C3 AC Charge Questions

[Pat K.]

The service manual has instructions based on the J-8393 charging station which I don't have. I have a set of gauges, vacuum pump, leak detector, cans of R12 Freon, and the accessory to pierce the Freon can to add to the system.





Does it matter which low pressure fitting I use to add the Freon?

Do I add one can of Freon without the compressor running?

Do I introduce the Freon as liquid or gas?

I was planning on checking for leaks after the first can entered the system. Can that be done without running the compressor?

If no leaks found, then start car, engage compressor and add the remainder of Freon?

Thanks,

Pat

[Larry M.]

Re: C3 AC Charge Questions

Pat:

Glad to see that everything is buttoned up and appears leak free. Regarding your questions:

Add one can (if possible) of R12 by opening both the high and low pressure valves into the system. Add only as a gas....no liquid. Purge the air out of the yellow charging line (at the gage manifold) before opening the valves up to the system.

I would add R12 at the POA valve rather than at the compressor itself. But either is acceptable. Yes, the low pressure connection is on the side without the muffler.

Do the above without running the engine. Then do your pressure checks with the system pressure at around 60-90 psig (temperature dependant). You probably will only use 1/2 can of R12 to get to this pressure. Don't take too long at this point, since the R12 can connection leaks a little and will lose some of your refrigerant.

Then if all is okay, start the car, engage the AC compressor, and add the additional R12. Have your windows open on the car, and the blower on high. Add R12 through the suction side as a gas only. Don't try to rush and add liquid....which could damage the compressor. Purge air from the charging line each time you change cans.

You may also need to heat the cans in a bucket of warm water during the charging to keep the pressure up in the can....since it will cool down as the refrigerant evaporates.

After adding a total of two cans, I would increase engine RPM to around 2000 RPM as recommended by GM Service Manual, and then add the last 1-2 cans of R12. Watch the sight glass on the drier for bubbles (you do not want to see bubbles since that indicates a low charge) and also try to add the stated amount of pounds of R12 for a "full" charge per the service manual. Should be around 3 to 3-1/2 pounds total for your Corvette.

If all goes well, the pressure at the POA connection should be around 30 psig, and the high side pressure should be around 190 - 230 psig. Temperature at the duct should be around 42-45 F.

Then disconnect the gages and take a drive!! Check again for leaks the following day.

Larry

[Pat K.]

Re: C3 AC Charge Questions

Larry,

Thanks for the info, but I'm still learning on the job and have a couple more questions.

You said "Add one can (if possible) of R12 by opening both the high and low pressure valves into the system. Add only as a gas....no liquid. Purge the air out of the yellow charging line (at the gage manifold) before opening the valves up to the system."

Do I just unscrew the yellow line to purge the air?

Then, open the valves, and use the can tap to introduce the gas at the POA?

You said "Purge air from the charging line each time you change cans."

Do I open the valve all the way on the charging line to purge the air?

I included pics of the equipment I'm using just in case my questions aren't clear.

Thanks,

Pat

[Larry M.]

Re: C3 AC Charge Questions

Pat:

Typically when you hookup the gage manifold to your AC system, you connect the blue (low psi) hose to the POA connection, the red (hi psi) hose to the compressor discharge connection, and the yellow hose to the vacuum pump. You open both the blue and red manifold valves and start the vacuum pump. After you pull the vacuum down, you shut both the red and blue manifold valves, and then vent and shutdown the vacuum pump.

(***) When you go to charge refrigerant, you disconnect the yellow hose from the vacuum pump and hook that end up to the valve on the R12 can. You then slightly unscrew the yellow hose at the gage manifold, open the valve on the can of R12, and flow refrigerant through the yellow hose and out to atmosphere at the manifold for a second or two to remove any air in the yellow hose. Then you fully tighten up the yellow hose connection with refrigerant pressure still on the yellow hose. Finally you are ready to open the red and blue valves on the gage manifold and add refrigerant to the system.

