but ignored it because I thought I might have read the gages wrong - this time I was watching.

Shut the engine off and after a few minutes opened the valves on the gage set to equalize pressure. initially 105 psi. After an additional 10 minutes 96 psi and after 20 min 93 psi on the gages. Ambient was still 80F.

Connected my old R12/R22 gage set. Engine off system press was 91 psi.

After 2 min of run time L: 25 psi, H: 200 psi fin 43F and vent 48F. After an additional 3 minutes L: 19 psi, H: 160 psi with a minor needle flutter (+/- 2 psi). After an additional 5 min L: 15 psi, H: 180 (flutter gone), fin 41F, vent 46F.

I began to increase the charge. L: 18 psi, H: 235 psi, fin 39F, vent 45F. L: 19.5 psi, H: 275 psi, fin 39F, vent 45F. L: 21.5 psi, H 310 psi (high pressure flutter restarted , now +/- 5 psi). At this point I stopped adding charge.

A/C still does not operate via the pressure switch. Voltage at the pressure switch connector is only 11.8v (Vbatt is 14.2 engine running)

Shut down the engine for a few minutes and restart. After 2 min run time L: 32, H: 275 (+/- 10 psi flutter) 51F vent. After an additional 2 minutes of run time L: 27, H: 290 (small flutter) 50F vent. After an additional 3 min of run time L: 22, H: 295 (small flutter) 46F vent. Finally after an additional 5 min run time L: 21, H: 295 (small flutter), 44F vent, resistance across pressure switch 1705 ohm. I monitored the resistance of the pressure switch after shut down and the resistance stayed in the 1700 to 1800 ohm range until the pressure was almost equalized and the resistance was then 18 ohm.

A lot of reading to get here. Constriction in the system causing the low pressure to drop about 10 psi over time? I would expect the pressure switch would be more digital (on/off) and not have such a great difference in resistance.

The system test pressures you're seeing, as well as the vent temperatures, still seem erratic and "off," which bring me back to questions about the binary switch. I can't tell from the photo on Page 1, but I presume that it's mounted on the high side, on or near the R/D—is that correct?

You said earlier, "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)."

The markings on the switch don't make sense to me, either, with the exception of "U.A.C." and "R-134a." I'm not sure what "HP 28" and "LP 2" mean. Surely they can't denote the cut-off and cut-on pressures for R-134a. I would expect those pressure points to be in the neighborhood of 400 PSI for the cut-off and 27 PSI for the cut-on.

I suggest contacting Tim at ACKITS.com, this site's sponsor, for a suitable binary switch for your application, change it out, and then re-check the pressures and vent temperatures.

Does the existing switch have a Schrader valve under it so that you don't have to recover the refrigerant again?

John I wonder if those units are bar.
2 bar = 29 psi
28 bar =406 psi

Good observation. Sure looks like it. Who uses Bar in this country? Certainly not me.

So now we can presume the markings are correct. The questions are, Is the switch any good, and if not, is it responsible for the wacky pressure readings?

I still can't reconcile the various low-side pressure readings and the vent temperatures, though.

JohnHere & tbirdtbird:

Thank you both for still following this thread.

"Does the existing switch have a Schrader valve under it so that you don't have to recover the refrigerant again?" I have no idea, never knew that some filter/dryers had a Schrader valve under the pressure switch. I think the switch was preinstalled. I hope there is a valve. Tomorrow I will loosen the bracket that secures the filter/dryer in hopes that I can find the Parker Hannafin P/N and in tern find out if the Schrader valve is present under the pressure switch.

"I suggest contacting Tim at ACKITS.com, this site's sponsor, for a suitable binary switch for your application, change it out, and then re-check the pressures and vent temperatures." Will do - I think I will also buy a spare filter/dryer and expansion valve. **If** I have to open up the system to replace the pressure switch then I would replace all three.

"...the binary switch. I can't tell from the photo on Page 1, but I presume that it's mounted on the high side, on or near the R/D—is that correct?" Yes, mounted directly to the filter/dryer.

"The system test pressures you're seeing, as well as the vent temperatures, still seem erratic and "off," Is normal that a system shortly after start up will have the low pressure drop for the first 5+ minutes - I don't recall seeing this before in my limited A/C servicing. Before adding more R12, the Low side was at 24 after 2 minutes and slowly dropped to 14.5. After adding more R12, the low side was at 32psi after 2 minutes and dropped to 21psi. Is this common/normal or does it imply a restriction at the expansion valve?

BTW, one of you had asked about bubbles in the sight glass - looked for but never seen any with the system in operation. Yesterday morning I drove the car from my residence to my workshop garage (about 10 miles, no A/C use) and after opening the hood I saw an occasional bubble in the sight glass on the filter/dryer. Bubbles slowly migrating in the direction of flow - guessing heat in the engine compartment was slowly moving the refrigerant. As soon as the compressor was running - no bubbles.

On an R12 system, with the system running, you add until the bubbles clear and then 2 more oz. Is that what you did (see John's comment July 19).

aribert wrote: ↑Sun Jul 24, 2022 11:30 am "Does the existing switch have a Schrader valve under it so that you don't have to recover the refrigerant again?" I have no idea, never knew that some filter/dryers had a Schrader valve under the pressure switch. I think the switch was preinstalled. I hope there is a valve. Tomorrow I will loosen the bracket that secures the filter/dryer in hopes that I can find the Parker Hannafin P/N and in tern find out if the Schrader valve is present under the pressure switch.

That's a good thing to know about should you ever again have to replace the binary switch. Lucky for you if the Schrader valve is there. If it's not there, a brass fitting is available in the market that goes between the R/D and binary switch that contains a Schrader valve. It'll save a lot of work in the future to install the fitting now

aribert wrote: ↑Sun Jul 24, 2022 11:30 am "The system test pressures you're seeing, as well as the vent temperatures, still seem erratic and "off," Is normal that a system shortly after start up will have the low pressure drop for the first 5+ minutes - I don't recall seeing this before in my limited A/C servicing. Before adding more R12, the Low side was at 24 after 2 minutes and slowly dropped to 14.5. After adding more R12, the low side was at 32psi after 2 minutes and dropped to 21psi. Is this common/normal or does it imply a restriction at the expansion valve?

Pressure drops like that aren't normal. Before looking into the TXV, are you sure that your testing apparatus (Manifold Gauge Set, hoses, etc.) aren't leaking someplace? The most common leakage points are at the hose ends. If all those are okay, an aftermarket TXV (instead of an OEM part) would be next on my "suspects" list.

Thanks for contacting us. I'm going to need an image of the TXV valve. It could be a standard universal. But an old under-dash system like that might have something different.

R12 is harder to find these days. I may be able to find a cheaper Drier. Is this the type of drier you are looking for?

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