r134 recharge, no complaints just a question

[teleski42]

1988 911 Carrera with updated AC to r134. Used a recharge kit with oil and a pressure gauge 2 weeks ago. Added 2, 16oz. cans to raise pressure to 25lbs. which is lower end of 25-40 lb. range. Blows plenty cold as evidenced by today's 85 degrees. I am satisfied. Question is with r134 system is 32oz. refrigerant the max. capacity? Thanks, Jim

[SilberUrS6]

Posted somewhere on PP is a pressure/temperature table for R134a in our cars. So, you put in as much as is required to get to the high/low pressure points at a particular air temperature. So, the pressure will be higher on a hot day and lower on a cold day.

[teleski42]

Thank you SilberUrS6. The chart was in the kit, it was a cooler day and my aim was 30-35. I could have checked it today given the hot temp. probably irrelevant because it works satisfactorily at least for now.

[SilberUrS6]

You're good to go!

[kuehl]

teleski,
The stock factory charge of R12 for that year was 47.6 oz.
In theory, with R134a you might use approximately 80-85% of that weight or 38 - 41 oz to start off your initial charge and then you adjust the charge.

First however, you need to start with the system evacuated, you've pulled a vacuum correctly on the system, check for gross leaks, and then you charge the system.
The final pressures are tested with the system idling, deck lid down resting on the service hoses, and you refer to a P&T chart (pressures and temperatures) comparing the ambient outside temperature on the chart with the corresponding high side pressure noted (that is just the basics).

As Eric pointed out the pressures will vary depending upon the outside (ambient) air temp.

[88911coupe]

Not to hijack the thread but I have a question about P/T charts...if you have 2 different cars side by side but both using same refrigerant (say R134), both properly functioning, but with obviously slightly different ac systems. Should you have similar ac vent temps with the same hi/lo pressures?

[kuehl]

Originally Posted by 88911coupe

Not to hijack the thread but I have a question about P/T charts...if you have 2 different cars side by side but both using same refrigerant (say R134), both properly functioning, but with obviously slightly different ac systems. Should you have similar ac vent temps with the same hi/lo pressures?

What are the differences?

[88911coupe]

Sorry, lets say two different cars, both with properly functioning systems. I guess what I'm really asking is are P/T figures that I see irrespective of the system? I seem to recall (can't find it now) a chart that showed pressures for the low side and then an "expected" vent temp, i.e. 28 psi on the low side would result in 28 or 29 degree vent temps. A follow on question would be if you see vent temps far out of line (typically higher than expected) then you could assume somethings wrong...correct?

[wwest]

Originally Posted by 88911coupe

Sorry, lets say two different cars, both with properly functioning systems. I guess what I'm really asking is are P/T figures that I see irrespective of the system? I seem to recall (can't find it now) a chart that showed pressures for the low side and then an "expected" vent temp, i.e. 28 psi on the low side would result in 28 or 29 degree vent temps. A follow on question would be if you see vent temps far out of line (typically higher than expected) then you could assume somethings wrong...correct?

To my knowledge the P/T tables can be used, are to be used, only to be sure that an R134a system is optimally charged. My memory says that the conditions should be with the system set for maximum cooling, blower on high, engine RPM elevated (~2000 RPM) and the cabin completely open to OAT.

That being said I don't think any of that can be related to vent temperatures directly

Too many other parameters involved...inlet air temperature, blower speed, etc.

Many newer systems target the evaporator core temperature at ~34F 24/7 and then moderate, modulate, the air outlet temperature using a REHEAT/REMIX method. So even with the evaporator core at 34F and with your setpoint at 72F the system outflow air temperature might well be only ~68F, just cool enough to sustain the cabin atmosphere at 72F.

[rusnak]

I want to echo the comment that you must evacuate the system prior to charging. Get a vacuum pump, use fresh oil, and pull a vacuum for at least an hour.

