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  #16  
Old 08-05-2013, 07:17 PM
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Okay, I got home and pulled out my gauges. Here are the readings:

Idle
High Side: 250 PSI steady
Low Side: 55 PSI (occasionally varying between 50 and 60 PSI)


2000 RPM
High Side: 225 PSI steady
Low Side: 27 PSI steady

These pressures look about right to me, with my only real concern being the varying pressure on the low side at idle.

Do these pressures fall within the correct ranges?

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  #17  
Old 08-06-2013, 03:02 PM
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The pressures look fine, ideal really. Afraid the R-4 just doesn't do enough work at idle to produce very cold air. Not sure whether that's due to the idle speed of the diesel, pulley sizing on the compressor, or displacement of the compressor.

At this point all you can do is make sure you're getting as much recirculation as possible. I'm not familiar with the 126; never owned one. But on the 123 the factory designed the a/c system to use 80% recirculated air and 20% fresh air. A common mod changes it to 100% recirculated air, which makes a noticeable difference in comfort on hot days. Not sure if anything similar applies to the 126.

Other than that, a good window tint helps a lot by reducing the sun load.
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  #18  
Old 08-06-2013, 11:51 PM
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Thanks for the information. I was afraid that might be the case. I'm glad that at least my system is set up and functioning correctly, albeit not as cold as I would like it.

I'll look into the 80/20 air to 100% air mixture setup. I've heard that the W126 functioned that way, and I vaguely remember reading somewhere that people have tweaked their systems to be 100% recirculated air during A/C or specific vent settings. I'll keep you posted.

Thanks again for your help. Now, perhaps it's time for some good window tint...
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  #19  
Old 08-07-2013, 12:21 AM
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Has anyone thought about a bad (or marginal) mono valve letting some heat in as well? It is possible that there is some heat being mixed in with the cold. Try clamping with padded vice grips, the heater hose coming from the mono valve. Take it for a drive and observe the vent temps. If no change from previous, then your mono valve is fine. Although it is too late for you, the Sanden in my 123 will freeze my arms when sitting and idling if the vent is toward me. I don't think the R4 in any condition can compare with the efficiency of the 5 cyl Sanden, let alone a 7 cyl. Not that I am wishing bad luck for you or anything, but WHEN (I don't mean if) your R4 goes bad, you will be very happy with a Sanden if you choose to retrofit. Just sayin'....Rich
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  #20  
Old 08-07-2013, 02:23 AM
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Quote:
Originally Posted by ROLLGUY View Post
Has anyone thought about a bad (or marginal) mono valve letting some heat in as well? It is possible that there is some heat being mixed in with the cold. Try clamping with padded vice grips, the heater hose coming from the mono valve. Take it for a drive and observe the vent temps. If no change from previous, then your mono valve is fine. Although it is too late for you, the Sanden in my 123 will freeze my arms when sitting and idling if the vent is toward me. I don't think the R4 in any condition can compare with the efficiency of the 5 cyl Sanden, let alone a 7 cyl. Not that I am wishing bad luck for you or anything, but WHEN (I don't mean if) your R4 goes bad, you will be very happy with a Sanden if you choose to retrofit. Just sayin'....Rich
The mono valve hasn't been mentioned, but I did consider it. The mono valve core was one of the first things I replaced when I bought the car 11 months ago (albeit with a used unit). The original diaphragm was torn, but I'll definitely check the one in the car again. It can't hurt.

I just found your other thread with the full Sanden retrofit kit. I just wish I would have found it a month ago. I would have snatched up one of the kits in a heartbeat! I see that your last kit is tentatively sold, but if you come up with another one (or the sale falls through), please let me know. From what I gather, you're right about the failure of the R4 compressor in that it's more a matter of WHEN, not IF it fails.

I'll see about the mono valve and report back.
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  #21  
Old 08-07-2013, 08:04 AM
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Originally Posted by seebeexee View Post
Thanks for the input.

On the 25-mile trip in to work this morning, with outside temperatures at 82*F, the vent temperatures got down to 42*F at one point on the freeway, but were mostly around 46*F at freeway speeds. As soon as I got on to city streets, however, the temperatures rose to the mid 50s, and just below 60*F when stopped at a light.
........

One thing you didn't mention is did you clean your evaporator? I added some 134a to my 85 R4 300 D and got down to 45 deg F at an ambient of 95 F (at idle). I think mine will go lower still with more tweaking. R12 system on yours should do better than that.

An easy way to check the mono valve is the hoses to/from it should not be hot when the temp dial is on full cold detent.
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  #22  
Old 08-07-2013, 09:16 AM
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The worst piece in the W126 AC is the condenser. Late '84 and '85 models received an upgraded split tube and fin type. This one has approx fifty 6mm tubes as compared to the early version which had half as many passes with much larger tubing.