Now, your setup is different, since your R12 can tapper has its own short red charging hose. Most don't. What I would do is go back to (***) above and completely remove the yellow hose from the gage manifold. Then slightly screw in the short red charging hose to the center manifold connection where the yellow hose once was. Then open the valve on the can of R12, and purge refrigerant through the short red charging hose to remove any air. Then tighten this connection. Then fully open the can charging valve. Finally open the red and blue manifold valves to charge the air conditioning system.

Normally the can tapper valve does not have its own hose and simply has a male connection for the yellow hose to attach to.

Pat: Each time you change cans of refrigerant, you must close the red and blue manifold valves and then remove the old can and tap a new can. Doing this will cause a small amount of air to get into the short red charging hose. You must open the R12 can tapper valve and pressure up the short red charging hose, and then slightly loosen this hose under pressure at the manifold connection to remove this trapped air. If you don't do this, that air will get pushed into your AC system and can cause problems. You want to avoid this. That is one advantage of having a 30 lb cylinder of refrigerant....you don't have to repeatly change cans and purge out any air. (Disclaimer: Since your red charging hose is short, the amount of air will be small and may not cause you a problem. But it won't help any either, and it is best if not injected into the AC system.)

Okay?? or more confused than ever??

Just be careful, since the R12 refrigerant vapor vented to atmosphere will get very cold and will cause frostbite if you let it stay to long on your fingers/hands. Not a big problem, just use care so that the loosened hose connection doesn't blow apart. That could be bad.

Suggest having a helper around to act as a third set of hands if needed, and to hold the bucket of warm water to keep the small R12 can from getting to cold and the pressure to low. (Another advantage of the 30 lb cylinder......the cylinder never gets to cold to properly charge the system)

Larry

[Pat K.]

Re: C3 AC Charge Questions

Larry - I'm fairly certain I got it, thank you very much!

[Pat K.]

Re: C3 AC Charge Questions

Larry - sorry, one last question.

You state "...Then open the valve on the can of R12, and purge refrigerant through the short red charging hose to remove any air. Then tighten this connection. Then fully open the can charging valve. Finally open the red and blue manifold valves to charge the air conditioning system. "

At this point I have a connection to the blue (low), red (high), and yellow (R12 can) right before I charge the system. Are the gauges smart enough to only allow gas to enter into the blue (low) side?

Thanks,

Pat

[Jim T.]

Re: C3 AC Charge Questions

Pat, Larry has given some very good instructions to follow to recharge your 69.
I am going to charge my 70 with some R12 from a 30# container and I have the guages. I do not plan on hooking up the high pressure line to my system. Years ago when anyone could buy the small cans of R12 off the shelf I did not have the guages and just injected the R12 into the suction connection at the compressor and judged the system filled when my sight glass cleared up on the dryer.
When I add R12 now with guages I am going to do it the same way except I will purge the R12 supply line going to the guages and also purge the blue line going to the compressor suction connection. I am not going to hook up the high pressure line and just use the sight glass.
The reason I am not going to hook up the high pressure line on my 70 is because I recently added R12 to my 85 Corvette using my guages. The procedure in my 85's service manual to add R12 does not use the high pressure hose/gauage connection and there is not a sight glass to view. The only check on the charging of R12 is to feel one of the lines.
Based on my 85's procedure for me this is just one more connection I will not have to use when charging my 1970. I always use gas freon, never tip the cans or container to inject liquid freon.
Larry Mulder if this is wrong please post.

[Larry M.]

Re: C3 AC Charge Questions

Larry - sorry, one last question.

You state "...Then open the valve on the can of R12, and purge refrigerant through the short red charging hose to remove any air. Then tighten this connection. Then fully open the can charging valve. Finally open the red and blue manifold valves to charge the air conditioning system. "

At this point I have a connection to the blue (low), red (high), and yellow (R12 can) right before I charge the system. Are the gauges smart enough to only allow gas to enter into the blue (low) side?

Thanks,

Pat

Pat: The INITIAL fill of refrigerant should be through the low pressure and the high pressure sides together. This way everything in the system comes up to pressure together.

