merchauser

spring maintanance involving my HVAC system has left me with many questions. my car is R12 to R134a converted, and I have trusted the WSM GTS R134a specs as a guideline. since the major components are the same, I thought the pressures and vent temps would correspond, but I am not sure that is correct.

most conversions suggest an 80 to 85% fill of R134 compared to original R12 capacity. for my car, 40.5 oz of R12 is required, while the GTS with R134 uses 37 oz. (this is WITH rear AC) that is 91% of R12 fill?

R134 pressures are always higher than R12, but can the WSM be trusted? and, WSM dash vent temps are very different from R12 cars to R134 cars. MUCH lower for later cars?

to anyone who has done the conversion, I would be very curious to know what your dash vent temps are, along with pressure readings at specific ambient temps. perhaps this is a good time of the year to ask if anyone is able to post information, or maybe some have recorded previous readings?

Speedtoys

"but can the WSM be trusted? and, WSM dash vent temps are very different from R12 cars to R134 cars. MUCH lower for later cars?"

Yes, they can be trusted.

Dash temps should all be the same if EVERYTHING is working properly under the hood and behind the dash and laboratory conditions are met sealing, evacuating, and filling the system.

System pressures for the most part ONLY set an evaporator temperature just below freezing, no use for anything lower, R12, R134, R22, etc..anything lower cycles the freeze protection, and then there's no point. Pressures themselves are important to look at the health of the system. How the low and high side relate to each other to expose problems.

If you have your pressures, esp the low side in the right range to give you the required temperature, with the right range of fill, everything else is about whats right and wrong behind the dash.

I dont see why dash temps would be any different between the two AT the WSM test conditions. There is arguably, perhaps, a difference at extreme temps in R134 compared to R12, but the peak healthy vent temp between the two is pretty much the same. I was in an R134 converted S4 car last weekend (The Forgotten On) and ice cubes fell out his center vent. The coldest 928 AC I've ever felt.

The end of section 87 has your charts..even a converted system will be in those bounds..its just a gas at a temperature. At the right fill a native or converted system will act and feel the same.

dr bob

Folks regularly cite the tails of old wives when conversion discussions come around to differences in system pressures R12 vs R134a. The WSM guidance is very accurate. If you compare the Mollier Diagrams for the two refrigerants, they are within measurement-error of each other for virtually all US driving conditions. R134a pressures can be up to 20 PSI higher at 120ºF ambient, and the slope steepens above that as ambient temps go up above 130º. Best solution is a clean condenser and correctly-working fans.

Jeff mentions a ride in Blake's well-converted car with ice cubes coming from the center vent. Mine does the same thing, and has for over 20 years converted now. Driving with the freeze switch in circuit, vent temps cycle between 33º and 37º. With the freeze switch jumpered as it was while we lived in SoCal, center vent temp was parked at 18ºF with 90º+ ambient conditions and 65MPH cruise. with fan on speed 2.

At 2000 RPM test condition with cabin fan on speed 2, suction pressure is around 25PSIG with center vent temps in the low 20's with the freeze switch jumpered. You can look at the Mollier diagram for R134a and see how nicely that's working -- with less than 6º delta between gas and air at the evaporator. The WSM diagrams for the GTS show similar results with the freeze switch in circuit, cutting off as the air temps approach freezing point.

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It's impossible to overstate the effect that humidity has on apparent system performance. The system has to work multiple times harder condensing moisture vs. extracting heat from dry air. The performance transition from vent temp above the local dew point to below that dew point is the real test of system performance. For folks in humid climates, the system uses a majority of its total capacity condensing water out of the cabin air. For those of us blessed with a generally drier climate, getting frigid-cold vent temps isn't nearly as impressive. That is covered in the WSM by the darker-gray performance bands on the diagrams. The results can be as much as 20º divergent in a car system.

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Folks who move here to the dry side of Oregon or to coastal SoCal generally suffer through an adjustment period to the dry desert conditions we enjoy. Dry cracked skin and lips, and even casual nosebleeds are common symptoms that slowly go away. Just as I suffer miserably in Florida humidity, it turns out that my lack of personal heat-shedding capability is compromised there.

merchauser

thanks, good to know. I thought that there would be some sort of "conversion" allowance for the different gases. also, it is confusing that the WSM chart for dash temps with R12 are considerably higher than the chart for R134. I have always believed that R12 was colder and more efficient?

can you get blake to chime in on high and low pressures as well as actual dash vent temps?

merchauser

thanks dr bob. do you remember your high side reading and ambient temp?

trying to understand HVAC better and wonder: will 2 identical cars require the exact same fill to get specific results, or will they need individual fine tuning and slightly different levels to get there? is the factory fill a suggestion/guideline or an absolute?

Speedtoys

"will 2 identical cars require the exact same fill to get specific results"

Yes, it's physics. They both have the same system volume, and to reach max efficiency at the evaporator and to make sure superheat conditions exist. (https://blog.ravti.com/knowledge-sup...g-b14741120174)

The right fill is by liquid grams, 1 gram is less than the weight of a paperclip. So the window is very narrow. This is why machines fill at the factory, and at the AC shop..they use weight. Evac, fill, done. Too much oil has a negative effect. Moisture in the system has a HUGE negative effect.

