A/C R134a pressure question - PeachParts Mercedes-Benz Forum

eatont9999 · #1 05-03-2012, 04:34 PM

Hello everyone,

I have a 1983 300SD with weak A/C. The other day, I decided to hook up my manifold set and check the pressures. With the system running, I was getting around 30PSI on the low side at idle. I added some R134a up to about 38-40PSI and there was no real noticeable difference. I noticed the high side pressure is only around 120PSI; I was expecting to see a much higher pressure here. The air blows out cooler than than ambient temperature but not enough to effectively cool the car on a hot day. It was about 80F out when I charged the system. It gets a little cooler while driving on the highway but that is to be expected.

So, what can I do to get better cooling aside from running propane or finding R12? Could the expansion valve/orifice tube account for the high side not being that high? Am I off my rocker or what?

Thanks for any help.

compress ignite · #2 05-03-2012, 05:38 PM

(I have NO Idea of the Parameters of your A/C system as Functioning)

1.System designed to run R-12. [and you're running (At lest partially) R-134]
2.Pressure Values you quote do not "Add Up", with Either Refrigerant.
3.The Only way to "Correct" for using R-134 in an R-12 system is a Larger
Parallel Flow Condenser.

vstech · #3 05-03-2012, 05:50 PM

if you have 48 psi on the low, and 120 on the high, you have either a bad compressor, or a stuck open expansion valve.

eatont9999 · #4 05-03-2012, 06:20 PM

Quote:

Originally Posted by vstech

if you have 48 psi on the low, and 120 on the high, you have either a bad compressor, or a stuck open expansion valve.

It is 38-40PSI on the low side, not 48PSI. I bought this car in January, so I have no clue what was done to the AC system before I bought it. I assumed it was converted to R134a at some point because it has the R134a fittings on it. It doesn't seem to leak, so I would imagine someone put the new green O-rings on the fittings throughout the system. The drier is not new and shiny but I also have no clue when the conversion was done. I always replace the drier when opening the system and so would a true professional.

Is there any way to check the expansion valve short of replacing it? If my research is correct, it is a PITA to get to.

The compressor spins freely and doesn't make any abnormal noises when on. Those are the only two ways I know of to verify the compressor is functional.

eatont9999 · #5 05-03-2012, 06:24 PM

Quote:

Originally Posted by compress ignite

(I have NO Idea of the Parameters of your A/C system as Functioning)

1.System designed to run R-12. [and you're running (At lest partially) R-134]
2.Pressure Values you quote do not "Add Up", with Either Refrigerant.
3.The Only way to "Correct" for using R-134 in an R-12 system is a Larger
Parallel Flow Condenser.

I'll take some pictures and post them up so y'all can see what the equipment looks like.

Graplr · #6 05-03-2012, 06:36 PM

Yes, the expansion valve is a bit#h in the 126s. You need to do a head stand under the steering column and you can only move then wrench about 10 degrees a turn.

First, you need to determine what vent temps you are getting (or post them if you have already figured that out). Stick a thermometer in the vent and go for a drive.

eatont9999 · #7 05-05-2012, 07:52 PM

OK, so driving at highway speeds, about 88F out, AC on max, I get 70F at the defrost vent according to my newly purchased AC thermometer. My defrost vents are always on for some reason. Probably one of those stupid dash-pots died. I figured out which one controls the center vents, so if I don't have a replacement dash-pod in my stash, I'll jury-rig it open at all times. I have the kick panels off, so the side vents don't work, either.

Pictures coming soon.

eatont9999 · #8 05-05-2012, 07:54 PM

Graplr · #9 05-06-2012, 08:45 AM

I would say the first thing you need to fix is your air flow issues. Your air may be getting heated by the heater core and that is could be why you are not cooling properly. Even if that is not the reason, it still would be good to fix the issue for easier diagnosing of why your system isn't working and you'd be able to rule one more thing out.

It's been awhile since I had my 84 SD but I had the same problem as you (blowing out defrost). It seems to be a common issue on this model. I think I posted about it on here. I'll have to see if I can find it. It may be a bad push button unit.

