A/C Diagnosis Help

I've gotten partway through diagnosing my A/C not working and before going down the rabbit hole further, I'd love a little advice on the next steps I'm taking. Before I loosen the airbox, there might be other things I can check.

Background
1979 308 GTS I've owned for 28 years
Used R12 until 2020 and was converted to R134a by my mechanic with 1.6 lbs of R134a
The conversion was just a new dryer and adding adapter ports to the compressor. Did not change compressor or fittings as far as I can tell

It has, at times, blown cold air so I know the system can work. But it doesn't work for long, which screams a leak.
In September 2021, my mechanic recharged the system for low R134a and did a leak check but didn't find anything. Evacuated system and added 1.76 lbs.
In July 2024, my mechanic saw low pressure, recovered the R134a and it was almost out but passed vacuum. He added dye, evacuated the system, and added 1.78 lbs.

So that brings me to today. I figure if the system is this finicky, I should just learn to fix the damn thing myself so I got a setup from HF of gauges, dye, vacuum pump, blacklight flashlight, and R134a.

Then the fun began as the true consequences of the R12->R134a port adapters became clear. I couldn't access the low-side port from the top and I can't access the high side port at all. But here we go down the rabbit hole

Car is currently on my lift with the wheel and wheelwell cover off.

First check - any charge in the system at all
I could get the low side port attached by dropping it from the top of the engine and attaching it from underneath.
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Car is off, ambient temperature is 93 degrees. Connected it with all of the valves off and then opened the valve at the compressor and got 104psi. Since the car isn't running, that's total system pressure not low side pressure. I think that tells me there is a reasonable charge in the system.

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So next step - is the clutch working

With the car off, I could rotate the clutch by hand.
Then I put a thermometer in the vents. It showed 93.7F.
Then I started the car, put the A/C coldness setting to the middle, turned the fans on high

Checked under the car and I could see the clutch plate turning with the belt. So the clutch seems to be engaged.

Then checked the radiator fan - it was running

Outlet temp after a few minutes , 90 degrees
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Ok, what about low side pressure

While running low side pressure started at 30.5 psi and dropped to 28 after a few minutes. That's definitely low. This chart says I should be seeing around 50 psi.


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I tried to check the high side pressure. Problem is that the high side port interferes with the airbox inlet.

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I thought removing the air intake rubber hose would do it, but still interfered. I can't yank the airbox to the side enough for the coupler to clear the corner of the airbox. So removed the airbox cover and realized I need to loosen those 16 nuts holding on the carb trumpets, and realized I could loosen 12 of them but don't have a tool to loosen the back 4. So I'm ordering one of the 8mm gearwrench ratcheting flex-head combo wrenches. (I have a ratcheting wrench, but it doesn't have a swivel)

So a few points of advice would be helpful

1) Would a low charge alone (28psi vs 50 psi) be enough to have the system essentially not functional and give only a 3 degree drop from ambient (93 ambient -> 90 degrees vent temp)

2) System obviously has a leak so tonight when it's dark I'm going to take a look around for some dye around the dryer area, the compressor fittings and clutch, and the condenser in the grille. Anywhere else I should check?

3) If I can't find any dye indicating the leak, my sense is to get a high side reading which means dealing with this damn interference. Is that the right thing to do next? My plan is to loosen the carb trumpet nuts and hope the airbox will then be able to be shifted. For those who have done this, will it shift or do I need to actually remove it? I've never removed the airbox myself and would prefer not to risk dropping washers down the carbs. It looks like that port could potentially be swiveled but its a tight enough space that it might require removing the airbox to do that and I'm not sure about loosening the fitting while the system is pressurized.

4) Anything else to check before getting a high side reading?



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Thanks for the tips and suggestions so far. I got in touch with Old Air Products and ordered some 90 degree connectors that may allow me to extend the high side and low side ports to make them easier to get to. Depending on when they arrive I’ll either use them to get a high side reading or will take off the airbox. Good suggestion on that strainer.

@Freddie328 - the ambient was 93F/34C when I measured the system pressure

tonight I did a UV check. Found a dusting at the fittings by the dryer - maybe a small leak or some escape up there. Same with at the connector to the front radiator.

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There was a bunch on the low side port but suspect that was from connecting/disconnecting the gauges so I’ll clean it off well and then recheck after it’s been cleaned but before I connect a gauge set. There’s an odd chance that there is some leakage between the compressor and the low side port adapter. There was none on the high side.

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Update. Short answer is the system is now charged and giving me 65 degree vent temp at 93 degrees ambient. High side and low side pressures are a touch low at 40psi low side/200 high side at 93 ambient but I've added about 720 grams/25.5 ounces/1.6 pounds and didn't want to over-charge. When we started this I was getting 91 degrees vent temp at 93 degrees ambient

Full details.

I got the book "Just Needs a Recharge: The Hack Mechanic Guide to Vintage Air Conditioning" - definitely recommended reading. Really consolidates everything I saw in youtube videos in one place.

