1986 Merc

[600SL]

I have a somewhat similar situation on a 1986 Mercedes 560SL. The engine was replaced with an engine from a 1996 SL600 M120 V12. So the system is a completely stock 560SL with exceptions being compressor from the M120 and I am using an aftermarket universal parallel flow condenser and running R134. The compressor form the original 560SL was a Denso 10P17C the replacement from the 1996 SL600 is a Denso 10PA17C. They do look very similar. One of the things I observed was that during initial charge the pressures were very high on both high and low side, like 375 and 70, until I jumped out the electric fan, at which time The pressures both immediately swung down to normal levels and I was getting 42° F. duct temperatures on a 70° F day. But since hooking up my new fan controller, I have lost all performance and now have duct temperatures of 58-60° F on a 70°F day. I believe I may have left my manifold gauges on for several days and possibly they were leaking. So this week I will be recovering and recharging. The original 560SL used 1Kg (35.3oz.) of Freon R12.

So the first questions I have are:

1) What is the best way to establish the charge when the volume of the system changes by an un-known amount?
2) If I had a system that used R12 and I replaced the receiver drier with a new OE receiver drier would the new OE drier be compatible with R134
3) Ar there bolt in expansion valves that are better rated for R134 available as replacements for the older systems. My particular valve uses SAE flair fittings on both ends and is externally balanced.
I'm going to try and move this cars problem to its own thread.

[Cornbinder89]

Ok, I think I got that to work
Best way to establish correct charge is to pull it down and start from zero with the correct amount. Anything less will be a guess.
Most driers are compatible with both refrigerants, and most will be marked as such.
Tx valves are NOT refrigerant specific. All they do is make sure the amount of refrigerant that is allowed into the evaporator is just enough that it all boils by the exit. It doesn't care about temp or at what pressure the refrigerant is at 32 degs. As long as the Tx valve is of the correct size for the evaporator it will be fine.
I do suspect that leaving the gauges on for several days would leak, they were never designed for permanent use on the system.

[600SL]

Thanks for the reply.

I will be trying this tomorrow night when I get my recovery tank in. Only problem I have is I don't know what the correct amount is, since it has a condenser of unknown size and some of the lines were changed to accommodate it. I have performed calculations to estimate the volume differences between the two condensers and I came up with the parallel flow condenser to be 95% of the volume of the original serpentine condenser. So best I can do is 35.3oz x 0.85 for R134 = 30 oz and assume the 5% difference in volume of the condenser amounts to 2% of the whole system so 30oz x 0.98 = 29.4 oz. So I can put in 24oz and tune from there. Also I will be using a gauge set with 6' lines, According to my AC 609 study guide a 6 ft line can fold up to 2 oz, so I may need a little more. Fortunately I will be using a 30Lb cylinder and scale so I wont have to be accounting for the loss of purging lines of individual cans. The other thing is the parallel flow condenser has a significant portion of its volume in the headers compared to the serpentine condenser. Not sure what the effect of that is. I was wondering if there was a better way to dial it in based on pressures, or should I just add charge until the duct temperature starts to rise then remove some. For example say I get down to a duct temperature of 48° at 24 oz and find that at 30 oz it is down to 44° and at 36oz. its back up to 48°. Then I could assume 30oz. is the correct charge.

[Cornbinder89]

A Tx system with a receiver as much more forgiving of getting the amount off a little than an CCOT system. It stores the excess refrigerant in liquid form in the receiver. You can't tell much of anything by duct temp. On a cool day with good air flow over the condenser, the amount of refrigerant needed will be less (Tx valve will be restricting the flow) as less is required in the evaporator for the heat load. The excess it held in the receiver. On a hot muggy day more refrigerant will be required in the evaporator to keep it from all boiling away before the exit, (Tx valve more open) and less will be held in the receiver.
If your are worried about the long hoses, pre-charge them with refrigerant before taking your weight measurement then the amount in the hoses is moot.
It has been my experience that switching condenser types has little to no effect on how much you should put in. I would go back with what the system calls for.
Don't over think this. The condensers job is to extract heat and change the hot vapor to liquid. Once the vapor has turned to liquid, it will remove a little more heat from the liquid (sub cool) but otherwise and area of the condenser not needed to convert the vapor to liquid is just adds to the receiver volume. In otherwords, it just stores liquid refrigerant. As long as the condenser has enough surface area to remove heat, any excess is not detrimental to the system. The area needed on a cool day will be a very small portion of condenser, on a hot day, it may take the whole area.
When you over-charge a system you effectively reduce the condenser size by partially filling it with liquid refrigerant. A small overcharge will be handled by the receiver, but as the charge is increased, condensing pressure will rise as more and more of the condenser volume is take up with liquid.
Duct temp is almost un related. As long as the Tx valve can pass enough liquid refrigerant to keep liquid boiling to the exit, duct temp will depend on the area of the evaporator, the air temp and humidity of the in coming air, and the exit pressure (temp).
A better aprox of charge is to observe the sight glass and condensing pressure (temp) when compared to the air temp across the condenser. It is best done on a hot day. You want to see condensing temp (pressure) as close to air temp passing through the condenser as possible (most times 20-40 degrees above air temp passing though the condenser), while looking for liquid refrigerant exiting the condenser when viewed in the sight glass. If you have rising condensing pressures and liquid in the sightglass, you have overfilled it. If you have liquid and steady condensing pressures around 20-30 deg above the air passing though the condenser, you are close. If you have vapor or vapor and liquid mix, and low high side, you need to add a little at a time, letting the system stabilize, and re checking pressure and sight glass. The hotter the day you attempt the better the chance of getting it right. Always better to come at it from to little than to try and draw it down (from to much) to the correct amount.

