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Proper vent temperature and pressure readings for specific Vehicle Pages: 12

y2k600f4 on Wed April 16, 2008 5:34 PM User is offline

Year: 1980
Make: Chevrolet
Model: Camaro
Engine Size: 5.7
Refrigerant Type: R134a
Country of Origin: United States

I've read a bunch of articles on this site regarding the proper procedure of R134a conversion and recharge which I am in the process of. I am trying to do it as right as possible. My question is what is the "correct" way to know you have the proper amount of R134a in your system ?I beleive the original R12 was 52 oz and I was targeting 80% or 42 oz. I planned on putting in (3) 12 oz cans for 36 oz and then adding a little at a time to achieve the proper refrigerant amount. Since it is hard to accurately add 6 oz or 1/2 can by weight I was going to use the proper vent temperature and pressure readings for this vehicle for R134a ??? Are these available ? Or do I need to use the 2.2 x the ambient temperature as a guide....targeting 2.0-2.5x ambient temperature with minimum vent temperature expected in the range of 36-40 degrees ? If there is somewhere to get the pressure and temps for this vehicle please let me know. If I am wrong in my assumptions and procedures above...please set me straight !!! Thanks.

GM Tech on Wed April 16, 2008 7:18 PM User is offline

Why not keep it R-12 and add the exact amount by weight and not worry about the vent temps? R-12 was what your system was designed for- and adding by weight takes away all the guess work.............

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

y2k600f4 on Wed April 16, 2008 7:39 PM User is offline

Bought all the parts to do a R134a conversion from AC Kits (adapters, oil, o-rings, gaskets, accumulator, orifice tube etc). System was not charged when I purchased the vehicle and decided to do a proper conversion plus R12 is $$$$$$$. Thanks.

TRB on Wed April 16, 2008 7:52 PM User is offlineView users profile

There is no specific procedure for charging an R12 system with R134a. You will hear some say times x. Some will say that is under charging the system. I say get as close to the original charge. Keeping your pressures at a point the system will cool. Normally that is around 80 percent depending on your ambients. The theory is if you don't have enough refrigerant to carry the oil. This can lead to a compressor failure, which I agree. But if you have to high of a pressure you going to have poor cooling and a compressor failure also.

This is one reason many of us just use r12. It's still available, if you repair the system as to not have leaks. It's not really the highest cost of a repair. All our products can be used with r12 or r134a. You just need to use the correct oil from the start.


When considering your next auto A/C purchase, please consider the site that supports you:

y2k600f4 on Wed April 16, 2008 9:11 PM User is offline

Thanks are right about the R12 being not the highest cost of repair. Since I am attempting to do this myself I think I can save $ on this project by using R134a; I will give it a try and hope it works out. Now I understand more the preference of R12.

NickD on Thu April 17, 2008 8:21 AM User is offline

Optimum charge can be obtained, but is a slow process, has to be done on a warm or hot day, at least 80*F, engine running at 1,500 rpm, doors open, one vent opened with a thermometer in it's mouth, pressure gauges installed, AC at maximum, and blower at full. There is some leeway as the accumulator can store and additional 3-5 ounces, but the key is to slowly increment the charge until you see no further improvement in decreased vent temperatures. At the same time you are watching your gauges, in particular the high side so the system won't blow up in your face. At 85*F, around 250 psi is generally considered as high as you want to go, or time to think about changing to a more efficient condenser and looking at your fans.

The system must be clean and this includes all the dirt buildup in the evaporator, you can't charge by pressures with poor airflow. With R-134a, can generally prefill to 80% of the R-12 capacity and go up from there.

Another goal you want to reach is having pure liquid feeding the orifice as foam doesn't cool very well, again this has to be within the limits of pressures. A sight glass helps, but you can feel the pulsations with a sensitive hand, a point is reached where these pulsations subside.

Also you want the outlet of the evaporator temperature to approach the inlet temperature, not any colder, you are flooding the system, again within the pressure limits.

Finally, charging a CCOT system that you have is a compromise at best, at higher temperatures you want less refrigerant to control high side pressures, but just the reverse at cooler ambients, you want more refrigerant to limit the cycling time, so you have to find a point somewhere in between.

