merchauser

great work. are you certain your chart for the low side is correct? the ranges don't match with the WSM?

Bulvot

Thanks. It should be correct now. I forgot to adjust the maximum for the low side when I copied the worksheet from the R12 spreadsheet. Keep in mind, that I've converted bar to PSI and Celsius to Fahrenheit. And I've stretched the graphs out horizontally a little.

For reference, here are the WSM pages with the R134a charts:

merchauser

^^^^^great job. thanks for posting this info!

merchauser

can it be assumed that if you charge at the low side of the allowances the compressor is likely to cycle more, and charging towards the higher numbers, cycle less?

dr bob

Yes that's a valid assumption. But don't charge toward the higher numbers on purpose, thinking that cycling the compressor less is a reason to cheat yourself out of otherwise-available cooling capacity. On a warm day, especially when the car has been sitting out, having that extra capacity is precious. A closer to perfect setup might delay freeze switch operation for the first maybe ten minutes after engine start, giving you maximum initial cooling capacity for the heat-soaked cabin, before there's a chance for the evaporator to ice over. With the system as it is, heat-soaked cabin air will typically use enough capacity to keep the freeze switch happy for a while anyway.

It should also be noted that the chart terminology can be a little misleading. The air side evaporator approach temperature is not ambient temp as described in the workshop manual diagrams, it's the inlet temp to the evaporator best measured in the passenger footwell. The auto AC only opens the fresh air vent after cabin temp approaches target temp, so you have max cooling and dehumidifying available. The test condition has the temp slider at max cold, so the fresh air vent stays closed throughout the test. This also clarifies the confusion some have with the industry guidance of "~~40º of temperature drop available" in the system. That number refers to how much temperature drop you see across the evaporator, which may not be seeing ambient temp at the inlet. Some see he guidance number and think the vent temp won't drop below 50º on a 90º ambient day, and that's not at all the case.

Calculating the efficiency of the evaporator is too complex to share in a forum post, thanks to the slew of variables involved. On the refrigeration side, liquid enters from the expansion valve, and heat from the cabin air is used to boil the liquid. that causes it to expand, so flow velocity increases towards the outlet as more heat is absorbed. The actual distribution of heat movement favors the liquid inlet end of the evaporator, good since inlet airflow favors the other end. Net is a transfer load that's fairly consistent across the evaporator, with coldest air passing the coldest section on the way to the vents.

From a central California tour, when it was only 85º ambient, but 20º center vent temp. Freeze switch was jumpered for dry Los Angeles conditions, forcing me to shut off the AC every half hour or so on this tour to let the ice melt.

Speedtoys

Do you have a WSM?

How can you be this far in what you've been doing, and this is the first time you have seem this?

You're handicapping yourself without a manual set...and easily cost yourself the purchase price.

Speedtoys

Three things cycle the compressor

1) Too low of a charge (it all leaked out)
2) Too HIGH of a charge...I mean, a serious mistake was made or the system is seriosly broken.
3) The freeze protection switch saying "stop".

This was noted a few weeks ago when you brought up compressor cycling, the 928 system doesnt cycle by any timed/reasoned means unless it senses freeze conditions...otherwise it could run 24/7 just fine.

dr bob

Jeff --

Paul was citing the chart pressures prior, as he has been trying to identify the causes of deviations from what he's seeing on gauges and the center vent vs the chart numbers.

Speedtoys

Thanks goodness, I was concerned. Hard to get so far without reference material.

merchauser

does an expansion valve dictate the value of low and/or high side pressure values? are expansion valves adjusted differently for R12 v. R134?

dr bob

The expansion valve does that but indirectly. It throttles the incoming liquid flow based on the temperature it sees in the evaporator outlet. The pressures change interactively based on heat loading and compressor (pump) flow.

There may be slight differences between R12 and R134a valves but their target temps are darn close. Being a true mechanical temperature valve, the action of the valve is solely proportional; it opens to a specific position based on measured temperature, and makes no further correction if the position doesn't cause the target temperature. The differences, at least theoretically, would be with the density of the two refrigerants. In practice these are not noticeable. Meanwhile, any valve you buy new today will be suitable for R134a, as the valves are used on other cars that are factory-fitted with R134a systems.

merchauser

a few weeks ago my numbers and temps were pretty damn good, but reading about other cars, it appeared I could do better I had no plans to start over, (again) but a leak in the clever low side port extension that I made, to avoid scrapes and burns, had a leak, so I was forced back to square one. since my expansion valve was 16 years old, I replaced it, though I don't think I had any bad expansion valve symptoms. didn't think it would hurt to eliminate another potential problem. I diligently vacuumed and recharged to 35 oz. (85%) and my pressures were now very different from a few weeks ago. previous was 18/235 @ idle and 10/255 @ 2k with ambient at 90'F, after my fill, I was at 40/265 and 35/275 with 90'. thought I might be overcharged, so I took a little Freon out, and no real difference.

finally realized, I am over my head, and took my car to a reputable AC repair shop. the bosch machine they hooked up to my car was amazing and I though it might mix me a drink! technology is amazing. $49. special included recovery, vacuum and recharge up to one pound. (recovery was 32 oz. 78%)

tech recharged to 36 oz (88%). numbers were 48/278 @ idle and 34/300 at 2k with 97' ambient. dash vent was 54'. again, high on both sides, and again, suggesting an overcharge, but not overcharged. I presented all the previous info and he suggested my new expansion valve was calibrated improperly and not allowing me to get lower numbers. he also said that he rarely sees valves go bad unless the compressor grenades. his suggestion was to reinstall my old expansion valve. he was thorough and pleasant, but told me this service was a lost leader and that he couldn't go much further unless I wanted the clock to start ticking at $125.hr. (he did spray the condenser with air, I guess to cool it down, but that did not change anything.) driving home, vent got down to 48'.

of note: I brought all pertinent WSM data, and tech could not recall seeing low side numbers so low.

I read about testing an expansion valve using a can of dust spray. hold can upside down, and spray valve disk with cold propellent and high side reading should drop by 50. had to return my loaner gauges, but I sprayed the disc and noticed to vent temp difference.

I still would like to see the pressure numbers from those who are reporting those incredibly low dash vent temps.

dr bob

I may pull the car out this weekend for some show prep, and put gauges on it for some comparison readings for you. I won't have any way to get ambient up past 70º though, so the readings won't be particularly relevant for you until fall in Florida or mid-July here. It's tough finding a 90º day here most years. Plus our mountain RH on those warm summer afternoons is less than 20% unless it's a thunderstorm, and for those we drop the temperature quickly.

Also:
During the testing/filling at the shop, how did the sight glass look?

Also-Also:
Are all the vacuum actuators working correctly?

merchauser

thanks. your low pressure will be very relevant since the WSM parameters are very narrow

the tech did say the sight glass had minimal bubbles

all vacuum working perfectly

forgetting the traditional parameters of percentage charging for retro, could I still be overcharged?

thanks

Speedtoys

Record YOUR pressures and YOUR weather conditions, and compare then to the WSM guidance..let us know what you observe compared to the WSM.