1984 300SD: A/C Vent temperature question
 

Here in Salt Lake City, we've been having record heat this summer, much like the rest of the country. This prompted me to get my A/C working as quickly as possible.

So far, I've replaced the compressor with a brand new one, a rebuilt ACC pushbutton unit, a brand new condenser (the tube-in-fin type, new TxV, new receiver/drier, new pressure and fan switches on the drier, 2.9 lbs. of genuine R12 Freon (I'm EPA 609 certified) and mineral oil.

I removed the old components, flushed the existing tubing and remaining components with an aerosol orange-smelling A/C flush, then installed all the new components. I added 4 ounces of mineral oil directly into the compressor during installation 1.5 ounces into the condenser, and 2.5 into the drier, with the drier being the last, and only exposed to the outside air for a few minutes before installing and pulling a deep vacuum.

The vacuum held perfectly for 5 days as i waited for my R12 to arrive in the mail. Upon receipt, I hooked up my A/C gauges and charged the system with the R12 (just less than 4 cans at 12oz each=2.9 lbs). The system has no leaks and has maintained it's refrigerant pressures for over a week without dropping any.

Now, the A/C works, condenser fan turns on, etc. but I have a concern. The air isn't as cold as I expected it to be. Other R12 systems I've used in past vehicles (non-MBs) have been superb, while my 300SD seems mediocre most of the time.

At night, with outside temperatures in the mid 70s, the vents put out air down to 40 degrees while moving, but around 60 while stationary. During the day, with outside temperatures at 95, the air at the vents gets down to 55 degrees while driving, but is at 70 while stationary.

I expected colder temperatures out of a system with so many new parts. Any suggestions on what I'm missing or does the system sound like it's functioning properly?

Thanks in advance.

you state, that your system held vacuum perfectly for 5 days, THEN you attached your gauges and charged the system with R12...

so, you had it in a deep vacuum for 5 days, without gauges connected?
if so, that is your problem. the R4 compressor has a orifice opening to the atmosphere to allow MINUTE refrigerant to leak out when not running, and under vacuum, it'll draw in atmosphere... moist atmosphere.
automotive refrigeration is supposed to be drawn into a vacuum for a minimum time, and immediately charged with refrigerant, or pressurized with nitrogen holding charge for leak checking. NEVER LEAVE MOBILE REFRIGERATION IN A VACUUM!!!!

please explain in detail how you evacuated and charged the system.

the problem with using cans of refrigerant is the connecting and disconnecting the cans lets in atmosphere...

atmosphere does not condense, or refrigerate at R12 pressures...

I thought venting refrigerant is illegal? At what rate is it venting?

And why?

Thanks for the responses. It appears I wasn't as clear or as detailed as I should have been. Let me clarify:

After installing all the new components, I connected my gauges, pulled a vacuum for 30 minutes to get the air (moisture) out, then pressurized the system with pure argon (since I don't have access to nitrogen). I left the gauges on the car and left the car to sit for 24 hours. I returned, found the pressures to be exactly the same as when I left it (I put little marks on the gauge face cover to indicate position). I concluded there were no leaks in the system, so then I pulled a deep vacuum for 90 minutes (down to 27 inHg) via my electric vacuum pump. Not needing the car for transportation, I left the gauges attached for those 5 days and let the car sit.

When my R12 arrived, I pulled vacuum for 10 more minutes, attached the cans to my gauge set (that's what I meant by "attaching my gauges" in my first post), and filled the system with refrigerant. My gauge set has valves used to open and close the lines, so I used them to close the system off during can changes. I even purged the fill line with the tiniest amount of R12 by cracking the screw-on fitting on the gauge body to expel any incidental amount of air in the line before opening up the valve to start the flow of refrigerant into the low-side (suction) line.

From what I've read on this forum, I believe I've followed the correct procedure with my A/C evacuation/filling, but correct me if I'm wrong. At no point was there any refrigerant in the system while it was under vacuum. What would be the purpose of having the compressor INTENTIONALLY leak refrigerant under any condition? Whatever the reason, that's not what's happening to me. I'm just curious.

My system is perfectly sealed. I have no leaks, and my refrigerant is all intact. At this point, I'm starting to suspect that perhaps my new condenser is sub-par, given the results.

Hmm...
new condenser should be fine, but is fan clutch working properly? the 126 is not the greatest a/c, but should do better than 70 at idle in the upper 90's outside temp...
My bet is on aux fan, and fan clutch.
which fan blade do you have? 9 blade plastic?
also, the system has a temp freeze switch that's adjustable, yours may be adjusted too high...

I have the 9-blade plastic fan. The fan clutch is working. It's only 3 months old. I replaced it when I replaced the radiator, water pump, and thermostat at the start of the hot season. I figured I'd cool the engine first before I worried about cooling myself. ;)

Admittedly, I skimped a little when I bought the condenser. It was only $85 brand new, when I saw others much more expensive. I hope that wasn't a mistake, but it appeared to be of the same build quality as the original I removed. The fins are flimsier, but there are more of them, and all of them are perfectly straight.

I've never heard of the temp freeze switch. I'd love suggestions on how to find/test/adjust it.

EDIT: I should mention that the aux. fan comes on after about 20 seconds of A/C operation, and as far as I can tell, stays on the whole time. I obviously can't tell what it's doing while I'm driving at high speeds, but it's always on any of the times I've checked it.