[600SL]

This sound like a sub cooling test typically performed on residential systems. I tried doing a sub cooling test last week, but I found it hard to do like it is done on the residential systems. Typically the residential systems are looking for ~10° sub cool. Sounds like your looking for 20°-40° sub cooling. The residential systems would typically measure pressure and temp at the outlet on the drier. Unfortunately Mercedes put the high side port on the inlet of the condenser. So If I assumed zero pressure loss across the condenser I was getting a sub cooling of 4 to 10°. Also how should the sub cooling test be performed. In residential applications it's with the AC in the building stabilized. I tried it doors and windows open, high blower at 2000 RPM as well as idle with windows closed blower on auto, and automatic temp stabilized. So it appeared to be low but I had nothing to go by.
Excellent Thank You.
Excellent Thank You.

[600SL]

Excellent Thank You.

[Cornbinder89]

No, I am not saying 20-40 deg of subcooling (temp of the refrigerant exiting the condenser below the condensing temp of the refrigerant), The condenser doesn't operate at 100% thermal efficiency. There has to be a temp difference between air passing through the condenser and the temp of the refrigerant. The closer you can get the refrigerant to the temp of the air the better, but it usually is around 20-40 degs above the air temp. There is no such thing as a zero pressure drop across anything, there has to be a differential in pressure to have flow, just as there has to be a temp difference for there to be a transfer of heat.
Look at it this way, What if you put a condenser on the system that was double the capacity of the minimum required, what would happen? As soon as enough heat was removed from the refrigerant to condense, it would turn to liquid, the remainder of the condenser would continue to extract heat from the liquid, but it would never reach the temp of the air passing over it. The temp difference between the condensing temp and the temp exiting the condenser is sub-cooling.
I don't even think about measuring sub cooling, It is generally a function of the system design and conditions under it is being operated. What I am looking for, is liquid at the receiver and a condensing temp that doesn't exceed 40 deg F above the air passing thru the condenser. The closer you can get the condensing temp to the air passing thru the condenser the better.

[600SL]

Got it. And when you say air passing over the condenser, are you talking shop ambient. That seems to be the same as using one of those temp pressure charts only you are adding the step of looking for liquid in the sight glass.I have tried those charts and can usually get either the high side or low side to be in but never both.

[Cornbinder89]

I'm not a fan of charts like you posted. The chart in the reference section show what temp the refrigerant is at for a given pressure.. Charts like the above are what the mfg expects to see for those conditions, I want to see what is happening right now and with the system in the condition it is in.
In a normally operating systems, the high side pressure equates to the condensing temp and the low side, the boiling temp. If the boiling temp is too high, or low, I would be looking at the evaporator, is the inlet and outlet of the evaporator close to the same temp? My general rule is: if I can't grab and hold the outlet of the condenser, I've got a problem there. It should never be so hot that it burns.

[600SL]

Well the results are in. The new recovery tank came in today and I recovered 11 oz of the original 36 oz of Freon I put in. So that meant that at some time or possibly still I had or have a leak. The only thing I can think of is that there is a good possibility I left the gauges hooked up for the week that I was waiting for my fan controller. At that time I did not have a leak detector. I have one now and cant find a leak but I think we will now for sure in about 2 weeks.

So the task still remained as to finding the correct charge I should be using for this system. So while I was down I did a little calculation of the volumes of the original and replacement condensers. It turns out that the new condenser is ~ 5% less volume than the old serpentine condenser. As a system that account for about 2% less for the entire system. Therefor using the 85% charge typically used for going from R12 to R134 I came out with 29.4 oz.

So with 88 to 89°F ambient temps I slowly approached 30oz. At 24 oz Most of the bubbles in the sight glass disappeared. The ambient temp duct temp, and pressures were all recorded with stabilized readings at 2000RPM.

