1986 Merc

[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

[600SL]

I think you onto something with the expansion valve. When I first charged it it was a 70° day. Notice in the picture my duct temp at ~43° and the OAT gauge reading 65°. The OAT was low because the system was cooling off the shop. It was a really strong system for a day. But even on 70° days it I cant get duct temperatures that low. I think there is a good chance that a piece of basing debris broke off and is now stuck in the expansion valve screen. Its worth taking a look. One thing I like about that one from Germany is it specifically states its for R134. The others don't mension anything. Hopefully they will sell me that one. Otherwise I will get my sister in law to buy it in Ireland and send it to me. In any case tomorrow it will be 90° here and I can take it for a good spin to see how comfortable it is.

In the mean time, many thanks for your help its been enlightening.

[Cornbinder89]

. One thing I like about that one from Germany is it specifically states its for R134. The others don't mension anything. .

I really wouldn't be too concerned about it mentioning 134a or not. What is important is that it meter the correct volume for the size evaporator and refrigerant tonnage. Since R 12 and R134a (as well as some others) take basically the same volume to do the same job ( notice I didn't say weight or mass), the exact refrigerant is irrelevant to the valve. The valves only job is to meter the refrigerant such that it all just boils off by the exit of the evaporator. It doesn't care what temp or pressure (within reason), only that the volume of refrigerant passed boils by the exit, not too much or too little. So while R 12 is around 28.5 psi @ 30 deg F and R134a is 26.1 psi, the same valve works for both, it just doesn't care what the temp or pressure is only that it meter the correct amount of refrigerant.
Some Mfg esp early on used pressure regulating devises to control evaporator temp (POA STV, EPR etc) these devices control how low the pressure drops in the evaporator and therefore how cool it will get, with these, they DO have to be set for the refrigerant being use, and the setting will be different depending on the refrigerants boiling pressure at the temp desired. Most mfg today that use a Tx valve, opt for a "frost switch" to limit the temp in the evaporator by de clutching the compressor. Early in auto A/C they compressor ran continuously when ever A/C was called for and the pressure regulator controlled how cold the evaporator got. I guess they felt it best not to clutch and de clutch at high engine speeds, but the systems were more expensive to produce and the whole idea was dropped in favor of the frost switch design , while it is harder on the clutch it is cheap to produce.

[600SL]

If I strike out with the one from the UK, I will order a Red-Dot. Although I'm having trouble finding someone who has it in stock and will sell to non commercial accounts.

When I went to GM school after the CCOT systems started to appear, I remember the instructors first words, "Your going to sell a'lot of clutches". Interesting to note in that day when GM was running POA systems with the A6 compressors, the clutch was a much smaller diameter then the typical Ford York/Techumsa cycling clutch systems. Nothing beat the old GM POA systems, in fact my dads 78 Lincoln Continental used it.

[Cornbinder89]

Yeah, if the A-6 has one weakness it is the size of the clutch. When it was designed in the late fifties to replace the A-5 (Gm's 1st compressor) everything had the compressor run continuously and controlled the temp by another means. Still, they used it for a long time with a frost switch until the CCOT and the R-4 (piece of junk)!
I was around for the beginning of the CCOT.
I've managed to burn more than 1 A-6 clutch, but most times it was due to some other problem, not just the clutch slipped, but the clutch slipped because of a bearing or compressor problem.

[600SL]

I guess they had to come out with the R4 because the A6 was bigger than the Vega engine. How GM went from the A6 to the R4 with a straight face, I will never know.

[600SL]

Did a little bit more testing today. I pulled the bulb off the expansion valve to see how it would react. I also measured sub cooling and super heat as best as I could.These are the results.

Base line bulb installed

Ambient Temp 77° Duct temp 54°

Subcool Pressure Sat Temp sub cooling = 6
High Side 205 132
Low Side 16 16.5
High Side Line Temp 126

Superheat Pressure Sat Temp Super Heat = 37
High Side 220 137
Low Side 14 14
Low Side Line Temp 51

With Bulb Disconnected

Ambient Temp 74° Duct temp 62°

Subcool Pressure Sat Temp sub cooling = 5.5
High Side 225 138.5
Low Side 34 39
High Side Line Temp 133

Superheat Pressure Sat Temp Super Heat = 4
High Side 235 142
Low Side 33 38
Low Side Line Temp 42

One thing I can say about this is that I can only measure pressure before the condenser, So I have no way to account for pressure drop through the condenser and receiver drier. I'm thinking I really have zero sub cool and I'm under charged or just don't have enough fan power. I may try later to test the sub cooling with the water on the condenser.