My 'Black' '85 300SD with its stock AC has no problems keeping the cabin uncomfortably cold at any ambient temp below 97f using R134. It has the late version split tube condenser.

Earlier W126s I have owned had marginal AC with R12.

You may have received the early version condenser which would account for the lack of performance. Also, these systems are sensitive to the amount of oil installed. Too much is not better.
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  #23  
Old 08-07-2013, 10:36 AM
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Quote:
Originally Posted by seebeexee View Post

I just found your other thread with the full Sanden retrofit kit. I just wish I would have found it a month ago. I would have snatched up one of the kits in a heartbeat! I see that your last kit is tentatively sold, but if you come up with another one (or the sale falls through), please let me know. From what I gather, you're right about the failure of the R4 compressor in that it's more a matter of WHEN, not IF it fails.

I'll see about the mono valve and report back.
I had 5 kits on a summer special, but all are sold. I still have everything available at regular prices. I hope to offer only 7 cyl compressors when my supply of 5 cyl units is gone.....Rich
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  #24  
Old 08-07-2013, 11:14 AM
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He has reasonable cooling at road speeds, it's the idle that's not so great. With a low side pressure of 55PSI there is no way to get good cooling at idle. Recall your middle school science class - the boiling point of a liquid varies with pressure. Lower pressure equates to lower boiling point. In the case of R12 the temperature at which it boils is approximately equal to the low side pressure (expressed in PSI) - one of the cool things about R12. Hence his evap is only at 55 degrees at idle, and the air coming off it is typically ~10 degrees warmer, depending of course on fan speed. Here's a link to R-12 & R-134 pressure/temp charts: http://www.ackits.com/aacf/ptchart.cfm

My 123 300D had the same OM617 & R-4 combo, and I had the same challenges. It was OK at speed - not great, but OK. At idle it really didn't cool well enough and the car really wasn't comfortable in the heat of a Dallas summer.

My solution involved selling the 123 and acquiring a 124.
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  #25  
Old 08-07-2013, 04:28 PM
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Quote:
Originally Posted by funola View Post
One thing you didn't mention is did you clean your evaporator? I added some 134a to my 85 R4 300 D and got down to 45 deg F at an ambient of 95 F (at idle). I think mine will go lower still with more tweaking. R12 system on yours should do better than that.

An easy way to check the mono valve is the hoses to/from it should not be hot when the temp dial is on full cold detent.
You're right. I haven't done anything with the evaporator other than flush it internally. From what I've read, getting to the evaporator seems to be a daunting task. Is there a fairly easy way to clean the outsides of the evaporator without having to pull the dash apart to get to it? The idea that the evaporator is dirty has been in the back of my mind since I began this project.


Quote:
Originally Posted by Whiskeydan View Post
The worst piece in the W126 AC is the condenser. Late '84 and '85 models received an upgraded split tube and fin type. This one has approx fifty 6mm tubes as compared to the early version which had half as many passes with much larger tubing.

My 'Black' '85 300SD with its stock AC has no problems keeping the cabin uncomfortably cold at any ambient temp below 97f using R134. It has the late version split tube condenser.

Earlier W126s I have owned had marginal AC with R12.

You may have received the early version condenser which would account for the lack of performance. Also, these systems are sensitive to the amount of oil installed. Too much is not better.
My condenser has about 28 round tubes going through the fins. The fins are very thin and flimsy, but I believe there are more fins than the factory condenser. If memory serves me correctly, my OE condenser is of the same type, and I believe my 300SD was manufactured during the first quarter or so of 1984.

As far as oil is concerned, I know that I have no more than 8 ounces of mineral oil in the system. I've seen varying amounts listed as the right amount (anywhere from 6 to 10 ounces as acceptable) from others online, but nothing from MB stating an exact amount when starting from scratch.


Quote:
Originally Posted by ROLLGUY View Post
I had 5 kits on a summer special, but all are sold. I still have everything available at regular prices. I hope to offer only 7 cyl compressors when my supply of 5 cyl units is gone.....Rich
Too bad, I guess. Do you have a link or site where you list all the parts you sell and have available? I'm not opposed to replacing some parts if they're far-superior to the OEM stuff.


Quote:
Originally Posted by jcyuhn View Post
He has reasonable cooling at road speeds, it's the idle that's not so great. With a low side pressure of 55PSI there is no way to get good cooling at idle. Recall your middle school science class - the boiling point of a liquid varies with pressure. Lower pressure equates to lower boiling point. In the case of R12 the temperature at which it boils is approximately equal to the low side pressure (expressed in PSI) - one of the cool things about R12. Hence his evap is only at 55 degrees at idle, and the air coming off it is typically ~10 degrees warmer, depending of course on fan speed. Here's a link to R-12 & R-134 pressure/temp charts: http://www.ackits.com/aacf/ptchart.cfm

My 123 300D had the same OM617 & R-4 combo, and I had the same challenges. It was OK at speed - not great, but OK. At idle it really didn't cool well enough and the car really wasn't comfortable in the heat of a Dallas summer.

