Chris Prack

I have done hundreds of conversions over the years. Here is your conversion requirments:

1) Evacuate R-12
2) Install 134 adaptor fittings
3) Pull a vacuum
4) install 80% of the R-12 charge with 134.

That's it. Nothing else is needed to convert it to 134. Too much 134 will make it cool poorly.

Turbo17

What about the oil? The official Porsche conversion instructions seem to state that at least 50% of the original R-12 compatible oil must be removed and replaced with R-134a compatible oil.

There's a chart that tells you how much oil to add when replacing each different type of A/C component.

I am about to do the conversion job, and I was planning to use ester oil, which is supposed to be more compatible with the older R-12 oil. I'm not certain how much to add, but I was going to try to follow that chart in light of the total oil amounts listed for the R-12 fill.

Chris Prack

Without removing every component from the car and draining them, how would you ever know you removed at least 50% of the oil??

Ester oil is compatible with mineral oil and PAG oil. The chart you are referring to is as old as the hills and not very accurate. First, it assumes the system has the correct amount of oil in the system to start with and that none of the oil was lost to a leak or during evacuation. Unless the compressor was noisy before the conversion I typically add an ounce to an ounce and a half before charging with 134. To some degree it's a crap shoot. Less is better than too much.

Start off with less oil and go back and add more if necessary. It would be nice if it were easier to know how much to add to start with but it doesn't work that way.

Turbo17

The tech docs were assuming you used a machine that extracted R-12 and oil and separated the two, allowing you to measure the oil removed. If insufficient oil was removed, it called for returning R-12 to the system (but not returning the oil) and running it for a while, then extracting again and measuring the additional oil extracted. You compared the total to the total factory recommended capacity to determine if 50% had been removed.

I was not saying that we can detect how much has been removed without such equipment.

Is it worthwhile to pull components and drain them to try to get old R-12 oil out? My system was last full of R-12 at least 15 years ago. The wife now wants A/C and I've agreed to get it running. I've got the tools, manifold, gauges, etc. I'm going to put a vacuum on it this weekend. I suspect I'll need to rebuild or replace the compressor, and certainly will need to replace the dryer.

How can you tell if it needs more oil?

Chris Prack

You will never remove all the oil buy repeatedly evacuating the system. Trust me here.

If you have to remove the compressor, measure the amount of oil in it and add that much Ester oil back into the compressor before you reinstall it. Add another ounce to the dryer and let it go like that. If you need more oil you can buy an oil injector pretty cheaply.

lee101315

How can you tell if it needs more oil? A knocking noise coming from the AC compressor would be a good sign

Chris Basically summed it up.

BikePilot

FWIW my 968 blows ~44 deg F air out of the vents (in FL so ambi is around 90f).

Expansion valve should let the low side come up when revving once the evap is cold enough to make it happy. If it doesn't I'd guess you've got a blockage or dead expansion valve.

PorscheFanatic202

I'm thinking i'll go ahead and evac the system and check for blockages.

Turbo17

Thanks for the comments. The compressor came off yesterday. Can you tell me what the third A/C port is for my '86 951? There's 1) one high pressure at the front dryer, 2) one just behind that, but up higher in the engine compartment - in the middle of an A/C line easily accessible on the top left of the engine compartment and 3) one low pressure right at the compressor.

Is #2 above used during evac/refilling?