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

kuene · #46 05-05-2018, 08:22 PM

About how much pressure should one use to test low and high sides respectively? I did a quick test today with 35 on low (it held for as long as I let it).

On the high side I went up to 100 but it always dropped down to about 35 after about 15 seconds. It would then sit at 35 for as long as I left it. I did have as many fittings sprayed with soapy solution but I couldn’t see any obvious bubbles. I will be doing a more thorough soap and visual leak test next time.

PS even though I’m opening up the system to replace drier, exp, compressor - I still vacuumed the system after I was done pressure teasring in order to remove moisture.

Should I leave it at vacuum in the meantime - until I test or replace components next?

Diseasel300 · #47 05-05-2018, 08:29 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 · #48 05-05-2018, 08:56 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 · #49 05-05-2018, 11:09 PM

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 · #50 05-06-2018, 01:33 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.

BillGrissom · #51 05-06-2018, 05:03 PM

Quote:

Originally Posted by kuene

... expensive r12 ...

If $100/lb for R-12 bothers you, plus an unknown price in the future, read-up on hydrocarbon refrigerants (Duracool, Enviro-safe), but ignore the scare claims from people who know nothing and/or AC shops with a vested interest. I have in all my vehicles now, even my 2002, some for decades. I am an engineer who has published papers on combustion so am allowed to think for myself. R-134A is being outlawed. PAO 68 oil doesn't absorb moisture and cause corrosion. I also use silicone brake fluid, and many ignorant statements about that too.

kuene · #52 05-06-2018, 05:15 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 · #53 05-06-2018, 06:27 PM

Actually that Murray compressor is NLA, checked it today.

greazer2b · #54 05-07-2018, 08:11 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 · #55 05-07-2018, 11:47 AM

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

kuene · #56 05-07-2018, 03:05 PM

Is it required (or just recommended) that I have an AC clutch spanner/wrench on hand when I remove/replace the compressor?

Diseasel300 · #57 05-07-2018, 03:14 PM

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

SD Blue · #58 05-07-2018, 05:10 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 · #59 05-07-2018, 05:20 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).

greazer2b · #60 05-07-2018, 06:57 PM

Quote:

Originally Posted by SD Blue

Check Spectra Premium 7-4076 on Amazon for PF condensor

Thanks! I saw two one was 300SE $139 other $169 which one they look same?
Also if I stick with my R12 would this still be best way to ho since I have R4 compressor?

My other 85sd I didn't replace the condenser but did the other things...dryer, Rebult R4 from eBay, expansion valve, and went to R134. That was 9 yrs ago and still working but I wish it was cooler. So that it is why I am thinking about different condenser in my other 85sd and keep it R12.
Thx John