dr bob

Been doing it at least that long, and even taught it some. Trust me. The sight glass is an OK indicator but far from the best way to charge. You are correct that a perfect system will be just at the bubble point there when it's working perfectly. But while you are standing in the driveway with the engine idling on a 75º ambient day it may be "just right", the same charge will not be "just right" for idling in 100º conditions with high humidity, or at 3000 engine RPM in any ambient condition. The expansion valve "adjusts" for most of this, but only based on evaporator pressure(temp) not for the high-side conditions where you have the sight glass.

Take a peek at the Mollinier diagram for R-134 and notice that it has a big 'knee' and in fact doubles baclk on itself slightly. Toss a couple darts at the chart, and tell me what the one perfect bubble-point condition in the system should be that will be perfect anywhere else on the chart.

----

Lots of folks have worked on these systems for years, and have a conceptual working knowledge of how they should work and what seems to work for them. But there really is some serious science behind it all. Really!

smyth1

going to disagree with you again.ac and or refrigeration systems are not charged to conditions.system is designed to have aspecified amount of freon to operate correctly any more or less and the system is not working to its full potential.the system will adjust itself to the changing outdoor and load conditions.the expansion valve senses evap suction line temp and will open or close depending on the load moving across the coil.the drop in pressure when the freon psses through the valve is what creates the freon to change from liquid to gas therfore droping in tempureture creating a cold evap coil.there may be some bubbles in the sightglass if the valve hunts during a change inload conditions but other than that the line fron the condensor to the evap should be 100% full liquid in any system

dr bob

Agree that, under any particular "perfect" condition, the line should be liquid. But gertting to just at the bubble point for all conditions is impossible. You state correctly that the expansion valve adusts liquid admission to maintain optimal pressure (temp) conditions in the evaporator. You also state correctly that it does not and in fact can not adjust for condensing pressure. So the "correct" mass of refrigerant in the system is a moving target based on many influences, only one of which you can control (compressor RPM).

So my questions to you: What should the ambient conditions and engine RPM be when you charge to lquid in the glass? When you come back on a cooler day and see bubbles, should you add refrigerant? If you come back on a hotter day and see no bubbles, should you bleed some out until you just see bubbles, then pump a little back in?

Hopefully you can see why the system is best charged by weight. All but one condition for any particular charge level will be a compromise. To get the best performance within the design capabilities of the system, you need to put a specific mass of refrigerant in the system. I'm not saying that charging to the sight glass is bad, it's just not that great, especially when the OP is discussing charging an empty system.

smyth1

yes charging by weight is the best method.chances are unless you are set up with the proper equipment you wont get it right.as far as the ambient temp goes a temp pressure chart is need for the type of freon that is used.air conditioning is designed to work in temps of 80 and up so charging a system in any cooler temps than that is difficult.

Calgary Ole

Jim
Your checklist is correct. I am not an expert but I was able to charge my system after reading hundreds of posts from experts like Dr Bob, Merlin, Dan, Wally, Blown 87 and many others.

I removed and flushed as many lines as possible,changed the o-rings and added the correct amount of oil, purchased a guauge manifold and pulled vaccuum.
The best way is to add refrigerant is by weight and you will not have to worry about overcharging. I filled via the low side by adding a can then starting the system to add the rest because it seemed the easiest for my first try at AC.

One issue I had on my first attempt was forgetting to purge the manifold line after the first can so I decided to release the refridgerant to start again. I caught the refridgerant in a cup so I could open a line to replace the oil that escaped. You would be suprised at how much blew out. I used a suringe and replaced the oil that escaped then pulled vacuum again and charged. The right amount of oil is key for a happy compressor!

A year later, I used the AC today which performed perfectly!

As scary as it seemed, it was relatively easy with some patience and help from the forumn.

jwillman

Thanks all.

I pulled a vacuum today and it held so I am pleased with the mechanical part.

Unfortunately my HVAC controller seems to have fallen apart so the saga continues in another post for that issue.

I'll pick this one up once I resolve that issue.

jwillman

Finished up the AC today.

First can of R134 bleed out through the lower dryer connection by the time I found it. I had not tightened it enough.