Hmm. aux comes on in 20 seconds? perhaps you are overcharged? too much freon is just as bad as too little. what are your pressures?
the freeze switch is under the center panel beside the accelerator pedal.

I don't think I'm overcharged. The aux. fan came on about halfway through my 2nd can of R12 and ran pretty much all of the time while charging the system. They're 12 oz. cans and I only had 4 of them. Combined, that's 48 oz of refrigerant. The sticker in my engine compartment states the refrigerant capacity to be 2.9 lbs, which is 46.4 oz. I put 3 full cans in, and most of the 4th, but there is still some left in the can.

The sight glass on the drier was bubbly/foamy till about halfway through the 4th can during charging, but is clear now.

I'm not sure of the pressures, but I can check later (I'm away from the car today). What should the pressures be on the high and low sides with the compressor both running and stopped?

If you don't mind, tell me a little more about the freeze switch. What does it look like and how is it adjusted? From your description of where it's located, it sounds like it should be near the TxV. I may have seen it and not realized what it is when I replaced the TxV.

AFAIK, the measurement of the can is in fluid ounces.

If so, the conversion shows that R-12 weighs .1 lb/fluid ounce.

If you need 2.9 lb. of the fluid, that would equate to 29 fluid ounces, or about 2.5 cans.

If I am correct with this theory, you're overcharged.

Maybe John can chime in..............

I also suspect that 4 cans in over-charged. In most of my cars, 2 cans is enough to get it cold. If too much, the low pressure will be too high, so the evaporation temperature (inside) will be higher. If you do end up venting a bit, do it while the EPA isn't watching, but don't feel guilty since the ozone hole theory turned out to be bogus or at least not man-made. They are quiet about it now, but the deformed frogs in Costa Rica turned out to be due to a virus that was always there and not uV radiation like the media reported. To avoid any guilt, use a hydro-carbon like Duracool or Enviro-Safe in the future, since venting those is no concern.

It's my understanding that refrigerant is always measured by weight (mass), not volume. Since refrigerant is packaged and shipped in liquid form, but dispensed as a gas (which is difficult to measure given varying densities as a result of varying temperatures and pressures, it is much easier to measure it by weight. Every A/C refrigerant sticker I've seen on a car identifies the required refrigerant amount in either pounds, kilograms, or sometimes both.

Refrigerant cylinders are always measured by weight, i.e. a 30 pound cylinder of R12 likely weighs 30 pounds full, but about 7 pounds empty, so the cylinder would be said to contain about 23 pounds of R12.

Additionally, the cans I used were specifically marked "Net Weight 12 Oz." not 12 FL OZ, so they are clearly being measured by weight.

Below is a picture of the cans for reference.

No mistaking that.

Clearly, you'll need just about four cans, as you stated.

One other concern I have is the vacuum level. 27" is nowhere near the level needed to get all the moisture out of the system. In fact, there are some that suggest that 29" isn't even good enough when the residual moisture is calculated.

I have no personal experience with those levels, however, it does appear that those who suggest that you really need to be above 29" have a decent argument.

Initially, I had that concern about not having all the moisture in the system out, but once I remembered my physics lessons of years past, I dismissed it. I'll explain...

Maximum vacuum achievable on earth is approximately 29.92 inHg. My vacuum pump is rated at 29.916 inHg and I left it pumping for long enough that it stopped moving the needle on the gauge and stayed there for over an hour while still pumping. (I suspect that the gauge is not as accurate as it could/should be.) Anyway, by splitting the difference and assuming my vacuum pump achieved 28.5 inHg vacuum, water moisture within the vacuumed system would boil (vaporize) at approximately 86 degrees Fahrenheit. Considering the ambient temperature was 105*F with 5% relative humidity during the vacuuming (with the engine compartment in direct sunlight), the A/C components such as condenser and lines would have reached such a high temperature that virtually all remaining moisture would have vaporized and been sucked out during the course of the hour+ vacuuming. Additionally, the desiccant in the drier should be more-than-able to remove any residuals after that.

I might be incorrect in my thinking, but I don't think I'm completely off-base. Someone please correct me if I'm wrong. Could moisture really still be part of my issue?

There's no problem with the vacuum or moisture in the system. He's located in Salt Lake, which is at 4226ft elevation. Air pressure and hence possible vacuum is reduced at elevation. At 4000ft air pressure is reduced to 12.2PSI from 14.7 at sea level. Maximum possible vacuum at 4000ft is 25.8 in-hg.

It might help if you post high & low side pressures at idle and 2000RPM. However, I think what you have here is a properly operating R4 a/c system. They just were not all that powerful.

Rather than refrigeration, the next place I'd look is the vacuum actuators and air doors. It's important to get 100% recirculated air in 100 degree temperatures.

Good luck.

Thanks for the input.

On the 25-mile trip in to work this morning, with outside temperatures at 82*F, the vent temperatures got down to 42*F at one point on the freeway, but were mostly around 46*F at freeway speeds. As soon as I got on to city streets, however, the temperatures rose to the mid 50s, and just below 60*F when stopped at a light.

Fortunately, all my vacuum actuator pods work with the exception of the one that controls the feet vents. I just manually close those vents and they stay closed. As for the center vents, I disconnected that actuator and plugged its vacuum line and manually tied the arm on those vents to keep them open all the time, so cold and hot air can blow through those as needed.

I forgot the gauges, so I'll test the refrigerant pressures when I get home. What pressures should I be seeing at idle and 2000 RPM?

Thanks in advance.