Correct high & low A/C pressures. - PeachParts Mercedes-Benz Forum

bolomiester · #1 07-09-2007, 08:02 PM

I just bought a 1983 380 sel, my first Mercedes. It's a project but is driveable and looks pretty good from 10 feet away. The A/C works and has been converted to R134a. Can anyone tell me what the high and low side pressures should be. It's currently at about 47 on the low side and about 162 on the high side. The cooling is ok but not great. Any info would be highly appreciated.

Bzoo · #2 07-09-2007, 08:07 PM

what is the average ambient outside air temp?

bolomiester · #3 07-09-2007, 08:13 PM

About 85 degrees.

gmercoleza · #4 07-09-2007, 08:15 PM

Those pressures are off - what's weird is that your low side is much higher than I would expect, given the fact that your high side is also very low. There is a chance that the TXV isn't functioning properly, but you might want to charge it up just to be sure - who knows, the low side might drop. The best way to charge is by weight on an empty system, but if you just want to top off I can give you a ballpark since I have pretty much the same converted system (85 380SE) and I charged it properly by weight (36 oz). I get consistent vent temps of 48 degrees moving, 62 idling. Pressures are approx. 30 low/340 high @2000 RPM with ambient air temp of 86 degrees and approx. 30% humidity.

gmercoleza · #5 07-09-2007, 08:15 PM

Check out this thread: http://www.peachparts.com/shopforum/showthread.php?t=155168&highlight=380se+r134a+ambient

gmercoleza · #6 07-09-2007, 08:19 PM

One other observation regarding the disproportionate high/low readings. The compressor may be weak, perhaps having difficulty creating enough suction/pressure. Just one other angle to consider. Your A/C is not going to blow very cold until you get the low side pressure somewhere closer to 30.

david s poole · #7 07-09-2007, 10:37 PM

you are running 340 on the high side at 2000rpm.either this is a misprint,your fan clutch is not locking up or the system is a little overcharged.

gmercoleza · #8 07-09-2007, 11:05 PM

Quote:

Originally Posted by david s poole

you are running 340 on the high side at 2000rpm.either this is a misprint,your fan clutch is not locking up or the system is a little overcharged.

Unfortunately the fan clutch is good and I am not overcharged. I used 36 oz. which is 85% of the 42 oz. of R12 capacity. I could let a few oz. out but the impact at the vent would be tremendous and the A/C would no longer keep up on those 100 degree days. The 340 psi was taken while idling, and I'm sure it drops when the car is moving, but it is still much higher than I would like it to be. 300 would be nice. Previous owner had the system professionally converted about 8 years ago by an A/C shop, they flushed everything properly, used PAG, and changed the TXV and drier. Somehow, despite the pressures, the compressor is still nice and quiet and the system hasn't grenaded in 8 years. When I first bought the car, hooked up the gauges and the pressure was pretty much the same. This system is just not a good candidate for conversion. I have a bunch of R12 on hand and will convert back one of these days...

saumil · #9 07-09-2007, 11:23 PM

Wow, 340 psi !! This is very very high. Low between 35-45 and high between 180-240 is more reasonable general values. 340 psi means either there is non-refrigerant in the system, could be excess oil, or air, OR there is a restriction somewhere.

bolomiester · #10 07-10-2007, 08:50 AM

I lowered the low side pressure to 40 and the high side came down to 155. It's now cooling very well. 44 degree output air at 85 degree ambient outside temperature.

I should have mentioned that while the system has been converted to R134a, I charged it with Enviro-Safe, a brand of HC-12. I've used this on a number of cars with great results. It cools better than R134a and runs at a lower high side pressure making it a little easier on the compressor. Plus it is compatible with mineral oil and the original seals and "o" rings. Just change the charge fittings and you're good to go.

david s poole · #11 07-10-2007, 09:39 AM

one very good way to lower the high side pressures is to clean the black paint off the condersor.a good rough and ready way without removing the condensor is to remove aux fan[s] and spray with mag wheel cleaner[weak acid] then wash off with hose after about 30 mins.this should lower pressure by about 15-20% and therefore vent temps as well.

gmercoleza · #12 07-10-2007, 10:10 AM

Quote:

Originally Posted by david s poole

one very good way to lower the high side pressures is to clean the black paint off the condersor.a good rough and ready way without removing the condensor is to remove aux fan[s] and spray with mag wheel cleaner[weak acid] then wash off with hose after about 30 mins.this should lower pressure by about 15-20% and therefore vent temps as well.

I've been meaning to try this but haven't had the time yet. One of these days...

saumil · #13 07-10-2007, 11:58 AM

A method I used for my home a/c is to look at the temperature of the low side hose with a special temp sensor that can wrap around the hose. The refrigerant temp for R134a corresponding to the the low side pressure can be read from the gauge. At maximum fan speed, adjust the pressure of the low side so that the temperature reading of the low side hose (from the temp meter) is about 15 to 20 deg lower than that read from the pressure gauge. The problem with the usage of this rule on cars is that the air flow is variable as opposed to fixed in home a/c, so for lower speeds the system may appear to be overcharged. But try this anyway.

brewtoo · #14 07-10-2007, 01:09 PM

Quote:

Originally Posted by saumil

A method I used for my home a/c is to look at the temperature of the low side hose with a special temp sensor that can wrap around the hose. The refrigerant temp for R134a corresponding to the the low side pressure can be read from the gauge. At maximum fan speed, adjust the pressure of the low side so that the temperature reading of the low side hose (from the temp meter) is about 15 to 20 deg lower than that read from the pressure gauge.

This is called Superheat for those who want to research further.

Also, you can use one of the infrared thermometers to check the temp of the low side hose. Check it on a metal part of the hose that sweats when the system is running, like near the charging fitting.

BUT, the Superheat charging method does not work on TXV systems, only on systems with a fixed orifice.