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Discussion Starter #1 (Edited)
For anyone interested in learning how the factory AC system works, I made an in-depth 30min video explaining how it all works in my 70 Nova.

In depth look at how the factory AC system operates and what's needed to update the system for R134a.

Explanation of controls 0-10:20
Driving test 10:20-12:55
Underhood components overview 12:55-25:36
Whats needed to use R134a 25:35-33:32
Factory AC vs aftermarket 33:32-end



Here are a few important thing to consider when using the factory AC and 134a
(1) The expansion valve (TXV valve) needs to be appropriate for 134a due to different superheat temperature needs. Most replacement TX valves are already setup for 134a, but if you have an original TX valve you should replace it. When using an R12 TXV with R134a, the evaporator will not be fully utilized because the refrigerant will be superheated more than necessary. The result is less cooling capacity, leading to higher temperatures out of the vent.
(2) The POA valve needs to be adjusted so the pressure in the evaporator is correct to keep r134a near freezing. The original pressure setpoint (29psig) keeps R12 just above freezing, but 134a at the same pressure will be at a higher temperature. This means if you don't adjust the POA valve, your evaporator will be warmer and therefore the air coming out of your vents will be warmer too. The pressure needs to be lowered to about 25psig. I have a video of doing that linked above
(3) The condenser should be replaced with a parallel flow condenser. The original condenser is a tube and fin design which is much less efficient at heat transfer and it becomes a major limiting factor, especially when using r134a. Get the largest parallel flow condenser that you can fit and get good air flow. If you use the original condenser, pressures will rise on the high side of the system and it may not be able to keep up with cooling demand in high ambient temperatures, again leading to higher vent temperatures.

Other things you can do to maximize performance:
(1) Make sure the high speed fan relay is working properly. This relay is located on top of the AC box and it is only turned on when the fan switch is set to high speed. If it doesn't work, your blower motor will not have the maximum speed.
(2) Make sure your fan is getting a good dedicated ground. The fan motor mounts to a fiberglass, non conducting box on the firewall, so it needs a dedicated ground wire to the body of the fan. You can run this down to the frame if you want to ensure best ground.
(3) Ensure all duct work is sealed. You can use household self-adhering weatherstripping foam to replace the original seals if they are not working. Air blowing around under the dash is not as effective as air blowing at the passengers.
(4) Use an electric radiator fan with a shroud. Getting good air flow through the condenser is critical, especially if you want good performance when the car is sitting still.
(5) Make sure re circulation function is working. When set to "Max", the factory AC has a vacuum valve that operates a re circulation flap in the passenger footwell area as well as one on top of the cowl. If it's working propertly, you will hear more fan noise from the passenger footwell area when the AC is set to "max". If it's not working properly, make sure the vacuum lines are connected to manifold vacuum and that the duct operators are working.
(6) Make sure the heater shutoff valve is working. There should be a vacuum-operated shutoff valve located on the inner fender that stops the flow of coolant to the heater core when the AC is set to max.
 

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Hey Clint . . . . . . . can I give you a "Double LIKE " : very nice job doing " Factory A/C " up-date .

Explained so even I can understand the whole system .

Thank you , jim

ps ; Very nice 70 Nova , you have . . . . . (y)
 

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Pretty sure I follow your stuff on insta, saw this then clicked and realized I went on YouTube and saved it earlier. Lol.


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Thanks I usually would not post such a long video but I wish someone else had before I started working on mine!
I’m a sucker for well explained stuff and having things broken down that I don’t know about. Didn’t think it was too long at all! Wish more things were able to be documented like that.


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For anyone interested in learning how the factory AC system works, I made an in-depth 30min video explaining how it all works in my 70 Nova.

In depth look at how the factory AC system operates and what's needed to update the system for R134a.

Explanation of controls 0-10:20
Driving test 10:20-12:55
Underhood components overview 12:55-25:36
Whats needed to use R134a 25:35-33:32
Factory AC vs aftermarket 33:32-end



Here are a few important thing to consider when using the factory AC and 134a
(1) The expansion valve (TXV valve) needs to be appropriate for 134a due to different superheat temperature needs. Most replacement TX valves are already setup for 134a, but if you have an original TX valve you should replace it. When using an R12 TXV with R134a, the evaporator will not be fully utilized because the refrigerant will be superheated more than necessary. The result is less cooling capacity, leading to higher temperatures out of the vent.
(2) The POA valve needs to be adjusted so the pressure in the evaporator is correct to keep r134a near freezing. The original pressure setpoint (29psig) keeps R12 just above freezing, but 134a at the same pressure will be at a higher temperature. This means if you don't adjust the POA valve, your evaporator will be warmer and therefore the air coming out of your vents will be warmer too. The pressure needs to be lowered to about 25psig. I have a video of doing that linked above
(3) The condenser should be replaced with a parallel flow condenser. The original condenser is a tube and fin design which is much less efficient at heat transfer and it becomes a major limiting factor, especially when using r134a. Get the largest parallel flow condenser that you can fit and get good air flow. If you use the original condenser, pressures will rise on the high side of the system and it may not be able to keep up with cooling demand in high ambient temperatures, again leading to higher vent temperatures.

Other things you can do to maximize performance:
(1) Make sure the high speed fan relay is working properly. This relay is located on top of the AC box and it is only turned on when the fan switch is set to high speed. If it doesn't work, your blower motor will not have the maximum speed.
(2) Make sure your fan is getting a good dedicated ground. The fan motor mounts to a fiberglass, non conducting box on the firewall, so it needs a dedicated ground wire to the body of the fan. You can run this down to the frame if you want to ensure best ground.
(3) Ensure all duct work is sealed. You can use household self-adhering weatherstripping foam to replace the original seals if they are not working. Air blowing around under the dash is not as effective as air blowing at the passengers.
(4) Use an electric radiator fan with a shroud. Getting good air flow through the condenser is critical, especially if you want good performance when the car is sitting still.
(5) Make sure re circulation function is working. When set to "Max", the factory AC has a vacuum valve that operates a re circulation flap in the passenger footwell area as well as one on top of the cowl. If it's working propertly, you will hear more fan noise from the passenger footwell area when the AC is set to "max". If it's not working properly, make sure the vacuum lines are connected to manifold vacuum and that the duct operators are working.
(6) Make sure the heater shutoff valve is working. There should be a vacuum-operated shutoff valve located on the inner fender that stops the flow of coolant to the heater core when the AC is set to max.
Great write up, and great video. I've done about the same with my factory A/C car that you've done to yours, except power it with an LS. Nice work!
By the way, I learned about the POA valve and made an apparatus to adjust it. My system works very well. I have a friend with a 69 Firebird (same system). I told him to either re-calibrate his POA valve or send it out to a company (I think Old Air does it). He didn't listen, and every time I see him he whines about how much it cost him to rebuild his A/C system to get unsatisfactory performance from it. The POA valve is very important when converting to R134a.
 
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