w124 AC - High Side Pressure - Blockage? - PeachParts Mercedes-Benz Forum

johnmharte · #1 07-02-2015, 08:28 AM

I've just finished an overhaul of my 92 300d's cooling system. Belt tensioner system, aux fans, ect...ect..

I also replaced a number of the AC components b/c the compressor would cut off after a VERY short clutch engagement. I figured it was due to the belt/tensioner system being busted and the compressor's free-wheel sensor cutting it off. That has proved to be the case b/c now my compressor is running like a champ as expected.

However, one question that I have for the expert panel.
I was seeing what I "think" are higher than normal high pressure side PSI readings.

Ambient temp was 75* out, but very very humid,
I'm in SC and it was actively raining, and I work in a steel building/storage unit deal.

low side: 29 - 40psi (compressor on - compressor off)
high side: 150 - 300 psi (compressor off - compressor on)

I really don't like that high side number as most references are saying that should be 170psi on the top end of the high side. What I'm reading is that high of a high side, might be a sign of blockage. Then again, the mercedes documentation says 24bar which is 348psi... so need a sanity check for the good people of the forum.

The good thing is I've got very very cold air. I'm putting a temp sensor on the vents today for "real" numbers.

I've got a new drier (behr china one that doesn't have a sight glass), new pressure switches, and new expansion valve. The system held 29-30" of vacuum for 10 hours so we are a tight ship.

A little confused by this one. Hell, I might even be a little low but I added 960g of r134a in a completely evacuated system. That was in the middle of the the two competing Mercedes documents. The R12-R134a conversion document says 900g and another document says 1000g.

Thanks for any help!

Images/Large Sizes
http://i.imgur.com/9rTaIL6.jpg
http://i.imgur.com/PFSx4SA.jpg
Imgur

Mercedes Documentation
http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Climate/83-608.pdf

Generic Mastercool document: http://www.mastercool.com/media/89660-INST_web.pdf

Phillytwotank · #2 07-02-2015, 09:17 AM

Are you getting cold air in the vents?

I'm fairly sure that the '92 year model still used R12 as the original refrigerant. My '92 has r134 fitting adapters on the service ports so i assume that it had been converted somewhere along the line before i got it Where as, the '93 300d in our fleet is definitely a native r134 a system. So, if you're running r134 in your '92 then higher pressure than what we see on a generic r134 temp/pressure chart would be normal?

97 SL320 · #3 07-02-2015, 06:29 PM

R134 can't be charged by using a sight glass because the liquid stream still will still have bubbles on a full charge.

Make sure you have enough air flow through the condenser ( Aux fan , engine fan ) , not enough air will make high side pressures very high.

Given the low side is at 40 and you have cold air in the car, I'd say the expansion valve is working properly. A stuck expansion valve is really the only place to have a "blockage"

lsmalley · #4 07-02-2015, 11:52 PM

Quote:

Originally Posted by johnmharte

I've got a new drier (behr china one that doesn't have a sight glass), new pressure switches, and new expansion valve. The system held 29-30" of vacuum for 10 hours so we are a tight ship.

Images/Large Sizes
http://i.imgur.com/9rTaIL6.jpg
http://i.imgur.com/PFSx4SA.jpg
Imgur

Mercedes Documentation
http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Climate/83-608.pdf

Generic Mastercool document: http://www.mastercool.com/media/89660-INST_web.pdf

I also just got the Behr (China version), the sight glass is actually just covered. There is a black rubber covering the glass, just pry it up.

johnmharte · #5 07-03-2015, 10:22 AM

Bingo, the Behr/China version here as well, no sight glass, but if that is just a cover... I'll pry it up. That' great to know, I though that was a metal piece screwed in. I saw your (ismalley) thread talking about german parts made in China with regards to the drier.

Plenty of air-flow, aux fans kick on when compressor is running. That all seems fine.

Air at the vents is cold. I was replacing injector heat shields last night so sorry for the delay in responding. I still need to put an actual gauge on it to get temps. I'll do that here today and report back.

I'm leaning towards add a few more grams of r134a if the vent readings are very low, like in the 50*'s. Given that 24-26bar high side (90% humidity) in the mercedes AC diagnostic guide. That's something like 346psi when I checked the conversion. That still just seems like a REALLY high pressure.

This was an original R12 system that was converted many years back. I don't see the adaptors on the hoses so I think it was a complete hose replace as well. I read about that the 92's were designed for either or "something" like that. I could be wrong so will recheck the hoses to see if it was the conversion kit or an even bigger swap over... but it is indeed R134a.

It just keeps raining here in SC so that's really screwing with the testing of the AC system.