At 24 oz - Ambient = 87° Pressures 245/16 PSI, Duct Temp 55°
At 28 oz - Ambient = 89° Pressures 250/16 PSI, Duct Temp 54°
At 30 oz - Ambient = 89° Pressures 255/16 PSI, Duct Temp 54°

I stopped at 30 oz since nothing appeared to be changing and it was in line with the estimated correct amount.

Attached is the MB performance chart for the 560SL running R12. Don't laugh when you see it. While the performance of this system is nothing to write home about, I am clearly beating the stock 560SL using R134. And that would be consistent with the 42° duct temperature I measured when it was only 70° ambient.

So what remains is do I have a leak or not. I will report back in a week or so.

[Cornbinder89]

Two thoughts:
High high side indicates a condensing temp of 145 to 150 so 55 to 60 degrees above ambient, way too high, I would say 40 above is the max I'd like to see and 20-30 is even better. So poor air flow or undersized condenser is indicated.
Unless Merc uses a valve in the low side to control frost instead of a frost switch or controller, you low size is 10 psi below frost temp, so something is wrong there as well. Could be a Tx stuck in "closed" or there could be a restriction in the lines somewhere, but the low low side is an indication that not enough liquid refrigerant is making it into the evaporator and it is all boiling way before it reaches the outlet.
I'd start by addressing the low side problem. You can "fudge" the high side during diagnosis of the low side by spraying (misting) cool water from a hose on the condenser, that will bring the high side down, while you observe the low side. If it is still low, than I would address that 1st. If it comes up to around 26 psi, I would wonder if the high side is still boiling before the Tx and you are getting a mix of hot gas and liquid to the Tx.
As I said before, I think system volume is a red haring, it doesn't change that much .

[600SL]

Two thoughts:
High high side indicates a condensing temp of 145 to 150 so 55 to 60 degrees above ambient, way too high, I would say 40 above is the max I'd like to see and 20-30 is even better. So poor air flow or undersized condenser is indicated.

I wasn't too concerned about the high side. It is about 2 bar (30PSI) higher than Mercedes would like to see on R12, but my understanding is that I should see higher pressures than when running R12. The frontal area of the universal parallel flow condenser is about 10% smaller that the original serpentine condenser. But I keep reading that parallel flow condensers are so much more efficient that it shouldn't be an issue.

Unless Merc uses a valve in the low side to control frost instead of a frost switch or controller, you low size is 10 psi below frost temp, so something is wrong there as well. Could be a Tx stuck in "closed" or there could be a restriction in the lines somewhere, but the low low side is an indication that not enough liquid refrigerant is making it into the evaporator and it is all boiling way before it reaches the outlet.

Mercedes Benz uses a switch to shut off the compressor. I did notice that yesterday I was unable to get the compressor to cycle at all as compared to several weeks ago when I tested it in 70°F ambient conditions, it would cycle. I wasn't sure if that might have been because it was too hot a day yesterday for it to cycle.

I'd start by addressing the low side problem. You can "fudge" the high side during diagnosis of the low side by spraying (misting) cool water from a hose on the condenser, that will bring the high side down, while you observe the low side. If it is still low, than I would address that 1st. If it comes up to around 26 psi, I would wonder if the high side is still boiling before the Tx and you are getting a mix of hot gas and liquid to the Tx.

Will try it. Tx valve is what I was leaning to. The original one from MB is no longer available and the one I replaced it with (the only one I can find) is from a company URO, with a poor reputation. If I have a problem with condenser flow, that is something I will just have to live with, as it is not possible to fit anything bigger or more fans into this car. You can see by the chart I provided in the previous post that the AC was a really poor performer to begin with. According to MB 63.5°F duct temp is acceptable on an 89°F day. I'm way ahead of that, but unfortunately that's what I'm stuck working with.

As I said before, I think system volume is a red haring, it doesn't change that much .

Seems to be plenty of margin. 24 Oz to 30 oz really made insignificant difference and I did go to 36 oz previously. I stopped adding Freon at 30oz because the low side was not responding, while the high side was rising.

[600SL]

Tried running it this morning. 84°F ambient this morning. I got the engine up to temperature with both fans running, didn't get it to stabilize but sprayed the water on the condenser when it was at 175/16, Both pressures went down ~ 140/11 and returned shortly after.

[Cornbinder89]

170 is aprox 123 deg for 134a, so still about 40 deg over ambient, 140 is about 107 deg, or about 20 deg over, but that was with water, where you would expect it do be much lower. The low side is way too low, and since we know the duct temp is no where near freezing, one can only conclude not enough liquid refrigerant is getting into the evaporator. Feel you liquid line from the outlet of the condenser to the Tx valve looking for a sudden temp drop, which would be an indication of a restriction before the Tx. Otherwise the Tx is suspect.
If it turns out to be too small a condenser, can you stack them two deep? Take the hot refrigerant from the compressor and feed the rear one, take the outlet (lower connection) and feed the upper front connection and bring the lower front to the receiver. It will require either a hard line that is swaged to go from -6 to -8 or a -6 hose with a step up fitting on one end. I am thinking I will have to use this on one of my trucks.
Is the Tx a block type or not? I just picked up a Tx for my '83 that is the non-block type for around $25. -6 inlet, -8 outlet externally equalized. could easly be adapted to any evaporator that had similar connections. 1 1/2 ton rating.