And how the gas works to go to liquid, and then again to a gas, is straight physics as well. (see above link)

The high side pressure you want is only useful as a reference to the low side to see if anything is wrong with a low pressure that is delivering a 2-5d sub freezing _temperature_ would be "normal". Outdoor temp & humidity affect high side pressures a LOT, so one persons reading is only somewhat relative to another persons "at the same temperature". Air density matters too, as coil cooling is a function of air mass over time (density and volume/speed). The low side will be a less variable value, the high side could be all over the place, depending.

Vent temps in a healthy AC system are basically the Evaporator temp (low side reading), plus the efficiency loss in moving air of "a temperature" over it. It will never be 100% efficient, as in 100d input air will never come out at the evap temperature. Depending who you ask, you can expect a 30-40d drop max. The you GAIN temperature by any not fully factory perfect components behind the dash. Missing/old foam seals, vacuum pods leaking or not working, etc.

IMHO, most AC refresh/conversion project fail in how sanitary the evacuation(Not long or deep enough draw), oil type & volume fill/overfill, and the recharge process where using individual cans can allow moisture to get in...and not fully emptying a can leads to accumulated errors in the process of knowing what is actually IN the system. (Keeping the cans in a bucket of HOT water ensures that they FULLY empty)

AC parts are easy to manage, refresh. The end to end charge process is unforgiving..simple, but small mistakes are very measurable in the end product.

You're not alone in "wondering"..absolutely not.

merchauser

great explanation jeff. thanks.

merchauser

is jumpered the same as bypassed? if yes, why would you want to bypass the freeze switch? won't this damage the coils?

The Forgotten On

If it gets enough ice on it to ice up the coils then yes. It isn't really an issue with shorter drives under an hour in a dry climate.

I mostly did drives like that but still left it in place. And as Speedtoys will tell you, I have cold AC

dr bob

I've frozen the evaporator to the point where airflow gets restricted, with no damage (so far...). As Blake shares, the issue comes into play in humid climates and longer drives. My black car in Los Angeles enjoyed the lower available vent temps especially on warm restarts with the car heat-soaked. Hot days in Los Angeles are in the early fall, temps above mid-nineties but low single-digit RH numbers. Plenty of heat to shed but very little moisture to condense. Contrast with your Florida conditions, where an afternoon RH less than 30% is unheard of. For those playing along at home, a daytime 50% RH in a desert climate is cause to watch for thundertstorms. No such thing as the 90+ days with sweltering 90%+ humidity at the same time that many states enjoy. I grew up hating that climate, so moved to SoCal for the dry climate (and relatively free state education at the time). Many seem to think of SoCal as a mecca for winter-haters, while I moved as a humidity-hater. Go Figure. I like to think I'm smarter now too.


Keep in mind that the cycle temps at the freeze switch are somewhat adjustable. Air temps cycling between 33 and 38 or so demand a carefully adjusted switch, and represent an evaporator temp that's at least sometimes below the freezing mark. The amount of safety room you allow needs to consider your use pattern in addition to system loads and typical approach temperatures. I don't mind a little front forming on the fins once in a while, so long as it has a chance to flash off when the compressor cycles off.

Everything in the system is a compromise. The center vent can blow freezing fog under the right condtions.

merchauser

does anyone have actual low and high side readings at idle and/or 2,000 rpm to post? I would really like to see what other people have observed.

Speedtoys

??

Why?

This is the part where "The internet makes this hard"...because its full of misleading data.

You know where it should be, why are anyone else's values in their test conditions useful to anyone else?


You know, with great detail, what yours should be on the day and weather you test it.

dr bob

my high-side readings vary with engine RPM and the amount of heat the condenser can shed. The pressures self-balance around those plus compressor capability and evaporator load. Absent exactly the same ambient and load condtions, no set of numbers I share from my car will agree with the numbers you see.

Your goal is to have the lowest possible low-side pressure you can while maintaining liquid to the expansion valves. That will give you the maximum available heat removal from the cabin. Adding more refrigerant beyond that point raises the whole system pressures including the suction pressure that determines the amount of cabin heat needed to boil the refrigerant in the evaporator.

928NOOBIE

I have to adjust my freeze switch...we did the conversion on my '86 and I've measured output in the upper teens with ambient temp somewhere in low 70's....its freezing up at lower temps but I would expect a properly adjusted probe could yield a reliable and plenty cold supply of air.

merchauser

we know the WSM numbers, but I would like to see actual real world numbers, especially from you guys who seem to have incredibly cold systems. would be good to confirm that your numbers are within the WSM specs, or could there be variants

hoping this info will be useful to others, as well as myself.

for my application, I am VERY close with my levels, but not perfect. in the driveway, my dash vent temps are around 44, and driving around town between 34 and 37. on the highway, its 39-42. the "feel" is pretty darn cold, and the cabin is comfortable. WSM says temps should be 34-37 for a sitting car @2k. so driving temps should be even lower? my high side at 235 is a little low and i may add some Freon, but my low side is a little low at 10, and i wonder if my expansion valve might be partially clogged, or sticking? but......i thought a failed EV would not produce cold temps, and a failing or intermittent EV would produce a varying of dash temps. sometimes cold, sometimes cool, etc.