That is why GMTech suggested sticking with R-12 and charging by weight, all this work is already done for you, but if you want to play, go ahead, use extreme care, you don't want to blow your head off in the process.

y2k600f4 on Thu April 17, 2008 9:44 AM User is offline

Nick, Thanks for taking the time to post a detailed description of charging a R134a conversion with temp and gauges !! I now see how it is more of an art than a science and understand why R12 is much easier to charge by weight. Hopefully I have not undertaken something I should of let the experts do !

mk378 on Thu April 17, 2008 10:08 AM User is offline

R-12 is easy to charge by weight because the car manufacture has already determined exactly how much R-12 is the proper amount. When you do a conversion the amount depends on other factors.

Ideally you want to flood the evaporator on a CCOT system, as long as the condenser is able to supply that much refrigerant to it. Two accurate thermometers can be used on the evaporator lines. The outlet line can actually get colder than the inlet because of pressure drop thru the evaporator. If you can't take precise measurements, charge until the evaporator outlet line starts getting really cold, but never go over the rated R-12 weight of R-134a because that will definitely be an overcharge.

Any car that has an engine driven fan should get a new fan clutch as part of getting the A/C going. You need peak performance from the fan to keep the condenser cool, especially with R-134a. It takes a lot more air than just cooling the radiator. Most old cars the fan clutch is very worn and though the fan still spins, it's not as fast as it should do.

mhamilton on Thu April 17, 2008 11:26 AM User is offlineView users profile

y2k, your F body a/c is the same system as the A and B body, I can tell you that you will be very close to the full charge with the 134 conversion. With the 3.25 lbs system you have the R4 compressor, a bit less than the A6 setup, but I would start with 3.0 lbs and go from there as others have said. Even with the OE condenser, will be very close to R12 charge when done.

y2k600f4 on Thu April 17, 2008 12:54 PM User is offline

Thanks mhamilton

ACtually I have the A6 compressor on my 80Z28; Others with similiar cars have told me that they are in the range of 42 oz with R134a when the original R12 was 52 oz. I was just trying to figure the best way to add freon using guages since I need ~3.5 cans and don't have the system to do it be weight. I guess I will just do the best I can to get close to 3.5 cans or 42 oz. Start with (3) and watch the gauges as you mentioned. Thanks.

mhamilton on Thu April 17, 2008 1:24 PM User is offlineView users profile

Oh, okay, so you have a setup simlar to the Corvette (A6 on a V8).

I don't know about 42 oz, that sounds awfully low (2.6 lbs out of 3.25). When I first did my '80 Malibu system I tried 3 lbs out of 3.5 lbs, and it didn't cool at all on the highway, cycled way too much. I ended up charging the full amount, then it would frost the windows on a 95 degree day. Do have slightly less cooling at idle, but expect that is the old condenser and a poor fan clutch. I guess your F body 350 has a heavy duty fan clutch, so that should help a lot.

Actually, I'm just getting ready to evac and recharge my system to replace a leaking oring. I will be following the same procedure of checking temps and pressures, but am expecting to get between 3.25 and 3.5 lbs. I'm sure you will be okay if you charge with cans, but I would get enough to go the full amount if needed.

Edited: Thu April 17, 2008 at 1:28 PM by mhamilton

y2k600f4 on Thu April 17, 2008 4:54 PM User is offline

Called up a local A/C shop....figured they can pull a better vacuum with their pro equipment and set me up with the proper charge and it won't cost that much. However they mentioned they charge at the R12 weight, telling me that the old way of thinking was the 80% or so of the R12 however what they have been finding is the reduced charge causes poor oil flow and burns out compressors. Don't think I will go with them...they do a ton of work and many many conversions. This goes against the experts in this forum, and I trust this forum (many experts) than one local shop. I guess I will call a few more shops...

TRB on Thu April 17, 2008 5:18 PM User is offlineView users profile

Nothing wrong with charging to the OEM specifications if the condenser can handle it. Problem is when you toss in X amount and end up with 400PSI on the high side.


When considering your next auto A/C purchase, please consider the site that supports you:

iceman2555 on Fri April 18, 2008 3:22 AM User is offlineView users profile

Some excellent advice was given stated that the best method was to insure that the evap is flooded...this is the best method to determine the correct recharge rate. Once the evap floods....doors open..max cool...high blower...doors open.....the system is properly charged. If high side pressure problem are encountered at this point....DO NOT REMOVE REFRIGERANT TO BALANCE PRESSURES.....the cooling system of the vehicle must be upgraded to insure lower pressures. Depending upon ambient temps....high side pressures may exceed what is considered 'normal'. Keep in mind that operational R12 type pressures are not valid for a retro fitted vehicle... unless the condenser is changed to a more efficient Parallel Flow Hi Eff unit. In this case most vehicle can be recharged to 100 % R12 specs.
The shop that recommended charging to 100 % is not far is essential that the system be charged completely...or the compressor will suffer....the good thing about your system is the use of the has a 'sump' system to maintain lubricant within the compressor. The down side of this compressor....the shaft seal...they tend to leak....R12 is a problem.....134a is compounded due to the smaller molecular weight of the refrigerant. If you have the compressor off the vehicle now...suggest to change the ceramic seal to a double lip seal of the modern GM compressor. May save having to do this later....
Good luck with the repair. lunch on for next week?

