Some AC numbers for the experts to look at

[otto huber]

After replacing my compressor and drier, I had a shop pull vacuum on the system for a couple of hours, then I charged my 1981 300SD with 3 12oz. cans of R12. The vents were blowing cold air and I decided to wait a couple of weeks and make sure that the system was holding the charge before adding a 4th can. The 4th can didn't make the vent temp much cooler, and now I'm wondering if I might have over charged the system.

Here are my numbers after having the AC running for about 1/2 hour in 79*F ambient temp:

Low side 35 psi
High side settled in at 180 psi after reaching 220 before the aux fan kicked in.
Vent temp was 50*F with 79*F ambient temp. I was hoping to get closer to 45*

The freon capacity for the 300SD is 3.3 lbs.(52.8 oz.), does this include oil?

[Rooster300SD]

My book shows 2.8 lbs or 45oz of freon for a stock W126.

Jeff

[otto huber]

The red sticker above the radiator says 3.3lbs. for 126, 2.6lbs. for 123 and 2.2lbs. for 107.

[Rooster300SD]

The sticker on your car trumps the MB Technical Data book.
I have an ACC Service Manual for the 126 and it says refer to the sticker.
Stick with the 3.3 lbs, maybe others can chime in. The red sticker on my car is missing.

The same book says to add this amount of oil in cc's when changing out these components:

Condenser 20
Evaporator 40
Receiver/dryer 10
intake line 10
Deliver line 10

add 40cc if the system suddenly empties: blow a line, ect.

Jeff

[otto huber]

I added 6 oz. of oil to the system when I changed the compressor and drier assuming that there was probably 2 or 3 ounces left over in the other components.

[Rooster300SD]

6oz oil squares with my book for a new R4 & rec/dryer

Jeff

[BillGrissom]

Your low-side pressure is fine. High-side is a bit higher than I like to see for 79F ambient, so I would monitor again in the middle of summer. When you turn it off and pressures equilibrate, I usually see ~70 psig on both sides at 80 F ambient. I use Duracool (HC) which runs very similar to R-12 (slightly lower pressures).

[vstech]

Pressures are a combination of compression, and heat content.

So going simply by pressure to determine charge is useless. All information needs to be included.

Ambient temp, cabin temp, cabin wet bulb temp and line temps.
Without all this info, the only way to charge a system is with a scale.

With a properly operating system, properly leak tested and properly evacuated, then weighed in charge, the pressures and related temps will be in line unless something else is amiss... Like a dirty evaporator, or condenser, or a bad compressor or expansion valve...

Understanding all the symptoms is a science.

[Rooster300SD]

Went to PNP today and saw three W126 cars
1st & 2nd gen gasser & diesel and they all said
2.9 lbs of R12 on their red stickers.

I think you might be overcharged with the 3.3 lbs.




Jeff

[otto huber]

Originally Posted by Rooster300SD

Went to PNP today and saw three W126 cars
1st & 2nd gen gasser & diesel and they all said
2.9 lbs of R12 on their red stickers.

I think you might be overcharged with the 3.3 lbs.




Jeff

Did you notice the year of that first generation W126? Maybe there was a larger AC component in an '81 than there was an '82, '83 or '84. I have a hard time believing that they put an incorrect sticker on a W126 the first year it came out. Even if my capacity is 2.9 lbs.(46.4 oz.), do you think an extra 1.6 oz. from the 48 oz. that I put in is going to make that much difference? I'd still like to know if that capacity rating includes oil.

[Rooster300SD]

Maybe the 81 had a different compressor or component.
Don't know what to advise you on that.
I believe the freon weight is independent of the oil charge.

How's the AC working now?

Jeff

[MBCAJUN]

Originally Posted by otto huber

....................... Even if my capacity is 2.9 lbs.(46.4 oz.), do you think an extra 1.6 oz. from the 48 oz. that I put in is going to make that much difference? I'd still like to know if that capacity rating includes oil.

Let me preface this by saying I'm not an expert however I've had good luck charging my a/c systems for years. Many people bleed the yellow hose before opening the low side valve on the gauges to avoid air entering the system when charging. I wouldn't be surprised if you lose a little more than a 1/4 ounce per can doing this, also counting what's left in the hose and can when your finished charging. This would put you spot on with the 2.9lbs or 46.4 oz. On the other hand if the 52.8 oz is correct you could be as much as 6 oz short. I treat the oil weight separate from the refrigerant. I think your low and high pressures are in the ballpark but the only true way to measure your refrigerant is to weigh it. These comments are strictly for discussion sake and shouldn't be taken as gospel. The proper advice is to tell you to take it to a professional. I don't but as I said in the beginning I've been lucky.

[otto huber]

Originally Posted by Rooster300SD

Maybe the 81 had a different compressor or component.
Don't know what to advise you on that.
I believe the freon weight is independent of the oil charge.

How's the AC working now?

Jeff

The AC is adequate with 80* ambient temp, but I have to set the fan on high and in the past I was able to set the fan on low and the system did a good job of keeping the cabin cool. I'm hoping to get the vent temps down to 45* on an 85* day. Maybe I'll add another 6 oz. If there's anyone in the Bay Area who wants to split a 12 oz. can, let me know. These can taps don't hold a very good seal.

[Zulfiqar]

Originally Posted by otto huber

The AC is adequate with 80* ambient temp, but I have to set the fan on high and in the past I was able to set the fan on low and the system did a good job of keeping the cabin cool. I'm hoping to get the vent temps down to 45* on an 85* day. Maybe I'll add another 6 oz. If there's anyone in the Bay Area who wants to split a 12 oz. can, let me know. These can taps don't hold a very good seal.

My old 12 oz can tap is a leaker now, I dont use it now but I have one 12oz can with it installed lying on my shelf, I used a small piece of rubber and a small dollar store screw clamp to seal that rubber. The can still is half full since I left it alone.

[DeliveryValve]

I remember a debate about 2.9 vs 3.3 lbs in past. If I remember correctly, the consensus was 2.9 lbs is the choice to use.


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