Pullled the condenser and tightened all fittings to the dryer and then did a another 4 hour vacuum. As pointed out by the experts a vacuum will not find issues that a nitrogen pressure test will find. System held a vacuum fine but bleed pressure immediately.

On the seconD go around I did a liquid fill through the pressure port with engine off. I put in ~895 grams of R134 and after a short wait started the car and turned on the AC and cold air

Not making ice cold but not bad considering I have some vac issues and not all the air is routed correctly. I did also remove the old vacuum heater valve and replace with a petcock style valve so I can turn off all hot coolant to the evaporator regardless of vacuum.

At idle I have 43 psi low side and 275psi high side. Low side seems a bit high based on previous recommendations. Any thoughts?

dprantl

What was the ambient temp? How did you charge, with cans or a 30lb tank? Did you purge air from the charge hose before charging?

Dan
'91 928GT S/C 475hp/460lb.ft

jwillman

Outside temp was ~90F. I used 12 oz/340 g cans and had a scale for the last partial can. I did purge the line each time I connected a can.

dprantl

Both low side and high side seem a little high for 90 deg F ambient. Some common reasons why this would be:

- Some air in the system
- Expansion valve stuck open/partially open
- Weak compressor
- Refrigerant slightly overcharged
- Too much refrigerant oil in the system
- Insufficient condenser and/or condenser fan efficiency

How is your fan clutch? Was the electric condenser fan running? Do you have belly pans installed so that hot engine air does not recirculate through the condenser? How are the pressures if you hold the engine at 1500RPM?

Dan
'91 928GT S/C 475hp/460lb.ft

WallyP

Refrigerant pressures are normally specified at 1500 RPM.

You appear to not have sufficient air flow at idle. Adding a relay to operate the compressor clutch and electric fan seems like a good idea.

Hilton

The factory manuals don't list how much r134a for a 6E171 - it was never a stock combination. However, they do list R12 values for both the 6E171 and 10PA20C, so you can work out a reasonable correction in refrigerant volume for r134a in a 6E171.

For R12 with no rear air, from WSM (p87-90):
6E171 takes 1050g R12
10PA20C takes 950g R12
so 10.5% more refrigerant in the 6E171.

For r134a, 10PA20C takes 860g (p87-129)
So add 10.5% gives 950g of r134a in a 6E171-based system with no rear air.

There's probably some thermodynamics laws which make this simple assumption less correct, but I did find getting charged with 950g of r134a instead of 865-ish made it cooler faster.

For completeness, in case this turns in in peoples searches for info on quantity, the mineral oil spec for 10PA20C is also lower - 120ml +/- 20, versus 280 +/-20 for a 6E171. Based on volume, I added the 160ml difference to the spec for oil in a GTS (160 +/- 20ml ND-oil 8), making it 320ml ND-oil 8 in a 6#171 system.

Oh.. other keywords for search.. quantity amount PAG POE ester

jwillman

Dan,

1. I bleed the service line between each can.
2. Expansion valve is new.
3. The AC shop that built my new lines tested the used compressor and said it was in very good shape and clutch had minimal wear.
4. The pump was drained (best I could) and I went with the book recommendation for oil capacity.
5. Condenser is used but I did flush it before install.
6. Condenser fan is operational and was running shortly after the AC started. I have a 75deg switch in ther radiator.
7. Clutch fan is there but not sure how to determine condition. Is there a test?
8. No belly pans. Car never had them since I have owned it (just a year).

I will check pressures at 1500 RPM latter this week.

Thanks all.

dprantl

Also when you read the pressures at 1,500RPM, put the fan speed on '2'.

Dan
'91 928GT S/C 475hp/460lb.ft

jwillman

So readings at 1500rpm and fan speed #2 with an OAT of 94deg F were:

At start low 34psi - high 235
5 min - 30 psi low - high 300

vent temp was ~55 deg

As a comparison I went to my 2010 ford F150 with fan at 2 and was at 55deg and on max it went to ~50 deg. That saud I have never been impressed by the truck AC but am more impressed by the 32 YO 928!!