[wwest]

Originally Posted by rusnak

I want to echo the comment that you must evacuate the system prior to charging. Get a vacuum pump, use fresh oil, and pull a vacuum for at least an hour.

I don't think rusnak means for "topping off", re-charging, refilling.

[tailwind22]

So the concept of doing a DIY charge a couple times a season just to freshen up the coolant is not advisable?

[wwest]

Originally Posted by tailwind22

So the concept of doing a DIY charge a couple times a season just to freshen up the coolant is not advisable?

How much..??

But generally speaking if you need a recharge a couple of times a season then something is leaking refrigerant at a serious level. Beyond 10 years is the norm for most systems.


My '95 LS400 is now at 270,000 miles and still has the factory refrigerant. Not unexpected, I might add.

[teleski42]

Just to be clear ( I appreciate Kuehl's comments which I have read on other ac threads) the car was purchased in Nov. 2012 and the upgrade done by the p.o. The tech who did the PPI said the system was in good working order and could benefit from a recharge. It was not blowing any cold air at the time. For this reason I elected to do the simple procedure and add 2, 16 oz. of r134 with the oil already added to the can. I did this with the car running, ac system on at the max. I stopped after 2 cans (32oz.total) to be safe. Today was mid 90's and it was satisfactory. Is there a risk in adding more r134? Thanks.

[rusnak]

Originally Posted by wwest

I don't think rusnak means for "topping off", re-charging, refilling.

Yes, exactly.

I took 2 cans to mean that he started with an empty system.

[rusnak]

Originally Posted by teleski42

Just to be clear ( I appreciate Kuehl's comments which I have read on other ac threads) the car was purchased in Nov. 2012 and the upgrade done by the p.o. The tech who did the PPI said the system was in good working order and could benefit from a recharge. It was not blowing any cold air at the time. For this reason I elected to do the simple procedure and add 2, 16 oz. of r134 with the oil already added to the can. I did this with the car running, ac system on at the max. I stopped after 2 cans (32oz.total) to be safe. Today was mid 90's and it was satisfactory. Is there a risk in adding more r134? Thanks.

Yes. You have got to use the High/ Low gauges set and fill with the manifold hose. Follow the pressure/ temperature chart. I've found that it's really very critical to evacuate the system when you are running R134a

[Ronnie's.930]

Originally Posted by rusnak

I want to echo the comment that you must evacuate the system prior to charging. Get a vacuum pump, use fresh oil, and pull a vacuum for at least an hour.

Isn't it more like 3 hours plus for a 911 due to all the hose, etc?

[rusnak]

Originally Posted by Ronnie's.930

Isn't it more like 3 hours plus for a 911 due to all the hose, etc?

Depends on the vacuum. Some guys say run it all night.

[kuehl]

Originally Posted by teleski42

I stopped after 2 cans (32oz.total) to be safe. Today was mid 90's and it was satisfactory. Is there a risk in adding more r134? Thanks.

The stock vehicle does not have a pressure switch feature to prevent the compressor from operating when system pressures and refrigerant volume (the 2 go hand in hand) are low.

The compressor relies on a sufficient amount of refrigerant flowing through it with the oil (the refrigerant carries the oil through the system) to keep it lubricated and cool. No lubrication or hot compressor turns into toast; compressor throws itself apart inside and the debris contaminates the system (plaque in your arteries, damaged heart). This becomes very expensive to fix: new compressor, new drier, tear apart the system, liquid flush everything; time and material.

Find the leak and fix it. Replace the drier and hoses with barrier hoses with a low high pressure switch. Be done with and with the time you save go golfing, hit the beach, spend time with the gal and kids, enjoy life.

[SilberUrS6]

Originally Posted by rusnak

Depends on the vacuum. Some guys say run it all night.

Really good vacuum pumps are expensive. The better the pump, the less time it takes to pump down. But it's true, there is a LOT of volume to evacuate.