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

NickD on Fri April 18, 2008 7:07 AM User is offline

Seems to be a difference in the terminology of the difference between a full evaporator and a flooded evaporator, but believe we are saying the same thing. Flooded means over full, least to me, would just be happy to have the evaporator full so that the outlet temperature is equal to the inlet temperature, doesn't get much better than that, and overcharging is just as bad as undercharging.

The accumulator frost line is yet another subtle indicator of a full charge, but increasing the height of that frost line really doesn't improve performance. What it does is give a reserve so that extra refrigerant stored in there can leak off first before you notice a difference. The same is true of the receiver in a TXV system, I prefer just adding enough charge to get the performance as if there is a leak, you know something has got to be done, that extra refrigerant in the accumulator or receiver is just wasted.

Always considered the A6 ceramic seal to be bullet proof, was a bad joke when GM introduced that infamous single lip seal just about the same time the EPA became interested in refrigerant leaks. The key problem is not the ceramic seal, but a groove is worn in the shaft that causes the leak. If you see a that groove there, time to replace the compressor, it's worn out, replacing the seal will not repair that groove.

TRB on Fri April 18, 2008 10:43 AM User is offlineView users profile

Originally posted by: iceman2555 lunch on for next week?

I would enjoy having you buy me lunch sir.

When considering your next auto A/C purchase, please consider the site that supports you:

mhamilton on Fri April 18, 2008 5:21 PM User is offlineView users profile

Y2k, I just finished recharging my 3.5 lb A6 system... I ended up going to 3 lbs 6 oz before the evap outlet was colder than the inlet. I'm sure you will find it similar once you get 3 lbs in your system. At about 1200 rpm, medium blower, got 25 psi low, 220psi high. That is with a rather poor fan clutch, car in the sun, ambient temp 85 degrees. Center vent temp was registering 45 degrees. It actually cycled with low blower, and got 40 degrees with the cycling point set to 19 psi.

It's terrible just how badly these cars leak engine air back around the condenser. I was shocked to find that all the hot air was blowing out under the radiator even with the hood open. GM put an air dam under there to try and fix that, but I am sure that more sealing around the core support will help idle cooling.

y2k600f4 on Sat April 19, 2008 12:31 AM User is offline

mhamilton thanks for posting that info, it will be a great reference for me. I am about finished re-installing the compressor with fresh oil, changing out the accumulator, changing all o-rings, swapping out the orifice and adapter, I hope to actually get a charge on the system shortly. Trying not to rush things (want it done right the first time), I just spent some time "wrestling" with the orifice did not want to come out nor go back in, don't know if that is something to be expected, but I it's back in (had to feed it in one section at a time with needle nose pliers).

TRB on Sat April 19, 2008 12:41 AM User is offlineView users profile

Sure you installed it the proper direction. Your color may be different, but the copper end should be heading toward the evaporator.


When considering your next auto A/C purchase, please consider the site that supports you:

y2k600f4 on Sat April 19, 2008 9:07 AM User is offline

I must of messed up my post !
Anyway I put it in the same way it came out; thanks for the post Tim !! This forum is a wealth of knowledge if this is your fist time working on an A/C system or a resident expert !! I believe the orifice I used was a White GM(.072) ; the ones I ordered....been reading about the benefits of using a Blue Ford(.067) tube on a R134a conversion but I did not order them and I guess it is too late now. I think the problem is a very so slight bend in the metal line where the orifice is; and after reading other posts I am think I am lucky it did not snap ! The new one is in , and the old one actually looked clean with no particles whatsoever in the screen. Hopefully all is good !!!

mhamilton on Sat April 19, 2008 9:32 AM User is offlineView users profile

I also kept the white 0.072" otube in mine. I still don't understand the benefits of the smaller orifice in terms of pressures and temperatures, but since it works with the white, with low evap pressures, no sense changing it now. All the GM big systems still use the white tube.

Sounds like you have it ready to go, I'm interested to find out how much you charge on that system.


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