[600SL]

I could have got the pressure down significantly if I kept the water on it. I'm sure I could have gotten a 10° spread. But once I got the story I didn't feel any more need to go further. Double stacking condensers is not an option in this car but more fan power might be. I just found a 8" diameter fan that the manufacturer claims to put out 2400 CFM. I could actually replace my 16" 2500CFM fan with 4 Perma-Cool 19128 and also add 2 Perma-Cool 7" 19127 2200CFM fans. The only problem with it is I would like to set up a test for it because I think the specs are not correct or at least not correct for a suitable pressure drop passing through a radiator and condenser.

I cant picture having a pressure drop on the high side with all new lines going to the TXV. But I will check, stranger things have happened. One thing I did have to do was reduce the discharge from the compressor from a -10 to a -18. So the last 1 ft of hose going to the condenser is -8 whereas the original was =10 all the way.

My Tx is not a block type.

I did find this one, and am checking to see if they will ship to US. https://www.autodoc.co.uk/hella/950949

Fans

https://www.summitracing.com/parts/prm-19128

https://www.summitracing.com/parts/prm-19127

[Cornbinder89]

Picture of the Tx looks a lot like the MEI 1610. It has -6 male flare inlet, -8 male flare outlet 1/4" female flare equlizer connection and rate for 1 1/2 ton.
http://www.partdeal.com/mei-external...ngth-1610.html
If you look at the cross in both, the Cat number is the same so I would guess it would work in your application
Around $30 mail order, Might be cheaper if you have a good refrigeration supplier in your area, Mine was $23 at Thermo-King Christensen, Sioux City, IA

We had a problem on here a while back, Guy had swapped in another system. Had all kinds of problems but it pointed to a restriction. Turns out he had a switch port welded into a hard line, and during the welding, the line was almost completely plugged off. He finely found it by cutting into the line and trying to pass a wire through it.

[Cornbinder89]

I'm surprised at the -10 discharge line of the compressor, even with two evaporators in semi's -8 is the norm. My Cabovers use -12 suction lines, I assume because of the long runs and many sharp bends. Still only -8 discharge line. I don't see how it would be the problem in your case.

[600SL]

I do have a braised in high side service fitting just before my condenser on a -8 inlet. I also have a braised in low side service port on the -12 line to the compressor.

The one I posted from the UK crosses to a 4 seasons https://www.autozone.com/cooling-hea...e&newYmme=true. For a few dollars more I would prefer to get the German one if I can because this is located where the sun doesn't shine. Below are all the other numbers that cross.

4SEASONS: 38604
AKS DASIS: 840050N
AURADIA: EXMS 008
AVA QUALITY COOLING: EXMS008
BEHR SERVICE: 7003906
CTR: 12 12 106
DELPHI DIESEL: TSP0585041
FEBI BILSTEIN: 08902
FRIGAIR: 431.10923
GERI: 971022
HELLA: 351235001
NRF: 38389
TTV: 8438602
VEMO: V20-77-0017
WAECO: 888-1100021

[600SL]

I'm surprised at the -10 discharge line of the compressor, even with two evaporators in semi's -8 is the norm. My Cabovers use -12 suction lines, I assume because of the long runs and many sharp bends. Still only -8 discharge line. I don't see how it would be the problem in your case.

Mercedes went to the -10 suction and -8 discharge when the switched from Denso to Sanden compressors in the late 90's. Prior to that it was -12/-10. Some of the cars through the 80's were using the GM A6 compressor.

[Cornbinder89]

I've good luck with MEI stuff, either that or Red Dot. 4 seasons not so much, There aren't many flare type Tx valves used anymore, tube O ring really has dominated the market. Flare was big in the early days of automotive AC as it was common in refrigeration back then.

[Cornbinder89]

Mercedes went to the -10 suction and -8 discharge when the switched from Denso to Sanden compressors in the late 90's. Prior to that it was -12/-10. Some of the cars through the 80's were using the GM A6 compressor.

Yeah, the A-6 was one of the best compressors ever designed. I use them on my cabovers, Size and weight as well as cost killed off use. I have even seen 4 of them ganged together to replace a Sutrak compressor on a motor coach! They are good to 40,000 BTU.
My cabovers use -8 discharge and -12 suction on the A-6