Graplr · #10 05-06-2012, 09:18 AM

I recall now that I had heat blowing out of my defrost at all times when I first purchased my SD. I couldn't figure it out. I was leaning towards my PBU being bad. My fix was to bypass the heater core by adding a two tees and a heater hose with valves so that in the summer I could close off the coolant flow completely to the heater core and then open it back up in the cooler months. If you can't figure out your problem perhaps you want to do this (at least temporarily) to see if the air getting heated by the heater core is your issue. Then hopefully you can figure out why the air is getting heated.

eatont9999 · #11 05-06-2012, 03:43 PM

I am pretty sure my monovalve is working because some of the heater hoses are cooler than others. I'm not sure of the exact routing but when I replaced all the coolant hoses, I cleaned up the monovalve and bench tested it to be OK. That is not to say it is still functioning. I would assume a good way to test if the heater core is getting flow would be to disconnect the electrical connector from the compressor clutch, turn on the AC controls, and see if I get hot air. Then I will know for certain if the heater core is the cause. I really don't think it is but I'm going to test it anyway.

So far, to me, everything is pointing to a stuck open expansion valve. If that turns out to be the case, I will replace it and several other components as well as a system flush and re-lube.

I got the center vents working by disconnecting the dash-pot actuator arm from the vent flap control arm located on the driver's side near the expansion valve. The center and lower side vents put out enough for me not to be worried about the defrost vents. I also removed the blower motor and put grease on the shaft bearings, so hopefully it will start without being smacked any more.

eatont9999 · #12 05-06-2012, 04:44 PM

I unplugged my compressor clutch connection with the AC on and I didn't notice any heat coming through the vents; I just got ambient temperature air. Now I think I am looking at spending some money on a few new AC components. The expansion valve really doesn't look too hard to get to. I think I will, when I get the money, buy the following and replace them on my AC system.

Expansion valve
Both cut-off sensors
Drier
Some type of flush agent
PAG 150 or Ester oil. -- leaning towards ester oil since the system was designed for R12 and ester. I doubt compressor O-rings were ever changed.
Schrader valves -- already have these
O-ring kit -- already have this (I will replace all the ones I can get to)
R134a -- thinking about propane as it is much cheaper. I just don't like the explosion factor.
Rent an evacuator.
Better manifold set that will allow me to vac on one side and ingest refrigerant on the other to purge any residual air. -- This is a maybe and depends on funds.

Graplr · #13 05-06-2012, 04:48 PM

Quote:

Originally Posted by eatont9999

I unplugged my compressor clutch connection with the AC on and I didn't notice any heat coming through the vents; I just got ambient temperature air. Now I think I am looking at spending some money on a few new AC components. The expansion valve really doesn't look too hard to get to. I think I will, when I get the money, buy the following and replace them on my AC system.

Expansion valve
Both cut-off sensors
Drier
Some type of flush agent
PAG 150 or Ester oil. -- leaning towards ester oil since the system was designed for R12 and ester. I doubt compressor O-rings were ever changed.
Schrader valves -- already have these
O-ring kit -- already have this (I will replace all the ones I can get to)
R134a -- thinking about propane as it is much cheaper. I just don't like the explosion factor.
Rent an evacuator.
Better manifold set that will allow me to vac on one side and ingest refrigerant on the other to purge any residual air. -- This is a maybe and depends on funds.

Harbor Freight has some inexpensive manifolds and vac pumps. The manifold gauges work well for awhile and then like just about everything they sell the components fall apart but if you are wanting to go cheap it is the way to go. I've replaced the red valve and the yellow hose so far but I am still ahead $$ wise.

eatont9999 · #14 05-08-2012, 09:12 PM

I have decided to go with propane for my refrigerant. I just need to figure out the best way to get it from the propane cylinder to the low side R134a port. I might have to build a custom adapter.

I still plan to replace the expansion valve, sensors, shrader valves, oil and O-rings.

eatont9999 · #15 05-27-2012, 10:39 AM

Update:

I decided to go with R134a because I didn't find a good, safe way to get propane/iso butane in the A/C system. R-12, which I would prefer goes for $51/can around here.

I changed and lubed all O-rings (most all of them were the older black ones) with new green O-rings. Replaced expansion valve, drier, and pressure switches on the drier. I lubed the system with 8oz of PAG46.

I vacuumed the system down for about an hour. It sucked in the 3/4 of the first can on its own, then I started the car and turned on the A/C to finish the rest of the can. I got 2 1/4 cans installed and I noticed my pressures were not right. I get about 60PSI low side and 275 high side. I was expecting 25-30PSI on the low side and about 250PSI on the high side.

The cooling I am getting from the vents is almost negligible. The low pressure line is slightly cool and the drier is hot. I see a lot of white foam-like activity in the sight glass. Any ideas what could be the problem? Do I just need to add more?

Any suggestions are appreciated.