a/c recharge advice

[dgc2]

I am recharging a completely evacuated a/c system on a 87 911. 24oz were added to the high port with the engine off. 12 oz were slowly added to the low port with the engine running and the compressor clutch engaged. Adding the next 12 oz through the low port results in freezing the coil. Temperature at the base of the expansion port reads 40 degrees. High side pressure is in the normal range (~150 psi). Low side pressure is above normal (~90 psi), indicating frozen coil is not passing freon.

Is there a different procedure for dealing with this situation other than waiting until the coil thaws out before attempting to add more freon? As a 609 certified HVAC tech, I am aware of low freon resulting in low pressure causing low temperature (boyles law). But I have never encountered a case where the coil freezes very quickly.

Thanks for any help.
dgc2

[dw1]

I am a bit confused by your message, for several reasons.

You don't say if you are using r134a or you scored some r12, but for r134a the refrigerant charge is approximately 38 - 40 oz, based on the r12-to-r134a equivalence. You wrote 24+12+12 (total 48) if I understood correctly, which might be good for r12 but maybe overcharged on an r134a system.

Also, I am not familiar with adding refrigerant via the high side in an automotive a/c system.

Regarding the coil freezing - what are the ambient conditions? Is the RH abnormally high?

[GH85Carrera]

The biggest "secret" to a cold air system on our G body 911s is starting with a really really really good vacuum. It is physically impossible to get "too good" of a vacuum. Run the vacuum pump for hours before you start filling.

And adding liquid refrigerant to the high side first is OK if you are in a big hurry. I never do it that way personally.

Is your system 100% stock. Just the OEM front and rear condenser, factory evaporator hoses and receiver dryer?

Only then can you use the weight by ounces, and if you are still using R-12. Any change from that will change the weight you need.

[dgc2]

Thanks for your comments. Everything is new except the rear lid condensor (which has been cleaned). This includes a new condensor fan. Yes I am using r12. And yes the system was under -40psi for 4 hours, and held a vacuum for 4 days. RH and temps in DC are ridiculous, but not extreme enough to limit charging. The frozen coil has me puzzled.

[GH85Carrera]

The best real test, is turn the fan to max speed, and go for a long drive on a hot day.

One of the very trickiest parts is to get the temperature probe in perfect spot to work perfectly. If it is just a tiny bit off that perfect placement, a good working system will keep getting colder until the entire evaporator freezes, unless it is 110 degree and single digit humidity.

Back in June, I drove my 911 to Palm Springs, CA. I went the southern route, and was in far south New Mexico, Arizona, and So Cal. 107 degree was the average daily high. I have to back the temperature knob back off of full cold or it will freeze up in anything but the hottest and driest temps.


If it is just in the 80s or low 90s my return line, low pressure line to the compressor is still cold enough to have frost on it.


My inside temps are 37 degrees or so, but I have to dial the thermostat dial back some or it does freeze up.

I know where that magic "just below condenser freeze up" point is on my temp control as evidently when I had to replace it, I missed the ideal spot.

Backing off the temp control is a really easy and free fix.

So, wait until it is max hot outside, go for a drive and start with the temp knob at 1/2 way point from off to max cold, drive for a while, and keep dialing it down until it freezes up. You will know because the air coming out the vents is warm, and very low volume. Just dial it back to full warm, and drive a few minutes, with the fan on high. When the air volume returns, go back to just a little less cold than the time it froze up. A little experimentation and you will find that perfect position to keep it from freezing.

Also, use the gauges to read the pressures to determine your charge level. Lows in the 40s and highs in the 250 to 290 depending on temps. If it is overcharged, your system will likely blow a seal or start leaking and that is bad.





Enjoy driving your 911 on hot days. By the way, my system has four condensers and the Griffith's hurricane blower, so you will not get the performance I get, but it will be better than no AC!

[PeteKz]

Hell, it's cooler there than it was in Portland yesterday!