R134a Retrofit on an '85 300TDT - PeachParts Mercedes-Benz Forum

col klink · #1 06-22-2007, 04:01 PM

What should the correct low-side pressure be for a retrofit with the engine at idle?

My honey-pot-pie had its A/C converted over to R134a six years ago by a shop. I bought the car last August and I would say that it performed better than luke cool. This spring I didn't get any cold air and it was running ~5 degrees over ambient.

I had assumed that I was running freon still and borrowed a coworker's gauges. I was excited when I realized they wouldn't work. I already recharged my 2000 Chrysler so it would be really easy to drop a new can in the Benz. I've done that and now it's blowing colder (beau cool?) but I think I could add another.

When I added the first can yesterday the gauge read a nice 35 psi at idle on the low side. This morning it read just over 25 at idle and then I had the wife rev her up to 2000 rpms and it dropped to 20 psi. I'm not so sure that there was leaking from yesterday to today but I'll take more readings tonight, before and after the trip home. Initially, when I was charging the system, there was high pitched squeal after the engine was turned off but that's pretty much stopped.

So, let's say I add more R134a -- I should probably add more oil too. Is it safe to assume that the shop used ester oil? It's my understanding that PAG oil is only used in systems designed for R134a.

Also, where in the hell is the high pressure port?

My hopes are that I can get one more summer out of this system. Next year I'll tackle replacing the compressor/receiver/dried/expansion valve.

Side question -- why does the expansion valve need to be replaced every time you purge/vacuum/recharge? I can see replacing the o-rings but why the valve?

winmutt · #2 06-22-2007, 05:49 PM

Quote:

Originally Posted by col klink

What should the correct low-side pressure be for a retrofit with the engine at idle?

My honey-pot-pie had its A/C converted over to R134a six years ago by a shop. I bought the car last August and I would say that it performed better than luke cool. This spring I didn't get any cold air and it was running ~5 degrees over ambient.

I had assumed that I was running freon still and borrowed a coworker's gauges. I was excited when I realized they wouldn't work. I already recharged my 2000 Chrysler so it would be really easy to drop a new can in the Benz. I've done that and now it's blowing colder (beau cool?) but I think I could add another.

When I added the first can yesterday the gauge read a nice 35 psi at idle on the low side. This morning it read just over 25 at idle and then I had the wife rev her up to 2000 rpms and it dropped to 20 psi. I'm not so sure that there was leaking from yesterday to today but I'll take more readings tonight, before and after the trip home. Initially, when I was charging the system, there was high pitched squeal after the engine was turned off but that's pretty much stopped.

So, let's say I add more R134a -- I should probably add more oil too. Is it safe to assume that the shop used ester oil? It's my understanding that PAG oil is only used in systems designed for R134a.

Also, where in the hell is the high pressure port?

My hopes are that I can get one more summer out of this system. Next year I'll tackle replacing the compressor/receiver/dried/expansion valve.

Side question -- why does the expansion valve need to be replaced every time you purge/vacuum/recharge? I can see replacing the o-rings but why the valve?

High pressure is practically underneath the car. They say its a replacement but honestly I switched from r12 to freeze12 and replace o rings. Didn't even purge the system (was completely empty when I went to fill). Been running 2 yrs GA summers non stop and zero problems.

Kuhlrover · #3 06-23-2007, 01:47 AM

The pressure you are asking about is dependant on what the temperature. You need to look at an R134a temperature chart. They are pretty easy to find on the web.

The problem you have is you don't know how much R134a is in the system. Too much is a bad thing and you don't want to risk overcharging it. If the compressor is cycling normaly and the air is cool enough to keep the cabin cool, then leave it alone.

Why do you want to add more oil to the system? Did some leak out and you need to replace it? Unless you have proof that you have lost oil, then once again, leave it alone. Too much oil is a bad thing.

TimK

col klink · #4 06-23-2007, 09:49 AM

Quote:

Originally Posted by Kuhlrover

Why do you want to add more oil to the system? Did some leak out and you need to replace it? Unless you have proof that you have lost oil, then once again, leave it alone. Too much oil is a bad thing.

I'll check out the temperature chart, thanks. And I agree that too much oil is bad, but since the oil is dissolved in the refrigerant, when the refrigerant leaks out so too does the oil.

I think it's pretty safe to assume that until I added a can a couple of days ago most of the refrigerant had escaped. Now I don't believe that all of the oil escaped as well, but the system as a whole must be low on oil.

Yesterday I added 1/2 a can (to the whole can the previous day) to bring to the low-side pressure up to 35 again and there's been a noticeable difference. We're almost up to having ice cubes fall out of the dash.

I'm crossing my fingers it holds for the rest of the summer.

But what is the official procedure for charging a system? Low-side pressure 25 - 45 psi at 1500 rpms? 2000 rpms? idle?

arew264 · #5 06-23-2007, 09:52 AM

You want low side to be 30-35 at 2000 RPM. This is with R-134, R-12 is different.

col klink · #6 06-24-2007, 11:35 AM

Thanks!

After running the A/C for a couple of days, I thought the high pitched squeal would go away, but no. I turn off the ignition and within 20 seconds you can hear a loudish squeal coming from the compressor.

Any idea on how much longer it will last?

col klink · #7 06-25-2007, 11:39 AM

I dropped in to the local Harbor Freight today to pick up an A/C manifold gauge set as they were on sale for only $40. Online you can get it for than $30 but I was anxious since I also coveted some nitrile gloves and a vent thermometer.

During the heat of the day (+90F) I'd get 68F at idle and as low as 58F on the Interstate. When the sun went down and ambient was 83F, I was able to get 62F at idle and in dropped to low 50's on the highway.

