Open Road

Hello, this is for my 1983 US spec 928S. My a/c system has not been converted so it remains R12. It has historically worked in the few years I've had the car but gradually gotten less cold when using. The compressor clutch quit engaging so I tested the electrical circuit first. I was getting 12v on both prongs of the freeze switch and 12V on one of the leads to the low pressure switch attached to the dryer. The clutch engaged when I jumped the two leads at the low pressure switch.

Next I put my manifold gauges on to check pressures and have 0 pressure on both high and low sides. No hiss at all at the connections so it's empty. I don't see any obvious oil residue evidencing a leak on the expansion valve or any of the connections I can see from the top side. I'll get it up on jacks in the next couple of days and look at the compressor for any obvious signs of leak residue.

I'm looking for any thoughts on best way to find the leaks. I've read about using nitrogen, or simply adding some refrigerant along with dye. Do I need to add oil before testing? Any input is appreciated. Thank you.

Also, I do have a vacuum pump and plenty of R12 once I get any leaks corrected.

I use a sniffer as the first means. A bit too pricey for DIY.

Clean off all the old dye before you recharge with dye.

Nitrogen and soap bubbles will work except for the evaporator. For that you need the sniffer. Or dye.

HVAC techs use nitrogen because they have to fill lines with it before brazing. You can use any dry inert gas - I have used CO2 and Argon with no ill effects. While some would recoil in horror at the thought, you are in the beginning of a process where you will be putting in a new receiver/dryer and pulling some part of the system apart. At this stage of the game you could probably use shop air and not do any harm.

Eplebnista wrote: ↑Tue May 12, 2026 1:36 pm At this stage of the game you could probably use shop air and not do any harm.

An interesting topic. Most HVAC folks say to never ever use anything but a dry gas. I too am not certain that using shop air is bad provided that a new R/D is installed and you vacuum the system thoroughly. On the flip side, I am curious if the impurities and water in shop air will react under pressure with the refrigerant oil. I've never researched it though.

I plan on using argon to test my system rebuild simply because I already have a bottle on hand FWIW

Helium would work too... if you're into balloons. But, I'm unsure if soapy water will make bubbles with Helium.

Stay away from using hydrogen for leak detection. (Pro Tip...)