evaluating an AC compressor when system is currently discharged - Page 4 - PeachParts Mercedes-Benz Forum

Diseasel300 · #1 05-05-2018, 09:29 PM

If you're using dry nitrogen, pressurize the whole system to ~200 PSI and look for it to drop. Dry nitrogen does not change pressure with temperature, so a pressure change positively identifies a leak.

35 PSI won't tell you a thing about anything. The high side of the system will easily get up to 300+PSI in hot weather with the compressor running.

You can't independently pressurize the low and high sides, they will equalize through the compressor and the expansion valve. You need to pressurize the entire system and watch it as a system.

kuene · #2 05-05-2018, 09:56 PM

Thanks. I didn’t think it would tell me much at that low pressure. I’m not using nitrogen because I’m a) taking a shortcut and being stupid, and importantly b) it’s an open system and I’m replicant several things including drier and compressor. Obviously I have a leak, so I would lose all the nitrogen immediately anyway.

so I’ll get the new components in there, new orings, and then test at higher pressure - the whole system of course. Is there any reason not to leave the vacuum on it while I wait between working on it?

ROLLGUY · #3 05-06-2018, 12:09 AM

Quote:

Originally Posted by kuene

Thanks. I didn’t think it would tell me much at that low pressure. I’m not using nitrogen because I’m a) taking a shortcut and being stupid, and importantly b) it’s an open system and I’m replicant several things including drier and compressor. Obviously I have a leak, so I would lose all the nitrogen immediately anyway.

so I’ll get the new components in there, new orings, and then test at higher pressure - the whole system of course. Is there any reason not to leave the vacuum on it while I wait between working on it?

There is no reason to vac before or after your pressure test if you are going to open the system to replace parts. All the time you have the system open, you are letting air/moisture in the system. That is no big deal, as you will vac the system after replacing the components and a leak test.

w123fanman · #4 05-06-2018, 02:33 AM

Supposedly the Murray R4 heavy compressors that you can buy new at O'Reilly's is made in the US and it's $193 plus $10 core. I'd try that personally. Flushing components is easier with them out of the car but still not difficult in the car. Most of the AC tools you would need to do this job you can borrow from AutoZone though you have to be careful with used gauges as they can be leaky which can be maddening to track down.

I think the main reason these compressors grenade themselves is too high of pressures in the system due to inadequately sized condensers. R4s like to fail, but it's far less common in my experience to find a dead one on an old GM than an old Mercedes out in the parts yards, it's usually 1 in 2 Mercedes and 1 in 10 GMs. I always convert my cars to parallel flow condensers and R134 but I also own lots of AC tools so it's easy for me to do.

greazer2b · #5 05-07-2018, 09:11 AM

Quote:

Originally Posted by w123fanman

Supposedly the Murray R4 heavy compressors that you can buy new at O'Reilly's is made in the US and it's $193 plus $10 core. I'd try that personally. Flushing components is easier with them out of the car but still not difficult in the car. Most of the AC tools you would need to do this job you can borrow from AutoZone though you have to be careful with used gauges as they can be leaky which can be maddening to track down.

I think the main reason these compressors grenade themselves is too high of pressures in the system due to inadequately sized condensers. R4s like to fail, but it's far less common in my experience to find a dead one on an old GM than an old Mercedes out in the parts yards, it's usually 1 in 2 Mercedes and 1 in 10 GMs. I always convert my cars to parallel flow condensers and R134 but I also own lots of AC tools so it's easy for me to do.

Where do you get parallel condensers? Is it drop in for replacement? (85sd)
Thx
greazer2b

ROLLGUY · #6 05-07-2018, 12:47 PM

Quote:

Originally Posted by greazer2b

Where do you get parallel condensers? Is it drop in for replacement? (85sd)
Thx
greazer2b

Many sizes of P.F. condensers are available on ebay. No they are not bolt in replacements, but are universal. But- for only around $60 delivered, a little labor and hardware will get the job done. I think I used a 22" wide by 16" tall in the last W126 I did. A 24"wide may fit as well.....Rich

SD Blue · #7 05-07-2018, 06:10 PM

Quote:

Originally Posted by greazer2b

Where do you get parallel condensers? Is it drop in for replacement? (85sd)
Thx
greazer2b

Check Spectra Premium 7-4076 on Amazon for PF condensor

kuene · #8 05-06-2018, 06:15 PM

Thanks Bill. If I lose this supply of r12 - I’ll likely switch over to something else like envirosafe or Duracool. Since I have the r12, and the system has never been converted I’m going to give it a go.

So - small update. Picked up some fittings at HD to mate my compressor to the manifold hose. I’ve had my system pressurized at 130 (I only have a 150 psi compressor and my air blower attachment is rated at 90). I was afraid to push it further as the low side gauge was getting pinned to the max (retard) levels.

It’s been sitting several hours and I don’t notice any leaks yet. Needle hasn’t visibly moved yet... maybe a molecule or two? In any case, I’m really hoping that whatever leak there is (and I’m sure there is a slow leak somewhere right?) will be resolved with the new orings and such. Waiting on some parts for now. We’ll see....

w123fanman · #9 05-06-2018, 07:27 PM

Actually that Murray compressor is NLA, checked it today.

Diseasel300 · #10 05-07-2018, 04:14 PM

Unless you're removing or installing the clutch, you don't need the spanner.

kuene · #11 05-07-2018, 06:20 PM

ok - so I can just rotate the pulley by hand on the bench to get the new oil circulated? I guess I'm not sure what force this would take. I've read in some compressor installation threads/instructions (one from here) to get that tool from autozone to circulate the oil in the new compressor.

Also, another couple of noob question.

1) When people refer to pulling a vacuum on the system for an hour - does that mean you are running the vac pump for the duration of the hour? or does that mean you are pumping down to a vacuum state, and then shutting pump off, and waiting an hour while it's held at vacuum? really basic question, but I just want to be sure.

2) Do ya'll recommend a top or a side can tap when working with old cans of r12. (sure, I wish I had a big canister and wasn't dealing with taps, but what I have are the old cans).

Diseasel300 · #12 05-09-2018, 12:32 PM

The orifice size is different on a 134a expansion valve and the power unit may be looking for a different head pressure. Short answer, yes, it'll work. Long answer, it's complicated. You may lose some capacity. If the R12 unit is still good, use it.

kuene · #13 05-09-2018, 12:45 PM

do they still sell a new part that is specific to r12? i'm just not sure how to test if the existing valve is 100% or not...

Diseasel300 · #14 05-09-2018, 01:22 PM

R12 disappeared in 1994, parts to service it are long gone. Your only source for an R12 replacement is new-old-stock.

The TXV can't really be "tested". If the system wasn't open to the elements, the powerhead wasn't leaking, and the compressor didn't grenade and send shrapnel through the system, it's probably still fine. Install new O-rings after you flush the system and see how it runs.

kuene · #15 05-09-2018, 01:33 PM

so I should install o-rings at the 4 connections in/out of the expansion valve? I really cannot say if its been open to air in the past for extended periods of time. It certainly HAS been opened up to atmosphere as of this past Sunday (I pressure tested with compressed air because I intended on replacing drier/valve/compressor. Do you think it would now be damaged based on these few days of being open?

I wouldn't want to fill charge with r12 if I'm going to have to replace this thing afterwards... finding a place to recover/hold r12 for me seems impossible in my area (so far). I didn't realize these available valves were not suited for r12 - bummer.