It's odd -- I would imagine that I'd get the lowest temps when on the highway above 60 mph because the condenser is better able to give heat to the outside and the compressor is getting higher RPM's. But the lowest temps I've seen at the vent are when I just come *off* of the highway -- that's when I hit 48, in the shade, with an ambient of 80F. That lasted for all of 5 seconds.

So obviously I don't have ice cubes falling on the floor mats but it's worlds better than having the windows open.

Before the last trip I added the last of the second can of R134a and that helped the performance. The low side pressure stopped dipping below 25 psi @ 2000 rpms. I'm thinking that I've added enough R134a to come close to capacity since it would drop to 10-15 psi previously when revved.

At this juncture I'm thinking of evacuating the R134a (HF has the R134a vacuum for $10, locally for $17) and adding back R12 to get better idle/stop&go performance. But I suppose I'll wait to do that until the compressor dies.

Speaking of which, I thought it went away, but after I turn off the car, the compressor emits a high pitch whistle for about 20 to 30 seconds. Is this normal? Is this compressor on its last legs? It runs rather quietly otherwise, well, next to the diesel it's quiet.

Side question -- any way to set the engine RPM at 2000 single-handedly? Something a little more accurate than a brick on the accelerator?

vstech · #8 06-25-2007, 11:54 AM

when you shut down the motor, the compressor "passes" the 200ish pressure to the low side, to equalize the pressure. totally normal.
switching back to R12 requires flushing all the 134 oil out of the lines, replaceing the TXV and the reciever as well as the compressor. 12 doesn't like the ester oil that 134 conversions use.

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arew264 · #9 06-25-2007, 01:36 PM

If you decide to switch back to R-12, install a parallel flow condensor while you have the system apart. It will condense much better than stock, and all you have to do is get one ($120 max) (make sure you get the right size) and get some piping made to make it usable with the existing hoses.

Hopefully someone who has done this conversion will chime in. I'm told it helps a lot, and it's not like condensors die every 2-6 years like the compressor.

col klink · #10 06-26-2007, 02:09 PM

Quote:

Originally Posted by vstech

when you shut down the motor, the compressor "passes" the 200ish pressure to the low side, to equalize the pressure. totally normal.

Swank, thanks. That's good news. I wasn't sure because the compressor in my Chrysler never makes that kind of noise.

This morning when it was 75 outside I was able to get 58 at the vents but at lunch today it was 90 outside and the vent barely dipped below 70. Not comfortable!

So, I'll hit the auto store for yet another can of R134a. The last time I added refrigerant the low side pressure was 25 psi at 2000 rpm -- I'll try getting it up to 35. If this doesn't improve the performance then it's time for some more serious work.

But before any major work, I believe I should add some oil. Let's say she takes a 1/3 of can of R134a tonight (total of 26 oz) -- this means that the system was basically devoid of refrigerant as I will have added the system capacity back in. Did the bulk of the oil leave with the previous refrigerant? Should I add 2 oz? 4 oz?

And if I add oil, it should be ester right?

arew264 · #11 06-26-2007, 04:15 PM

With oil in systems that weren't flushed, your guess is as good as ours. That's why flushing is so heavily advised (I found this out the hard way when I killed a compressor with too much oil before - flushed it after that).

Yes, you should use ester. Ester is a middle oil. It lubricates fairly well (not quite as well as PAG or mineral oil), and it will mix with BOTH mineral oil and PAG.

That 30-35 PSI is just a rough estimate, it does depend on the conditions, as others have stated. Look it up online, there are charts. I would guesstimate that that 30-35 is going to be at roughly 75*F, so try to work from that. Remember, slightly undercharged will be okay in the long run, and overcharged will shorten the life of your compressor.

col klink · #12 06-27-2007, 01:02 PM

Yesterday's drive home in the humidity and +90's ambient was pretty miserable -- the vent didn't drop below 64.

So last night I dropped in my third can of R134a (and 2 oz of ester oil) and this morning my vents were blowing down to 42! Granted the ambient was 70 but it's giving me hope for the hot days.

The low side pressure was 28 or 29 psi at two grand. With the latest addition that brings up the total refrigerant to 37 oz (there was one ounce with the oil). Multiplying 2.9 lbs by 16 ozs and by .8 comes out to 37.12 ounces. So my systems was pretty much devoid of refrigerant.

vstech · #13 06-27-2007, 01:04 PM

or you are leaking out quite rapidly...

col klink · #14 06-27-2007, 01:12 PM

I'm really hoping that isn't the case!

But time will tell.

saumil · #15 06-27-2007, 10:19 PM

If you went this low on your refrigerant charge when you started, the only logical explanation is that you have a leak. Now, if there was a leak when you started, there is air in the system. I did not see that you have evacuated the system at any point, so what you have in the system is air and refrigerant. This means, your high side pressure must be excesively high, compressed air cannot help in cooling, it just makes the output of the compressor hot. Measure the low and high pressure at idle, what you should have is 30-40 on low and roughly 160-220 on the high side.

If you have the gauge, you can also readout the temperature on the low side which corresponds to the pressure (the inner rings tell you this). Measure the temperature on the low line, near the valve, wrap your vent temp meter with an insulating tape on the low pressure metal hose. You want this temperature to be between 15 to 20 deg F below the temperature that you will read from the gauge. If the difference is larger than 20 deg, you are overcharged. If the difference is lower than 15, you are undercharged. These are rough numbers but have worked for me.

The squeaking in compressor could indicate that you are low on oil or you have excessively high pressure due to air.

You will have have to evacuate at some point to get rid of the air and then use stopleak (i.e. heavier grade oil) and recharge. Ester is the oil to use for compatibility reasons if you had R12 before and now have R134a.