Automotive Air Conditioning Information Forum

Hello,
I have a 1978 Pontiac Trans Am that originally had factory air. I've replaced all the system parts (Compressor, Condenser, Evaporator, Hoses/lines and Electrical). This will stay as a R12 system.

The Pontiac Service Manual states that the high pressure line should not be connected to the system for evacuation or charging. I understand this is to avoid any problems that may be introduced if that line was connected. That said, I would like to monitor the pressure on the high side either during the charging process or just after. I'd like to know that the system is within factory specs as much as possible.

I don't have a port or valve on the high side liquid line but I do have a new muffler that has a port (pictured below).
Is there an adapter available that would let me attach the manifold high side hose to this muffler?

Thanks,
TacoTownCharlie

I disagree with the service manual. The accepted procedure for vacuuming, charging, and testing is to attach both hi and lo side gauges via a manifold.
You vacuum from both sides. You can charge all the vapor to the low side, but a waaay better plan is to charge liquid to the high side until it won't take any more, then top off at the low side by vapor. This way you are not starving the comp for oil during the lengthy low side only charging. Of course the refrigerant should be weighed in to the correct charge level.

The threads on that port look weird, not c/w either R12 or R134. Tim the site owner may be able to offer assistance here, or perhaps one of the other consultants.
Are you able to query the vendor where you bought the muffler from as to just what that fitting is?

The reason they say don't use high side is so you don't try to charge through it with pump running....Duh. they are playing it safe.

Those threads on the port you want to use are easily recognized as metric 134a threads- the same that are on a 30 lb tank of 134a refigerant. So your new muffler is made to be 134a compatible (as a replacement) The easiest thing to do is use a 134a gage set with the center (tank hose) plumbed into the high side (red) gage and attached to your new port. Or you can leave the center hose in the center position of your 134a gage set and meerly open the valve to the high side (with high side quick connect closed) and read the pressure like that. You can use another gage set with r-12 threads (english) to read your low side and feed in your r-12.

Hmmm,
no wonder I did not recognize that fitting. My gage set does not have 134 ends, it has so-called R12 fittings and I adapt up to 134 quick couplers. I use a brass tank adapter for my center hose. In other words R134 to R12 machined adapter. Available on line or perhaps Tim the site owner has them. This way I retain the functionality of all 3 hoses. I also have a valved Tee tree on my center port of my set. That way I can connect up the tank and system for vacuuming and charging. When vacuuming, I am pulling down everything. Then I valve off the vacuum pump, and open the tank valve to charge. I never have to worry about losing vacuum in order to charge. I also have a valved Tee on the tree for my micron gage.
I do it this way because I also do residential AC where R12-type fittings are still used. I'll post a pic later.

You can never have too many adapters...

For what it's worth, I've never seen a test port with metric threads like that, either. Might not be a test port at all. My R-134a manifold gauge sets, as well as my hoses, small-can tap valves, and 30-pound cylinders, all have 1/2"x16 ACME threads, either male or female. All of my R-12 MGS's, hoses, and associated equipment have 1/4" SAE flare fittings. I wonder whether GM is the only user of that type of fitting, which escaped my notice.

Just got off Google and wikipedia
https://en.wikipedia.org/wiki/Trapezoidal_thread_form
If that male thread is truly metric, then it is called trapezoidal metric thread. It is close to the 1/2" ACME we are all familiar with, but not exactly the same.
Shame on GM for making MVAC worse than it already is.

Wow, looks like I have an interesting issue (Fortune cookie: May you live in interesting times).

I may take a step back and consider a replacement high side line that has a more suitable port. The system is empty so now is the time if I want to make changes.

Really appreciate the comments, I'm sure I'll have more questions as I move through the process.

Thanks,
TacoTownCharlie

Looks like a pressure switch fitting to me, that portion below the threads is for an o-ring

DetroitAC wrote: ↑Thu May 19, 2022 3:19 pm Looks like a pressure switch fitting to me, that portion below the threads is for an o-ring

Yep.

TacoTownCharlie wrote: ↑Thu May 19, 2022 11:49 am I may take a step back and consider a replacement high side line that has a more suitable port. The system is empty so now is the time if I want to make changes.

Check with Tim the site owner he may have something available