Resurrecting 1970 GM AC with POA

Friendly format provided to inquire about automotive a/c systems.
Archived Forum

Moderators: bohica2xo, Tim, Dougflas, HECAT

TheBandit
Posts: 24
Joined: Fri Aug 17, 2018 2:31 pm

Re: Resurrecting 1970 GM AC with POA

Postby TheBandit » Tue Aug 21, 2018 12:47 pm

Here is a look at the exit of the POA valve showing the adjustment screw and lock nut.

Image

Can someone confirm if the threads are standard direction? i.e. I would turn the lock nut counter clockwise to unlock.

Also which direction on the adjustment screw (CW or CCW) will lower the pressure?
TheBandit
Posts: 24
Joined: Fri Aug 17, 2018 2:31 pm

Re: Resurrecting 1970 GM AC with POA

Postby TheBandit » Tue Aug 21, 2018 12:49 pm



bohica2xo wrote:For heater valves on GM stuff I use a vacuum unit, like the AC Delco P/N 155536 It was used on G vans, and P vans from 1990 to 1996.


Thank you for both suggestions! I will look into these.
TheBandit
Posts: 24
Joined: Fri Aug 17, 2018 2:31 pm

Re: Resurrecting 1970 GM AC with POA

Postby TheBandit » Tue Aug 21, 2018 12:59 pm

Liquid Line

Last night I loosely mounted the drier and started looking at liquid line routing. I was happy to see I should be able to route the line tucked inside the fender and out of sight by using 45 degree fittings on either side of the drier and a 90 degree fitting at the TXV.

Image

Image

Image
71403
Posts: 42
Joined: Fri May 27, 2016 12:46 pm

Re: Resurrecting 1970 GM AC with POA

Postby 71403 » Tue Aug 21, 2018 2:36 pm

TheBandit wrote:Are you aware of a vacuum operated bypass valves that would be suitable?

Four Seasons 74781, 5/8 and 3/4 hoses, apply vacuum to close. Stock app is 96-2000 GM trucks.

TheBandit wrote:
bohica2xo wrote:Obviously you already know you can't use the PCM to run the compressor, because it needs the feedback from the 3 wire pressure sensor - and it wants to cycle the compressor based on that information. Run the compressor from the OEM 1970 circuit.

I agree and this is my plan. Also I believe my PCM and operating system (GM E38 '07+ Silverado) would require a body control module (BCM) for the AC request, which I do not have. I would prefer to use the original controls which are very simple and setup to run the compressor continuously.

E38 ECM does require a/c request over the data bus but there are aftermarket solutions for this with no dash control modifications needed. On most 58x calibrations I've seen the radiator fans will operate on refrigerant pressure alone even without a/c request functional. There's also an anti-slug startup routine in these ECM cals, low mount compressor can suffer that in the right conditions.

Cycling systems use a pressure switch on the low side to cycle. The high side pressure sensor (3-wire) is used by the ECM/PCM for fans et al in both cycling and TXV systems.
TheBandit
Posts: 24
Joined: Fri Aug 17, 2018 2:31 pm

Re: Resurrecting 1970 GM AC with POA

Postby TheBandit » Wed Aug 22, 2018 9:25 am

Thanks for the info 71403. Out of curiosity, do you know what the ECM does as an anti-slug routine?
71403
Posts: 42
Joined: Fri May 27, 2016 12:46 pm

Re: Resurrecting 1970 GM AC with POA

Postby 71403 » Wed Aug 22, 2018 10:46 am

For anti-slug GM briefly pulses the compressor clutch during cranking.
TheBandit
Posts: 24
Joined: Fri Aug 17, 2018 2:31 pm

Re: Resurrecting 1970 GM AC with POA

Postby TheBandit » Thu Aug 23, 2018 8:27 am

High Pressure Line

Many years ago I picked up compressor fittings from Docs Blocks just before they went out of business. I got both straight and angle/tube adapters and ended up using a combination: straight on the suction side and right angle on the high pressure outlet. Unfortunately the right angle adapter pointed directly at the radiator hose.

Image

Image

The tubing on this outlet adapter is #8 which has a 1/2" outside diameter (number/dash sizes correspond to sixteenths i.e. #8 is 8/16" or 1/2"). I didn't have a 1/2" tubing bender, but I found a cheap hand bender at the local parts store to get the job done.

Image

Now the outlet points in a favorable direction and I will be able to route the high pressure hose with a service port fitting toward the front of the car.