My solution involved selling the 123 and acquiring a 124.
The vent temperatures are decent (not great) if the ambient temperature is below 80*F or so. Yesterday, the temperature outside was 95*F and the A/C vent temperature was only 58*F while cruising, which jumped to 68* while stopped, so not that great. This morning, the outside temperature was 74*F and vent temps got down to 40*F, which was nice. 40*F at the vents seems to be the lowest I've been able to reach, regardless of outside temperature.


Previously in the thread, the idea that the freeze switch might be incorrectly adjusted. Does anyone have any additional information with respect to the freeze switch? Does it even sound like it might be the culprit (or at least part of the problem)? I think I found it up under the driver's side kick panel, but I'm not certain, and I definitely don't know what to do with it to adjust it.

Thanks for all the help and suggestions. I forgot to check temp of the tubes going in and out of the mono valve when I arrived at work, so I'll check them after the drive home and report back.

Thanks.
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  #26  
Old 08-07-2013, 05:09 PM
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Quote:
Originally Posted by jcyuhn View Post
He has reasonable cooling at road speeds, it's the idle that's not so great. With a low side pressure of 55PSI there is no way to get good cooling at idle. Recall your middle school science class - the boiling point of a liquid varies with pressure. Lower pressure equates to lower boiling point. In the case of R12 the temperature at which it boils is approximately equal to the low side pressure (expressed in PSI) - one of the cool things about R12. Hence his evap is only at 55 degrees at idle, and the air coming off it is typically ~10 degrees warmer, depending of course on fan speed. Here's a link to R-12 & R-134 pressure/temp charts: http://www.ackits.com/aacf/ptchart.cfm

My 123 300D had the same OM617 & R-4 combo, and I had the same challenges. It was OK at speed - not great, but OK. At idle it really didn't cool well enough and the car really wasn't comfortable in the heat of a Dallas summer.

My solution involved selling the 123 and acquiring a 124.
Are you saying at idle, R134A cools better than R12, but not at higher RPM?
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  #27  
Old 08-07-2013, 05:14 PM
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Originally Posted by seebeexee View Post
You're right. I haven't done anything with the evaporator other than flush it internally. From what I've read, getting to the evaporator seems to be a daunting task. Is there a fairly easy way to clean the outsides of the evaporator without having to pull the dash apart to get to it? The idea that the evaporator is dirty has been in the back of my mind since I began this project.
On my 85, I made a funnel out of the soda bottle and placed it under the evap foam drain tube, took the blower off and sprayed evap cleaner in there then a garden hose to hose it off.
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  #28  
Old 08-07-2013, 05:17 PM
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Originally Posted by funola View Post
Are you saying at idle, R134A cools better than R12, but not at higher RPM?
No, definitely not. But if you have 55PSI low side at idle, you're not going to get decent cooling from either refrigerant.
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  #29  
Old 08-07-2013, 05:31 PM
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Originally Posted by funola View Post
On my 85, I made a funnel out of the soda bottle and placed it under the evap foam drain tube, took the blower off and sprayed evap cleaner in there then a garden hose to hose it off.
I'll have to try that. Doesn't the blower come off from behind the glove box? Also, where is the evaporator drain tube located? Also, as far as getting the hose in there, where do you spray it from (below, or is there access from above)? I'd love to get in there and see how much crap I can get get out. I was told that the car sat for many years somewhere in Texas or Utah before I got a hold of it, so it's probably got a fair amount of stuff in there.
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  #30  
Old 08-07-2013, 05:50 PM
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Originally Posted by seebeexee View Post
I'll have to try that. Doesn't the blower come off from behind the glove box? Also, where is the evaporator drain tube located? Also, as far as getting the hose in there, where do you spray it from (below, or is there access from above)? I'd love to get in there and see how much crap I can get get out. I was told that the car sat for many years somewhere in Texas or Utah before I got a hold of it, so it's probably got a fair amount of stuff in there.
Don't know how the SD is layed out but on my 85 300D, I removed the panel under the glove box, removed 3 screws or bolts and the blower drops down, unplug connector and the blower is out of the way to spray evap cleaner and water (huge amounts) in there. I removed the drivers side center console panel (by your right knee) to get access to the foam tube. The soda bottle funnel is to direct water flow so it all goes out of the car instead of splashing all over inside.

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