AC 134a Working Pressures

[ccz145a]

As I put the truck back together, my project this weekend is to get the ac working again. I converted it to 134a about 5 years ago and it did fairly well, but I wasn't a member of the forum then and would like to know what others have done. I bought a new accumulator, orifice tube, and o-rings, hoses are still in good shape and so is the old a6 compressor.

My old vac-pump gave up the ghost and never did pull more than 22 in Hg when it worked (it was a freebie that I stamped my own reed valves out of a beer can.)

So now I have an air operated vac-pump, some ester oil, and a case of r134a.

Every part of the system is out of my '75 c10 454.

So, what's wrong with my plan.

1. Put the system back together with new o-rings coated with ester oil.

2. I lost 2 oz of oil from the old accumulator, so I plan to put that amt in the new one.

3. Hook up my gauges. (scale for r12 and r22)

3. Pull vacuum for 30-45min then close valves and see if it holds.

4. If it holds, load up my first can of r134a and purge the feed line, and load it in. then load another can the same way.

5. There should be enough pressure in the system to run the ac with the engine running now, so start the engine and turn on the ac wide open.

6. Last time I loaded refrigerant until working pressures on the r12 scale of ~250 on the high side, and ~55psi on the low side. It felt pretty cool by then so I stopped.

These pressure readings are way over what the r12 system would run by about 20psi.

Now how would you guys do it?

Thanks

[ccz145a]

I've been thinking on it, and the low pressure switch should let me just start the engine and then dump in the freon and it will just kick in the compressor when it can.

Let you know how it goes. It was 85 and sunny today.

[VileZambonie]

OK operating pressures are going to drastically change based on humidity more than temperature.

Before you charge the system AFTER you vacuum it if you are using pressurized cans install an 8oz charge of ester. Charge the system to about 80% of the original capacity to start and then watch your pressures.

You can look for a pressure/temperature/humidity chart online. I have a few good ones but am late for a funeral.

85° with low humidity expect low side around 37psig and high side anywhere from 275-325psig. Again the humidity will drastically change high side pressures so keep that in mind.

[ccz145a]

AFTER you vacuum it if you are using pressurized cans install an 8oz charge of ester

Funny you should mention that, I did buy an 8oz can of ester on Saturday morning. Saturday afternoon I pulled vacuum before the weather stopped me. Still holding...

If you get a chance post the chart, the one I found online is temp based only. I will keep looking and If I find one I will post it here.

Compressor, hoses, condenser, evaporator are all from the '75 year. I had to use the evaporator housing from the '84 donor and the wiring harness is '84.

The 75's controls are a little different than the 84, and I have some questions. The 84 housing has no provision for the compressor cycle switch that was on my 75. This switch had a temperature bulb that nestled up next to the evaporator. Well, the 84 housing and wiring won't allow this without modification, so I would like to cycle the compressor using the 84 method, since the wiring is all there, but I do not know how the 84 cycled the compressor.

Did the switch on the accumulator do this function on the 84? I have a 2 conductor pigtail in the harness that will connect at the accumulator. I originally thought this switch was a simple pressure cutoff and I was going to lengthen the leads and put it on the high side cutoff switch present on the 75 system. The 75 cutoff is on the high side, right before the orifice tube.

[VileZambonie]

If you are using the cycling clutch orifice tube then you need the low pressure switch on the accumulator. Ditch the TXV and the low pressure switch will allow it to cycle when the low side pressure drops.

[ccz145a]

TXV? Is that the cycle switch with the bulb on it that was in the 75 evap housing?

[VileZambonie]

A TXV is the thermal expansion valve. It is the thing with the capilary tube. So if you are making it into a CCOT system just either abandon it or take it out then swap over the parts from the CCOT system.

[ccz145a]

Thanks. Got the parts, now. Even got the cycle switch rated for 134a.

[ccz145a]

One more question... My gauge set is old and has scale for r12 and r22, both quite different values at the same needle position. Is there any calculation i would need to perform to the readings to correct for r134a?

[ccz145a]

One more question... My gauge set is old and has scale for r12 and r22, both quite different values at the same needle position. Is there any calculation i would need to perform to the readings to correct for r134a?

uh, i feel pretty stupid... psig is around the outside and the temp for various refrigerants is what I was talking about. Of course there is a conversion to be done, and the answer is in the chart posted earlier.

[VileZambonie]

yeah PSIG is PSIG so whatever the pressure is it would read. I believe Autozone had a nice set of conversion gauges and hoses for a good price in their tool flyer a while back since R134A reads higher

[ccz145a]

**Update** Charged up the system soon after the last post and it blows oh so beautifully cold.

ambient temp: 85 semi humid ~70%
low side: ~44
high side: ~350

Thanks for the pointers!

[Captkaos]

Did you get the leakdown fixed?

[ccz145a]

Yep. Messed up o-ring on the cycle pressure switch... It held vacuum, but not positive presure, go figure.