When we last left the situation, I couldn't get the high side access port connected to the gauges. So today I loosened the nuts on the carb trumpets to allow the airbox to be lifted up a bit. That worked. I didn't need to completely remove it. Key tool was the GEARWRENCH 8mm 4 Degree Swing Arch 12 Point Flex Head Ratcheting Combination Wrench. I initially tried a nut driver drive but it got hung up. It's still likely needed for the nuts closest to the corners of the airbox but I learned that my carb trumpets have none. Somewhere along the way a mechanic decided I didn't need those four (or they are too much of a PITA) and so each trumpet is held down by 3 nuts rather than 4. Is this a "mechanic trick" or a problematic situation I should remedy?

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Anyway, I completely removed the nuts closest to the airbox inlet and left the remaining ones just loosened to the top of the stud. Then I could lift the inlet of the airbox that covers the high side access port enough to get the gauge set connected. It's super tight and it's tough to then manipulate the knob wheel on the port connector but I got it done. I'm glad I didn't need to remove the airbox because if I needed to evacuate the system, I could still drive it in this situation to a shop and then quickly get it back loosened so they could hook up the evacuation machine.

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Once everything was connected:
System off pressures -
low side 92 psi
high side 100 psi
ambient at that time 85 deg

Then got the car started, turned the AC to Max, blower on max.
Then pressures were
low side- 24 psi
high side - 110 psi
Both low

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First looked at the sightglass on the dryer. Saw flow and a lot of bubbles. Was clear liquid. No streaks like it was just oil.



So with the high side and low side both low, figured let's go fill up the system.
Bottle #1.
Connected the bottle to the bottle valve, then to the yellow hose. Opened up the bottle, then vented a little pressure from the relief near the gauge to clear the ambient air from the line. Followed the instructions in the vintage A/C book to very slowly open the low side gauge valve. Kept pressure below 30 and slowly opened the low side valve until it would stay around 30 with the valve completely open. Bottle got cold and slowly fed in. After a few minutes, used the trick in the book to immerse the bottle in warm water so it would go a little faster. Took about 15 minutes for the bottle to go in. Bottle full is 415 grams. Took it off and it was 117 grams - so put in about 300 g. at this point, the gages were 30 psi low side and 150 psi high side. The vent was blowing 79 degrees F.

So on to bottle #2
I didn't weight things well here. They were weighted with the fittings and hoses on and that turned out to be too inaccurate. Might have started at 550 and ended at 280. Bottle felt similarly empty. Let's say I put in another 270-300g. Now I'm at 40psi low side and 200 psi high side. Vent is blowing 67 degrees F.

The pressure table said at 85 F I should be targeting 45-55 psi low side 225-250 high side. So I kept going.
Now I know the spec for the system is 2.4 lbs of R12, which I calculate to be 1.93 lbs of R134a, which is 30.8 ounces. But that's a completely evacuated system. In the past, my mechanic has put in 26-27 ounces when charging the system (unclear if that was the delta or if that was the full charge. Since that is what was on the parts list, it makes sense that was the top-up amount and they didn't charge me for the freon they recovered). So I'd put in about 600g by this point. (though I thought I'd put in closer to 24 ounces - lesson learned to weigh each bottle before and after without any of the hoses and valves and keep track).
Took about 15 minutes for that bottle to go in.

Bottle #3
Put in about 120 grams. So total of 690-720 grams or 25 ounces.
The gauges really didn't move during this insertion. It was at about 41psi low side and 205 psi on the high side when I removed the bottle. But I thought I had put in more like 27 ounces and was worried about over-filling. Guess I could have done another 80-110 grams for a total of 800. Maybe that would have gotten me to 45 psi and 225psi. Not sure. The dryer sightglass is still showing some oscillation though the book says you can't use that as the fill indicator with R1234a like you could with R12.

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Final score:
Vent temp - 65 degrees
Ambient in the garage - 93F

But at this point, I learned that the harbor freight can-valve was damaging the cans. That seems to be a known issue. The can I removed didn't re-seal and was hissing. Rather than release some pollutant to the atmosphere, I drove the can to a local service station that happened to be open and gave them the can and I they drained it with their evacuator. Threw the HF valve in the trash.

So that's where I'm sitting.

I could just button up the system from here. I could grab another bottle and a new valve and throw in another 80-110 grams.

A few final questions for the group
1) For a system without an upgraded compressor or any hardware and has been converted to R134a, is 65 degree vent temp at 93 ambient typical performance? I checked my brand new car for comparison and it puts out 46 degrees. But this is a 50 year old converted system so I'm not expecting the same.
2) Should I keep pushing to get the pressures higher (like 45/225 or even more like 50/260 with 93 ambient)?
3) If #2, is there any other diagnosis I should do on why both the low and high sides are a bit low? Or do you just keep adding freon?