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AC R134a conversion

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Joined: 29 Jul 2001
Posts: 8818
Location: Royal Oak, MI, USA

PostPosted: Sat Sep 02, 2023 9:02 pm    Post subject: AC R134a conversion Reply with quote

Just went through re-starting the system on my '82 931 last night. Got nice cold air - but have a little bit of electrical cleanup etc to do, and verify performance in the heat later today.

Key points - charged with 26oz of 134a, based on 944 numbers from Rennlist. Aiming for 6oz of ester oil - need to drop another 3oz in there, one of the cans somehow, inexplicably, came with a LH thread! Chinese junk FTW!

We disconnected everything, flushed the evaporator and condenser, but only blew out the lines with compressed air. There was quite a bit of funk and gross oil in the system. The compressor, we hotwired the clutch to engage and cranked the engine (coil unplugged, so not running) while blowing first flush solvent then air through to clean.

Key point there being not to run the solvent through any flex lines, to avoid damaging them.

Evaporator was removed from the cabin to flush, to reduce the mess.

Then install the conversion fittings with new Schrader valves, assemble everything with new green o-rings, and pull a hard vacuum while we ate dinner. Sealed up great, so charge (jumping the low pressure switch initially to get it to cycle the compressor) with the oil and 26oz of 134... and off we go!

I did also find the compressor clutch grounded to the compressor, which wasn't grounded to the engine, had to add that lead... which needs a rework in the light of day. And both the clutch and pressure switch terminals need a good cleaning - another project for today.

Happy to once again have air in this car; had it before, but didn't do a good, thorough job (with flush and hard vacuum pull) last time so it wasn't awesome and didn't last forever (as I missed a couple of o-rings, on the condensor as I recall).
Vaughan Scott
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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Joined: 05 Apr 2022
Posts: 221
Location: Roberts, MT

PostPosted: Sun Sep 03, 2023 5:10 am    Post subject: Reply with quote

Great write-up; you've covered about everything I can think of, having done a few of these jobs. Crazy about the LH thread - I think that new garbage, 1234YF, uses LH thread. Funny how R134 was going to save the environment, and now, 25 years later, it's being killed off because it's bad for the environment... For a while, the Wal-Mart store brand R134 was US-made; now it's coming from India, but I noticed a few cans of the US stuff last time I looked, so I bought some to stockpile.

I think the flush is the most neglected element of many conversions. I can't remember where, but I heard somewhere that if you don't flush it out, the old mineral oil will coat the cooling fins and tubes in the condenser and evaporator, reducing their efficiency (which is already worse than with R12). I've thought about running solvent through the compressor like you did, but always been afraid it'd result in metal-to-metal contact with the pistons. Last time I did a flush and re-charge (on my 560SEL), I cranked it over with a drill, re-orienting it several times, which hopefully got most of it out.

Anyway, AC is nice to have! I'll probably just rely on one of my other vehicles when I need it rather than add it to mine, but still, you're in a tight spot if you want to take a summer road trip and don't have it. I hope yours gives you many years of service. The one thing I didn't see mentioned is what shape the compressor is in. The shaft seal tends to be the weak spot, even if the compressor is fine mechanically, and it can be replaced on a lot of compressors. If there's no sign of leakage though, it might not be worth replacing "just because."
1980 931 diamond in the rough
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