A/C won't take charge - PeachParts Mercedes-Benz Forum

angst · #1 07-01-2014, 03:23 PM

Years ago a semi retired hvac acquaintance of mine “converted” my r12 to 134. I could not get a clear answer from him about what oil was used and how much was swapped out.
Things have been going fine for 5 years now. Each summer I add a can of 134 and it is good to go just slowly loosing pressure over the course of a month or two.

This year I tried to do things right. I changed the dryer and drew a vacuum down for 30 minutes.

Afterward I can not get it to take a charge. From negative pressure it would suck in about 1/3rd of a can then no more without the low side pressure gauge going way above the intended range (45psi or so?). The compressor clutch engages but nothing seems to be happening in the cooling system.

I read about engine off charging from the high side which I have tried. Once a day after it sitting overnight I can get a little shot in there.

So, three days in and I have only managed to get about 2/3 of a 12 oz can in there plus a 1oz can of oil that has like an ounce or so of some other additive. The pressures seem to be in the operating range and the clutch is engaging About 130psi on the high side and about 55psi on the low side (at around 90f ambient outside temp)
What am I doing wrong? Incorrect install of the dryer wiring perhaps? Should I just allow the low side pressure gauge to climb up above the range while charging it?

rscurtis · #2 07-01-2014, 05:28 PM

I would suggest that you have a problem in one of your hoses or Schrader valves. In Tennessee, I would convert it back to R12.

DrLou · #3 07-01-2014, 05:40 PM

Do you know if the acquaintance replaced all the seals in the system? This would be necessary in a 12-to-134 conversion. As a first pass, sounds like you have a leaky system...

And, I've never had to fill refrigerant from the high side on a properly-working system - yikes, don't even like the sound of it!

angst · #4 07-01-2014, 06:49 PM

If the compressor clutch is engaged and spinning then the compressor is running, right? Or do I need to be jumpering it to start it pulling refrigerant in?

vstech · #5 07-01-2014, 07:11 PM

Ok, if the system is hitting 55psi with less than a can in, something is wrong. Take pics, post them, and let's see what you are doing.

ROLLGUY · #6 07-01-2014, 07:28 PM

I suggest finding and fixing any leaks! It will not do any good to keep putting refrigerant in, and having it leak out again.

angst · #7 07-02-2014, 09:44 AM

vstech following the a/c thread in your signature led me to look at a temperature pressure chart.
From those charts and interpreting the readings chart it would seem like I have a blocked expansion valve. I have high pressure on the low side and low pressure on the high side.
Ill hook up the gauges this morn and update this post with the results.

Update. Engine off after overnight sitting 80f ambient
High 60psi
low 40psi

Running with a/c on
high 100psi
low 5 psi then 65psi then 5 again. I dont know what is going on there. I am starting to suspect that my high side gage valve is leaking.
Was able to get it to take a few more ounces of charge up to 45 or 50psi. Air from vents is still a couple degrees above ambient.

Another update: talked to my HVAC friend who said it is ok to let the low side gauge go up to 100+psi while you are charging it and this is natural. As long as your bottle off gauge reading of system pressure is in the regular range it is ok to have the needle up high while filling. This could change everything and would explain why I am not getting anywhere. When I try to charge I am turning it off before the low side gauge goes over 55psi or so.

Update once again: that seemed to do it. I have managed to get another half bottle in (around roughly 17oz total so far) and the high pressure side is coming up and approaching 250psi. Im now hearing the familiar squeak that the compressor has made for years and the aux. fan is kicking on.
The filling went real slow. So slow that I could not feel a temperature change at the bottle. I used the sight glass on the gauge set to show me if it was filling or not.
In the driveway I started seeing vent temps about 5-10f below ambient and on a highway test it got down to 65f out of the vents on a 90f day.

So this thing just might be licked. A couple more checks and modest fill additions to get the low side pressure to the target and I am thinking I will be back to regular performance.