W126.125 A/C not efficient 300SDL - PeachParts Mercedes-Benz Forum

Carson357 · #1 10-08-2008, 05:36 PM

i have recently replaced the dryer and both a/c pressure switches and it has a full charge of R12. but it still seems inefficient, any ideas? high pressure is around 250-300psi and low is at 40psi, i feel almost embarrased asking for help with this, but thought you all could help with some ideas

Arthur Dalton · #2 10-08-2008, 08:53 PM

Condenser fan come ON ???

Richard Wooldridge · #3 10-08-2008, 09:05 PM

Is that condenser clean? You might try a very gentle power wash on it if it is a little clogged. If you continue to have problems with high pressures on the high side, you might consider a parallel flow condenser, it will improve the overall operation immensely. (Your low side pressure seems about right, but your high side pressure is too high.)

Carson357 · #4 10-08-2008, 11:43 PM

the fan does come on, and i did clean the condenser very well the last two times i went to the high pressure car wash. sorry was off on the pressure for high side is 200-250 psi

Richard Wooldridge · #5 10-09-2008, 12:56 AM

One other thing that can cause low efficiency is if there's air in the refrigerant system. This could be caused by poor maintenance procedures, for example if the system was opened for whatever reason and was not pumped to a full vacuum. Another possibility is contamination in the expansion valve, or a clogged drier. One more thing that I've seen is a high pressure line that starts deteriorating on the inside and partially collapses or "swells" closed.
Just some ideas from my past experiences...

Carson357 · #6 10-09-2008, 01:14 AM

Quote:

Originally Posted by Richard Wooldridge

One other thing that can cause low efficiency is if there's air in the refrigerant system. This could be caused by poor maintenance procedures, for example if the system was opened for whatever reason and was not pumped to a full vacuum. Another possibility is contamination in the expansion valve, or a clogged drier. One more thing that I've seen is a high pressure line that starts deteriorating on the inside and partially collapses or "swells" closed.
Just some ideas from my past experiences...

thanks for the info, i ran the vacuum for 30 minutes, i am leaning toward the expansion valve, the drier is new, is a Behr unit. the line up to the firewall gets nice and cold, i think i will replace the expansion valve and see how it goes.

Arthur Dalton · #7 10-09-2008, 09:58 AM

What does the eyesight look like?

Carson357 · #8 10-09-2008, 08:28 PM

Quote:

Originally Posted by Arthur Dalton

What does the eyesight look like?

site glass is good, i just did a recharge, took 2.9 lbs according to the underhood sticker

Brian Carlton · #9 10-09-2008, 08:34 PM

I'm hardly an expert in these systems, but, I believe you've got to get some data on vent temps. With the fan on low speed, what's the vent temperature?

If the vent temp is too high, and the compressor is cycling, the problem is not with the efficiency of the system..........it's with the setpoint of the evaporator temp. switch. These are known to be set quite high.........to the tune of 40-42 degrees or so..........and the ultimate vent temp might be close to 50 degrees.

Dave wrote a post about putting a resistor in series with the evap temp switch to bring down this setpoint. It's got to be close to freezing if you want good performance when the ambient approaches 100F.

Carson357 · #10 10-10-2008, 07:46 PM

well today i decided to get a "second" opinion apparently the old faithful Blue Point thermometer is off by about 10 degrees, the coldest i could get it to today was close to 50degrees, this was today after i pulled the lower dash and was actually face to face with the expansion valve!! i hate doing those on 126 chassis, especially the ones with the bracket around them.
so brian if you could provide me with more info on the resistor mod i would be eternally grateful.
Ray

sixto · #11 10-11-2008, 04:02 AM

How clean/dirty is the evaporator core?

Is the monovalve completely closed in cooling mode? A simple test is to compare dash vent temp with rear cabin vent temp. IIRC the rear cabin vent is upstream of the heater core.

Sixto
87 300D

Carson357 · #12 10-11-2008, 05:15 PM

Quote:

Originally Posted by sixto

How clean/dirty is the evaporator core?

Is the monovalve completely closed in cooling mode? A simple test is to compare dash vent temp with rear cabin vent temp. IIRC the rear cabin vent is upstream of the heater core.

Sixto
87 300D

mono valve is completely closed, i have determined that the compessor is cycling too often, waiting to hear from brian about the resistor mod.

Brian Carlton · #13 10-11-2008, 10:13 PM

Quote:

Originally Posted by Carson357

mono valve is completely closed, i have determined that the compessor is cycling too often, waiting to hear from brian about the resistor mod.

Here you go, Ray:

http://www.peachparts.com/shopforum/showthread.php?t=131919

Please post back on the thread if you're successful.

I'm really curious to try it on one of these.

Carson357 · #14 11-07-2008, 03:24 AM

from GSXR's info i ended up soldering 3 resistors in series, One 100k plus two 1M = approx 82k total. doing this brought the vent temps from over 50 degrees down to 40-42 degrees, major improvement

as he suggested I used 1/2 watt resistors from Radio Shack, bought a pack of 1M and 100k. A dollar each, i had 2 sensors so i modifed one at 91k and another at 82k, 91k was too conservative and compressor would still cycle to soon, 82k was better, thinking of trying it with one additional 1M and see where it goes. i think we all need to do this, also this may help those who get the poor cooling after R134A conversion

One 100k plus one 1M = approx 91k total
One 100k plus two 1M = approx 82k total

Quote and picture from his info below
If you are at 49F vent temps now, and your ETS is on the far/warm side of spec like mine was, I'd try a parallel shunt in the 80-85k range (90k if you want to be a little conservative). I took a photo of the mod, soldered at the stock connector housing, but haven't uploaded it to my website yet. Stay tuned, I can probably do that soon... I also have the more complete original write-up from Stu Ritter or Richard Easley. I'll post that too.

(I'm still not sure why an "in spec" ETS gives such poor vent temps... my theory is that something in the climate control brain pushbutton unit is also on the far/warm side of spec, and the two tolerances add up to lousy vent temps! Not sure if I can prove that though. )

compu_85 · #15 07-08-2012, 11:29 AM

My car cycles the clutch to keep the vents at about 49*F. I just tried this resistor mod. Some observations:

-On the G2 W126 the plug end of the ETS is burried up in the dashboard by the ignition lock. Hard to get to... I just stripped some wire insulation by the sensor itself.
-I was able to lower the low vent temp down to 40*F
-The clutch would pull back in at 46*F

I think I'll leave the resistor mod out on my car... that 6*F swing is too much for my taste. I have to wonder if there is dirt buildup on the other side of the evaporator in front of the temp sensor, "tricking" it into thinking the evaporator is colder than it is. Not pulling the dash right now to confirm that.

-J