Fox 86 GT mustang A/C R135 upgrade from R12

You're going to need to transfuse the compressor oil with one that is R134 compatible.

Put the system back together, suck it down, and flush the whole thing with the new oil.

There are adaptor kits for fittings.

Service to 80%. More is dangerous.


When I did mine, I flushed the [entire] system and reused all of my stock components (or rather the A/C shop did).

JTRSMC said:

How do you change the compressor oil? And when you say transfuse what do you mean? Do you mean a oil that mixes with the old oil?

Click to expand...

I mean to completely remove the old type and flush it out and replace it with the new R134 compatible oil. They generally do [not] mix although I've heard that there are now some oils that are compatible with both (unverified).

You don't need to replace it. The stock pressure switch is adjustable. If you unplug the connector and look down into it, you'll see a screw. That sets the pressure point. With R12 it's set to around 25psi. WIth R134a, you want to lower it down to about 20psi.

Each 1/4 turn counterclockwise will lower the pressure by about 1psi, so you will need 1.25 turns to drop it to 20psi, and you can confirm this when you install the A/C manifold guages and watch the pressures. If you don't want to mess with adjusting it, you can replace it with one from a parts store as the new ones come set for R134a.

service doc here. Reads the PDF and look at the last page which gives recommended fill amounts for the various components. With R134a retrofits, you want to use a compatible oil for use in a system that previously held r12

The main reason you charge to 80% of the R12 fill spec is because the assumtion is you are not going to get 100% of the R12 oil (mineral oil) out of the system. It doesn't mix, and it's just going to settle into the lowest part of the system (condensor) and just sit there. It will reduce the amount of system internal volumn, hence the 80% fill recommendation. Oil doesn't compress, so putting in too much refrigerant will overcharge the system.

Now, the original design A/C condensor for the Mustang is no longer made. The modern replacements are an improved design that will work for R12 and R134a, but they have less internal volume. As a result, recommended charge needs to drop as well because of this. The label suggests 42 ounces and 10 ounces of oil (reduced to 8 in 1989 despite no system changes). I would target an initial fill around 30-35 ounces with 8 ounces of oil and then adjust based on pressures you see.

If your A/C accumulator has been sitting open for years, i would go ahead and replace that as well. That's the line from the compressor back to the bottle on the firewall.

As for the compressor, you will need to try and remove all the existing oil you can. Have the maifold ports open, turn it over and spin the compressor and try to pump all the oil out. Now, keep in mind if this compressor is original, and your system leaked out over the years, the shaft seal on the compressor is likely bad and needs to be rebuilt. Your a/c charge will likely vent out within a year.

Here's a good pressure chart to use.


As for pulling a vacuum on the system, you'll want to use a good pump and give it plenty of time. You are trying to achieve 500 microns or less, but some say 1000 microns is enough for auto AC due to the permeability of the hoses. A lot of youtube sites claim all you need is 30 mins of vacuuming, but for my car with all brand new stuff, it took almost 2 days of running the vac pump to really get it down to 500-1000 microns. There was just so much moisture continuously boiling off.

Also, not sure where you are located, but given that it's winter now, i would recommend waiting for a warm day before charging. I think 55*F or higher is what is recommended. I would wait to pull a vacuum before charging as letting a system with any leaks sit while under vacuum will pull moisture in.