W124 A/C how much 134A and pressures - PeachParts Mercedes-Benz Forum

mkrazy1 · #1 06-06-2008, 11:16 AM

I have been going through the forums and trying to do searches but it won't allow AC... hmmm and if I search on 134 convert I haven't found what I am looking for. - Anyway, my 87 300D was converted to 134A prior to me getting it. It was 90 yesterday with 55 percent relative humidity and it didn't cool very well on the way home, so I hooked up the guages to see what the pressures were. I am not an A/C person by any means so I need some insight. The high side was 310 and the low side was 30 with the engine running at about 2000 rpm. The sight glass looked white, so I am not sure if that means its low or not. I have no clue when it was converted to 134 and what was done. What I am trying to figure out are:

What should the high and low pressures be at that temperature/humidity/rpm.

I shut it off and let it cool down for an hour or so and the pressures evened out to about 100lbs on the high and low side. Does that sound about right? The engine was still pretty warm.

If I evacuate the system and flush it, should I change out the evaporator and anything else? How much 134A and oil should be added, and what type oil?

Also, another general question... I have a 30lb canister or 134A. If say the system needs 36 oz of 134A, how do I measure 36oz. I figure I can use a bathroom scale, but that isn't that accurate.

Matt L · #2 06-06-2008, 11:21 AM

That high-side pressure of 310 is out of line. Either you have too much refrigerant, too much oil, or air in the system.

The resting pressure is meaningless.

mkrazy1 · #3 06-06-2008, 11:49 AM

Okay, what is in line? Should I just remove some freon, or should I evacuate the system, flush, and recharge? If so, how much oil and 134A and should anything else be changed out, such as the drier.

Also, I see from a post just done after mine, that I need to get a digitial scale for measuring the weight. I'll look for one that reads ounces.

JimFreeh · #4 06-06-2008, 11:49 AM

Static pressure of 100 sounds a bit high, I'm used to seeing 85-90.

Is your suction line cold?

Jim

Matt L · #5 06-06-2008, 12:01 PM

The only thing that static pressure tells you is the average temperature of the refrigerant in the system. Ignore this completely.

If the system wasn't flushed when it was converted (that is, a "death kit" was used), you should flush it.

JimmyL · #6 06-06-2008, 12:09 PM

Quote:

Originally Posted by Matt L

The only thing that static pressure tells you is the average temperature of the refrigerant in the system. Ignore this completely.

If the system wasn't flushed when it was converted (that is, a "death kit" was used), you should flush it.

Based on temp it seems like the resting/static pressure gives a decent indication of freon level. There are no charts for it, but if you check things for awhile and keep your data, it seems to show an indication.
Now, it isn't necessary as you are going to check running pressures if you hook up your gauges in the first place, but just saying that it appears to be indicative of charge level. [a course indication, not a fine indication]
With temps at or near 100, with R134a, a high side of 300 is not outta the question, even with appropriate charge. That is why R134a is so hard on the system when it is really hot.......
BTW, you measure 36ozs by putting in 3 12oz cans......

Matt L · #7 06-06-2008, 12:19 PM

Static pressure is not an indication of anything at all. It will be equal to the vapor pressure of the remaining liquid refrigerant. If it was lower than the vapor pressure, liquid will boil until equilibrium is reached. Similarly if it was higher, gas will condense.

vstech · #8 06-06-2008, 01:24 PM

310Psi head pressure is not too far off in a hot car at 100°F outside...
I bet you aren't getting much cooling at those temps though... do the normal things to improve things, clean the front condenser coil with chemical that will dissolve bugs, blast out ANY obstructions from the motor side of the coil, and verify it's totally totally clear, and all fins are straight... also verify the aux fan is working properly.
with a 134 conversion, you have to ignore the sight glass, it's for 12... ya can't put enough 134 in there to fill the sight glass unless its below 80°F outside...
if you want it to cool, you will have to replace EVERYTHING. Compressor, reciever, expansion valve and flush the lines, and replace the oil and recharge with R-$12!!!

mkrazy1 · #9 06-16-2008, 11:22 AM

Well, I finally figured out what's going on. I was puzzled because it seemed like the AC was cooling okay again, then it wasn't... I finally figured out the compressor would sometimes stop and not restart until I restarted the car. I measured the air gap (after searching thank-you) and it's about .9mm. I called the local Autozone and they have 3 different clutch removal tools and they don't know which one I need. Anyone have any idea which one? Thanks as always!!