However, after this initial charge of 1/2 can or so and you start the car and energize the AC compressor, YOU THEN MUST FILL ONLY THROUGH THE SUCTION OR LO PSI CONNECTION (Blue hose). The reason is that your can of R12 will have around 90 psig of pressure. The low pressure side is around 30 psig or so, therefore R12 flows from the can into the system. The high pressure side will run around 200 psig, so nothing will flow from the can into the system. You DO NOT want to open the red valve once the compressor is running, since the refrigerant will try to go from the high pressure in the system back into the can.....which can rupture the refrigerant can. These cans are only good for around 150 psig or so.

To repeat: Initial fill of system with AC compressor OFF is through both the blue and red hoses. This allows system to come up to pressure equally throughout. Once the car is started and the AC compressor is running, KEEP THE RED VALVE CLOSED and only add refrigerant through the low pressure side (blue valve and hose). If you open the red valve with the compressor running, you stand a chance of overpressuring the refrigerant can.

Larry

EDIT: Pat, if you want, you can call me and I will be more than happy to review all of the charging steps with you and answer questions that come up. Let me know if you need my phone number. Larry

[Pat K.]

Re: C3 AC Charge Questions

Larry,

Thanks for clarifying. I understand the charging process now, but it might be a good idea send me you phone number via PM since you never know what can crop up.

Thanks again!

Pat

[Larry M.]

Re: C3 AC Charge Questions

Pat, Larry has given some very good instructions to follow to recharge your 69.
I am going to charge my 70 with some R12 from a 30# container and I have the guages. I do not plan on hooking up the high pressure line to my system. Years ago when anyone could buy the small cans of R12 off the shelf I did not have the guages and just injected the R12 into the suction connection at the compressor and judged the system filled when my sight glass cleared up on the dryer.
When I add R12 now with guages I am going to do it the same way except I will purge the R12 supply line going to the guages and also purge the blue line going to the compressor suction connection. I am not going to hook up the high pressure line and just use the sight glass.
The reason I am not going to hook up the high pressure line on my 70 is because I recently added R12 to my 85 Corvette using my guages. The procedure in my 85's service manual to add R12 does not use the high pressure hose/gauage connection and there is not a sight glass to view. The only check on the charging of R12 is to feel one of the lines.
Based on my 85's procedure for me this is just one more connection I will not have to use when charging my 1970. I always use gas freon, never tip the cans or container to inject liquid freon.
Larry Mulder if this is wrong please post.

Jim:

You are not wrong, and what you have stated will work fine. The reason I did not tell Pat to purge the low pressure (or the high pressure) lines to remove air is because his system was under vacuum from his vacuum pump. So the air was already removed. Pat's first picture actually shows vacuum on his gages.

Your comment about only connecting the blue low pressure hose is also a good one if you are not familar with AC systems. It will still permit correct charging and will keep you out of trouble from the high pressure. The downside is that you don't know what is going on on the compressor discharge side.

However, if you are starting from scratch with no refrigerant in the system, it is recommended to add the INITIAL charge through both the suction and discharge sides until the system pressure comes up to around 60-90 psig. Then close the discharge valve, start the engine and compressor, and operate/fill only through the suction side (blue valve and hose). As a matter of fact, if someone were to use only ONE connection for adding the INITIAL charge, it should be through the HIGH PRESSURE SIDE. In this way the refrigerant flows from the high pressure side to the low pressure side of the system as designed. But I (and many others) find it easier to just add refrigerant to both high and low pressure sides and build initial pressure equally throughout the system. Then I close the high pressure (red) valve and only work with the low pressure (blue) valve.

Without getting to far off base from Pat's original post, I believe that the GM FACTORY actually charged the system completely as a liquid through the high pressure side connection.....just dumped the entire 3 lb charge into the condenser and receiver/drier........to save time and get a consistant fill. If any liquid got into the compressor, I guess they felt it would vaporize with time and not cause a compressor liquid slug during initial compressor startup. This can also be done at home, but in my opinion presents too many risks for the benefit of saving some time. Perhaps someone who worked at St Louis during this time can comment on what really happened on the factory floor for these AC cars.

Larry

[Larry M.]

Re: C3 AC Charge Questions

Larry,

Thanks for clarifying. I understand the charging process now, but it might be a good idea send me you phone number via PM since you never know what can crop up.

Thanks again!

Pat

Pat:

PM sent. Larry