Koda

Took my 72 Chevy for a drive tonight; last time I drove it was months ago. Last time I worked on an AC system was with a fancy reclamation unit on this car, and my buddy and I just hooked it up and let it work. Same kind we use as backup at work. The monte carlo forums I'm on aren't as good as here, and I avoid the hoi polloi at chevelle forums on principle.

Anyway, AC is blowing slightly cool. It was converted to R134A in 02 when we got the car back on the road, and charged in 09 or so. In 09 or so, before it was charged, it was blowing ambient, with the compressor turning, and the in-town mechanic we had do some stuff back in 02 charged it up for me and stated it had pressure of about half a pound which was somewhat acceptable leakage for 7 years. Now it's blowing slightly cool, and this is 6 years after it was charged, so I am guessing it's low on refrigerant.

My plan is to go buy approx 4 cans of R134A on an estimated need of 3lbs. The car is sporting the red and blue capped quick connects of the R134A conversion, and both valves looked ok, with the low (blue) side being clean, and the red (high) side having a slight scum of green, which I guess is some seeped oil. The blue one is on the POA valve and the AC pressure gauge that I assume I can get at Autozone along with the freon will go there, and I am pretty sure I check pressure with car on, max, warmed up at some RPM the CSM tells me (think it's 2k).

I believe with the cans you get a connector hose, hook it up to the can and don't pierce it yet, hook it to the car, crack it at the can fitting to purge it, then pop the can with the piercer and let it evaporate and flow into the system for ten min or so with the car running and AC on. Check blown air temp and system pressure and add more if needed.

Did I miss anything?

twintracks

Yep, You got it. The directions on the can are pretty good. I'd start with two cans. That's usually enough.

oldspackrat

IIrc, you might not want the fan speed on the highest setting, but one notch lower maybe? That way the air is not moving too fast across the evap. Also I like to put a thermometer in the center vent to see how cool the air gets.

Ozzie

You're right about this forum having some good talent.
You seem to be on the right approach. It's best to use a real gauge set so you can watch both the high and low side pressures and the sight glass if there is one. If the pressures are still reasonable when the sight glass clears add another 1/4 pound and stop. If there is no sight glass use the charging pressures and duct temperatures shown in the shop manual. If all components are performing correctly, and there is no air in the system they should match reasonably well.

rocketraider

Does car still have the factory POA valve or has it been updated for 134a? We did a 72 Chevelle in summer community college auto-tech class maybe 15 yrs back and never could get that system to blow colder than about 50 degrees until the owner finally sprung for a 134a calibrated POA. If the system has performed well up till now though you'll probably be OK by charging it.



A set of 134a gauges is cheap enough. I think it will be money well spent.

Koda

Twin, Pack, Ozzie: Advice noted, will do, I appreciate it.

Raider, I am not sure on the POA valve. It SOUNDS the way you are describing it that it has the original R12 POA, as, when we charged it, the coldest it would blow was 60 degrees. Granted, it was 100 out there on that day. Now, I'm satisfied with that level of cooling, but it's not correct.

Eventually I'd like to go back to R12, once I have time to really work on a minor resto of the car, but I will put up with getting it back to the blowing 60 performance as is.

droldsmorland

Id recommend purchasing the gauges or take it somewhere that will check the high/low PSIs. With low Freon the pump shouldn't be turning as the high or low switch are there to prevent pump damage to either situation. Slightly low/high it will engage the clutch but for short bursts.
Have you properly adjusted the pilot absolute valve? The POA Valve regulates the pressure inside the Evaporator. It is factory set for R12's optimum pressure of ~29.5 PSI. To get maximum efficiency from R134A it has to be adjusted for a lower pressure. Looking into the inlet of the valve (through the fitting where the hose to the Evaporator goes), an adjustment screw with a locknut can be seen. Loosen the locknut and turn the adjustment 1/4 turn counter-clockwise. This adjustment lowered my vent temperatures.
Lastly verify that your heater control valve is working properly to null the hot coolant to the heater core. Also verify that the blend door is sealing properly when in the cold position.

Koda

I will get the gauges.

I think the valve is untouched. I will see how much of a pain the adjustment is to get to. Would rather not vent the system if I have to. I may be not understanding your location; I'll look it up.

Blend controls worked and felt the same as always yesterday. I'll verify the door and the control valve (will have to find that one.)

Thanks.

pogo69

i will just add that '' A/C is great when it works'' thats what someone told me years ago when i was asking questions about why it wasnt cold...smart ***

Fun71

I didn't think a '72 with a POA system had a low pressure cutoff switch.