Right now after all my injector work I've got the lovely car not shutting off and a bit of nailing that I'm debugging. Going to due the fuel delivery pump o-rings and washers next... before really getting crazy on that side of the project (engine/fuel/air-intake)

johnmharte · #6 07-09-2015, 08:07 AM

Never got below 63* at the vents. So, I've evacuated the system again. Will start over tomorrow. I'm concerned I may have gotten some air in the system so just wanted to be sure the next re-fill is clean, where I close the manifold completely between cans. I'm thinking I forgot to close the manifold during one of the can changes.

ps2cho · #7 07-09-2015, 11:04 AM

Sounds like air. I chased a similar problem and figured out it was my harbor freight gauge set. Once I replaced it it no longer was running at high pressures.

I get 50f vent temps while driving in 105F ambient with 55psi low and 340psi high at idle. In the mornings at 80F ambient I can get 38F With r134a.

With your ambient I would aim for 35psi low and 280 high at idle.

johnmharte · #8 07-09-2015, 11:14 AM

What exactly was the bug in the Harbor Freight set? That's what I'm using as well.

I do have one of those consumer connectors for the low side to feed r134a back in. I may just run with that instead.

ps2cho · #9 07-09-2015, 11:51 AM

Not sure maybe a rubber gasket was not holding so after I pulled a vacuum it sucked back in air afterwards.
All I know was when I took yo an AC shop to charge they had no problems doing it. Which led me to believe it was either the gauge or my vacuum pump. I replaced gauge first and that was it.

I was hitting 400psi after only 1.5 cans! And the system needs about 2.5 cans roughly.

vstech · #10 07-09-2015, 06:01 PM

Air.
Leak checking under vacuum is a no no. .. switching cans on a recharge allows air in also...
where in SC are you that it's 75? We're around 100 all month!

johnmharte · #11 07-10-2015, 02:29 PM

I'm borrowing a buddy's manifold gauge set for the next refill. Will see if that is an issue, the harbor freight version. Mine is doing exactly what yours did ps2cho, high reads crazy high.

vstech, the "leak checking under vacuum"... I thought after assembly, while the system is evacuated. It is one of the steps to put 29bar of vacuum on the system to make sure it holds.

The switching cans, I don't really have any other option... little 365g cans. What I think happened was the last can, I forgot to get atmosphere out of the feeder line, and that would've put air into the system when opening up the low side of the manifold to continue the feed.

I'm in upstate, Greenwood, SC. It is stupid hot this week but last week, was amazing.
81 and 84* for the highs. But.. today.... 99*
Weather History for Greenwood, SC | Weather Underground

Right now...working on the fuel injectors and fuel pump o-rings.
https://picasaweb.google.com/113269285636978370799/1992Mercedes300D25

Phillytwotank · #12 07-10-2015, 02:38 PM

99 degree ha's in Winnnsboro SC today....

97 SL320 · #13 07-10-2015, 09:48 PM

Air isn't that big of a deal when switching cans. If concerned, after connecting the can, and piercing the top, loosen the hose at the gauge manifold, open the can valve slightly to purge the line then tighten the hose. You only need a little puff as the hose volume is small and only a small volume of air will enter the hose in the first place.
I just change cans and keep going.

vstech · #14 07-11-2015, 04:56 PM

Nope... vacuum is best for moisture removal and refrigerant evacuation. .. lousy for leak check. Pressure is needed from a non condensing gas.

97 SL320 · #15 07-11-2015, 07:03 PM

A vac check is OK to test for gross leaks but 2 things will occur when under vac:

Blind bolt holes / trapped voids / oil will continue to gas out making the vac level change for the worse.

Compressor seals really are not rated for vacuum.

The best way to check for system integrity is to fill with a dry gas. Usually Nitrogen is used because it is convenient in an industrial setting however it is a large molecule and the rate of leak will be reduced.

A way accessible to anyone recharging their system is to: ( this works better with R 132 due to the cost of R12 ) Leave the AC and engine off. With the freon can in an upright position to draw gas, open the valve and let system fill. The pressure will equalize at some point. Shut off the can and note the pressure. Disconnect the gauges and let is sit for a few hours / days then check pressure again. Be aware that freon pressure is low at cold temps and rises at high temps so take that into account.

You won't be using much freon doing this as you are only adding gas, not liquid and the AC system isn't turning your gas into liquid. Next time I charge a system I'll weigh the can and see what it takes to gas a system.

If you charge a system with a running compressor and get liquid in the system, your engine off pressure will never drop until all the liquid has boiled off.

Freon ( and any gas that has been turned into a liquid like propane , CO2 , nitrogen , oxygen. ) will have a vapor pressure related to temperature. Take some liquid freon, dump it into a bottle and put the cap on. The pressure will rise then stop. At this point forces that want the freon to boil off into a gas are countered by the gas pressure in the vessel. This is an equilibrium state.

If you add or draw liquid off, the pressure will be the same once things stabilize. So , this is why using freon gas only is a good way to test for a leak. Any leak in the system will result in an immediate loss of pressure.