Image

Image
TheBandit
Posts: 24
Joined: Fri Aug 17, 2018 2:31 pm

Re: Resurrecting 1970 GM AC with POA

Postby TheBandit » Mon Aug 27, 2018 9:02 am

Hose Crimping

This weekend I borrowed a Mastercool 71550 AC hose crimper and made all the hoses for the system. Here's what a typical fitting looks like coming out of the tool:

Image

This tool is very simple to operate. It comes with dies for sizes 6,8,10, and 12. I marked my fittings and hose on the car to make sure they had the right orientation, then held them in the tool while tightening the large acme screw at the top. This brings the dies together which put indentations around the fitting to clamp the hose. There is a mark on the tool which tells you how far to crimp.

The first hose I made goes from the drier and runs under the fender around to the TXV.

Image

I put a high side service port in the discharge line just after the compressor. I've got a lot of hoses (2x transmission, 2x AC, coolant, coolant overflow) and the starter cable routing through this area and it's starting to get crowded. I had to disconnect the transmission lines while I was doing this so I could rearrange things. If you look on the left of this photo near the heater bypass loop you can see the larger suction line which connects to the compressor with a 45 degree fitting.

Image

The discharge hose goes from the compressor and loops up to the top of the condenser as shown below.

Image

I used a 90 degree fitting at the bottom of the condenser for the liquid line going back to the drier. I had to sneak it between the transmission cooler inlet and return lines.

Image

I saved the expensive shepard's hook fitting for last.

Image

Image

Image

Here is how the engine bay is shaping up now that the AC is fully plumbed.

Image
TheBandit
Posts: 24
Joined: Fri Aug 17, 2018 2:31 pm

Re: Resurrecting 1970 GM AC with POA

Postby TheBandit » Wed Sep 26, 2018 10:19 am

I decided to get back to work on the AC project.

Here is my to do list:
- Adjust POA valve
- Flush evaporator and POA valve
- Install o-rings throughout
- Install hi pressure switch
- Add PAG oil
- Wire compressor and fan relay
- Install compressor belt
- Evacuate and charge

Starting with the POA valve, I recently picked up the 1970 chassis service manual which has all kinds of great info on how the AC system works as well as diagnostics and repair. Per the service manual, the valve setpoint from the factory was 29.5psig at sea level and that appears to be where mine was adjusted. To get roughly the same evaporator temperature from R134a as I would have had with R12, I would need to adjust the POA down to about 27psig, however after a little discussion and reading up here on the auto ac forum I've decided to target a lower pressure of 26psig. This will give a freon temperature of about -1C or 30F which is technically below the freezing point of water, but with warmer air flowing over the evaporator it's still very unlikely to ice up and should give better cooling performance.

Here is a video explaining the POA adjustment. I used a 3/8" socket to loosen the lock nut and a 7/32" socket to adjust the setscrew.

https://youtu.be/B95_q7Z8DIE

It was really hard to see the gauge or line up the camera phone while taking video, so I went back and snapped a photo to show it indeed was set at 26psi.

Image

Satisfied with the POA adjustment, I took it off the system and got setup to flush the evaporator. Flushing removes any residuals of the old lubricant and R12 residuals that may not play well with the new R134a, as well as cleans out any debris that may have accumulated while the system has been unsealed for the last twenty years. I used an aerosol flush called A/C Pro "Power Clean and Flush". Below is a photo showing how I set it up. The flush is fed into the outlet of the evaporator and I used clear poly tubing at the evaporator inlet to a plastic container under the car. That allowed me to monitor the color/cleanliness of fluid coming out. Once I ran all the flush through, I blew it out with compressed air.

Image

Below is a video showing some of the process. When I went to air blow the system, some of the flush was forced out of the small oil bypass line so I added another tube to route that down to my collection container. The function of the oil bypass line is to make sure oil in the system is always allowed to circulate back to the compressor even if the POA valve is mostly closed.

https://youtu.be/vMpsEwh7Uqg

Here is my collection container showing the amount of flush that went through the system and the resulting color. The amber color of the fluid is mostly from the first few seconds of flushing when the flush came out pretty dark.

Image

Image

Not photographed, I also flushed the POA valve on the bench into a bucket.

Can anyone advise me on what amount of compressor oil to put into this system? Since it has all custom hoses, a 14"x24" parallel flow condenser, and a different compressor (Denso 10S17) from the factory system, I'm not sure if the factory oil amount is appropriate. The compressor is used but the oil has been drained from it. The evaporator and the POA valve are original to the car. The drier, condenser, TXV, and hoses are all new.
Dougflas
Posts: 214
Joined: Mon Apr 18, 2016 6:10 pm

Re: Resurrecting 1970 GM AC with POA

Postby Dougflas » Sat Sep 29, 2018 6:09 am

I have not been here in a while due to computer issues. I probably have OEM fittings if you need some. Also. I see your questions have been answered. As far as oil capacity, remember the only component that requires oil is the compressor. The OEM compressor required about 11 oz of mineral oil if memory serves me correctly.

Return to “Automotive Air Conditioning Forum”