Still tinkering and fine tuning a/c-pressures - PeachParts Mercedes-Benz Forum

zenuff · #1 07-22-2010, 07:57 PM

Well I thought the a/c was dialed in on my 79 240D but today the vent temps were less than usual. The outside temperature was around 93 today. I know it'd normal to have a higher temp at the vent on a extremely hot day, but what is normal? It was enough to keep me somewhat cool but it wasn't that great.

I checked the pressures and it was running 60 psi on suction side and 225 on pressure side at idle. I accellerated just a bit and it would pull down to 40 psi on suction line and up to 250 psi on the pressure side. It never seemed to go much beyond 250 but the low side would keep coming down to around mid 30's.

Well I thought it was a little low on freon so added some of my r420a. Just a little bump at a time, I took it real slow and the vent temps were coming down.

I ended up with about 50 at the suction line and 250 at the high pressure side.
Is that normalish for 90 degrees outside?

Went for a test drive and it was very cold especially on the highway. I really don't want to overcharge it so I'm checking with the experts here.

Seems to be working better for now.

What do you guys think?

smiffy6four · #2 07-24-2010, 07:00 AM

If it's nice and cold, I'd leave it where it is. Sorry, don't what the factory temps should be.

BTW, where is the low side fitting? I have an 83 240d, and can't locate the low side fitting to give it a fill.

ashedd · #3 07-24-2010, 10:40 AM

I feel your pain...

Yesterday was 92 with 91% humidity. The poor TD was bearable but it didn't get cold till around 8pm. I read a bunch of stuff online and checked the pressures. After a series of purging and filling I just decided to recover all the freon and start over.

I got my static pressure at 84ish and low is 25 high is 185. Most importantly the inlet and outlet on the evap is the exact same temp. I know the pressures a a hair low but I am going to leave it alone and see how it does.

Took a long road trip last weekend and it got down right cold in the car. I am thinking that high heat and humidity is a bit too much for the design of the system. What I really need to do is get the a/c working in my '87... I bet it would blow ice cubes with r12.

zenuff · #4 07-25-2010, 07:51 PM

Well after many weeks of hard work on this a/c system I'm going to have to start over with r12. The r420a is not working well for me. It seems that in order to get a decent vent temperature you have to put up with excessive head pressures. I mean so bad it will stretch out your compressor belt beyond repair. In my testing i've found that if you run comparable pressures to r12 you will not have adequate cooling, and if you add what the factory charge calls for the head pressure will be dangerously high, especiLly if it's smoking hot outside.

I'm lucky I haven't killed this compressor. I'm going to redo it and buy some r12 and quit messing around. I think one of my biggest drawbacks ia mycondensor coil isn't efficient enough to get rid od all the heat with the r420a. Btw r420a is 88% 134a so I assume I would get similar results in a 134 conversion. I did clean the condensor coil and rebuilt the fan to get as much air throught there as possible, and it did help bring the pressures down some but still not enough. And I cleaned it multiple times with multiple cleaning agents.

These are just my results your may vary.

BoiseBenz · #5 07-25-2010, 08:23 PM

If you spray water on the condenser and your performance (vent temps, pressures) improves, that indicates insufficient air for the pumping available. A 240D "main fan" is direct drive (no clutch)?

holtonmusicman · #6 07-26-2010, 01:01 AM

I didn't even know R12 was still available? And R134A cannot be run at the same temps as an R12 System, so if you pressures are the same the R134A will not fully expand (which is what causes the cooling) in the evaporator.