Koda

It does have a thermal limiter.

2Olds4U

Koda, with the 134A, as a rule of thumb, high side should be about 2.5 times ambient temp. Low should stay around 30-40. These are at idle.
Go to HF and get a set of gauges. They're cheap. They may not last forever, but they will give you enough info to get by on.
Make sure the clutch on the fan is working properly. If you can stop it at idle, it's past tense. Wear gloves (welding type) when you do this.
I've also heard of people pulling the thermostat out of the evaporator about a 1/4 to 1/2 inch to keep things cooler.
134A systems usually use about 75% of the refrigerant an R12 type uses.

Koda

Clutch is good on fan. I like the spin when cold and off and it stops in less than half a turn check.

Thanks for the numbers.

Ok, so 75% of 3.75 lbs is 2.81 lbs. I have read I add "as much as the temp keeps dropping and a quarter lb more." Or should I add to get to a certain Hi and Lo pressure like you indicate? 90 degree heat is 235 psi high side by your figuring?

I am satisfied with the previous performance, so I'll just charge it.

steverw

Classic Auto Air can recalibrate your POA valve for 134 or fix it if it needs repair. Keep it R12 if wanted. Im going to send them mine soon to check, as it was open when I got it. The hose was missing. Im going to send my evaporator too and have them pressure check it. Just because of its location, best to do it before the fender is on.

Koda

They'll get my business for my 442. The Chevy is currently 100% operational, so I am going to throw R134 at it and see if it will improve.

2Olds4U

I don't know if you know it, Koda, but R12 and R134A use 2 different oils, and they are NOT compatible. You won't be able to just throw 134A into an R12 system and expect it to work for any length of time.
The system will have to be evacuated and then the required ester oil added and then recharged with 75% of the amount of R12 required.
Forget the nonsense about replacing seals, driers, etc.
The only thing you may have trouble with is getting the proper amount of refrigerant into the system without a full evacuation.
If I run into a charging problem I can add .2 lbs at a time until I see the proper pressures. You can easily overcharge a system and that will make the system blow warm air and also send the compressor into hysterics.
You know you've gone too far when you hear it start knocking.

Fun71

I think he knows this already.

Koda

Thanks, both.

It is a R134A converted system with R134A style quick connect fittings and seals, and the original R12 set POA. I will slowly add refrigerant until I reach the proper pressures.

If I cannot get it to absorb enough, I'll take it for a drive back to the mechanic who worked on it with me and take a Friday off and drive up and back. I want to get an AC station one of these days.

droldsmorland

With everything converted and working properly I still had so so AC. The 1/4 turn counter-clockwise adjustment lowered my vent temperatures from 57F to 46F on a 90F day. So it matters.
Im about to put a POA eliminator kit into another car that was R12. We will report back on how well it goes.

2Olds4U

Sorry, missed it. My bad. I wish I was dead.

Koda

We still love you, man.

Koda

Got a gauge setup and a can adapter.

Initial conditions were ambient temp 80, low pressure 8 psi, high pressure 100 psi, AC was blowing 75 degrees on max, 3/4 blower fan speed, doors closed, right side vent.

Added two cans through the gauge setup. Final conditions were ambient 80, low pressure 33 psi, high pressure 215 psi, AC blowing 55 degrees, same setup.

Gauges and cans worked well. Car sounded the same throughout, except that after one can, the compressor whirred a bit more. It wasn't a quite a rattle, and nowhere near what I would call a knock. Accessory belt was quivering a bit, which usually signifies a load. I think it had not much in the system to push around and sounds different now that it does. I used the Autozone standard R134A cans; I don't think they have oil in them. The noise was different, and additional, but not loud or ominous or percussive.

I'll go for a drive tomorrow. 55 degrees is wonderful. Thanks for the help.

Fun71

80º ambient temps sound wonderful to me.

Koda

Yeah, the humidity sucks though. The whole POA assembly was sweating bullets.

Went for a drive, car actually froze me out, so I'm pleased. Next time I charge it (anticipating 6 years from now) I'll put in some oil, either in a can of R134 or dribbled in separately before charging.

Koda

Update: AC was not performing well this spring. I blame not starting the car enough to circulate oil. Added 1 can pag46 oil, 2 cans (12 oz x2) of R134A and got the high side to @252 psi at about 90 deg ambient in my garage. Blows 55 degrees again. This is not an ideal solution, but will work for the power tour here in a few weeks.