[Cornbinder89]

Am I reading your info correctly? 1st pressure then temp of the line for each reading? Are you really seeing 14 deg on the low side line? Are these temp readings with a thermometer on the line or what you are getting from a pressure/temp chart?
I'm wondering if you have a Evaporator problem?
Your high side pressure/ temp readings are spot on for the refrigerant (134a) same for the low side. If air is bypassing the Evaporator or if the evaporator has an internal problem.

[600SL]

I see the system has very kindly reformatted everything and took out the spaces I put in to make the data in nice columns. The data is translated like this.

Subcool Pressure Sat Temp sub cooling = 6
High Side 205 132
Low Side 16 16.5
High Side Line Temp 126

=

Subcooling is calculated as 6° from the saturation temp of 132 - the measured temp on the high side line of 126
High Side 205 PSI side has a saturation temperature from charts of 132
Low Side 16 PSI has a saturation temperature from charts of 16.5
High Side line temp is measures with a thermocouple clamped to the high side line after the RD

[Cornbinder89]

Ok, Have you measured the suction line temp after the evaporator? One thing I see right off the bat is your condensing temp is 55-60 deg above ambient. There is just no way, with condensing temps that high, that the system will ever provide good cooling. I'm not too concerned about sub cooling, I am about condensing temp.
I notice when you pull the thermostatic bulb the low side pressure rises to closer to duct temp. So I see two problems, one is with the Tx valve and the other is the condensing temp.
I would pull the Tx and look inside carefully. I wonder if it is clogged with debris from a bad compressor or drier? They are cheap enough (the Tx) to just replace.
Your not getting the heat out of the refrigerant, and without doing that, it will never cool properly. You need to get enough liquid refrigerant into the evaporator, which doesn't seam to be happening now, and you need to get the temp of the liquid refrigerant closer to ambient.
There is a 3rd possibility but I find it remote, that is the air is bypassing the evaporator and the frost device is not working and the evaporator is really at around 16 deg. For this to be true you would have to have multiple problems and it still doesn't address the high condensing temp.

[600SL]

So today I boosted the charge to get a 13° sub cool. No amount of charge would increase the sub cooling beyond the 5-6° until all of a sudden it went up to 13°. My theory was that the 5 to 6° sub cooling I was getting was really 0 because the pressure was taken before the condenser. Suddenly I got to a point where it started to respond but no difference in performance. The results today are getting worse both before and after changing the charge. Now 66° duct temp both before and after recharge. I'm liking your expansion valve theory. I think the little screen in the inlet of the expansion valve may be clogged with debris from fabricated and welded custom AC lines.

Today's results after setting sub cooling to 13°

Sub cool measurements Ambient temp 81°
High side 280 measured before the condenser
Low side 28
Measured temperature after the condenser 141°
Calculated sub cooling 13°

Super heat measurements Ambient temp 82°
High side 295 measured before the condenser
Low side 25
Measured temperature after the condenser 61°
Calculated super heat 33°

So super heat is way up high today as if I had the bulb disconnected from the suction line like I had the previous day.

The temp measurements are shown in the attached pictures for super heat. I only have a 1 channel thermocouple so I did on run for sub cool and 1 for superheat.

The sub cool temp was taken bu putting the TC under the red clamp shown on the liquid line.

Note the super heat may be high but there is a ton of super heating going on inside that engine compartment.

Next week I will have a multi channel temp meter and will attempt to clamp the TC's to the inlet of the TXV and outlet of the evaporator. I'm also thinking of fitting a high side port on the liquid line.

Im also considering an inline filter like this. https://www.amazon.com/ACDelco-15-10...s%2C167&sr=8-6

Might you have any recomendations.

[Cornbinder89]

All you did by overchargeing is reduce the area in the condenser available to hot gas and instead filled it with hot liquid. You end result is that the heat is still there and still way too high. You are going in the wrong direction. Super heat and sub cooling aren't going to solve this. They are things used when engineers are designing the system. You have the system you have, so lets deal with that.
1st I would change out the Tx, not expensive and it could be partially plugged. Next I would focus all my attention on getting the heat out at the condenser with the correct (30 oz?) charge in the system.
Just because getting the heat out at the condenser isn't the easiest thing to achieve, doesn't mean it isn't the problem.
At 280 psi the condensing temp ~155 deg F or over 70 deg above ambient! You are headed to burst hoses and compressor that will fail from too much heat and not enough lube.
Either the condenser is too small for the system, you need a condenser that is at least capable of 1.5 ton, or the airflow thru the condenser is not good enough or is re-cycling heat from the engine.
A/C can not survive with head pressures like you are seeing. Try standing in a spray of 155 deg water and see how cool you feel!
It seams to me you are grasping at things because you don't want to face the fact heat is not being shed.
Best mobile A/C system I have was designed for R12 running R134a, has a 1.5 ton Tx and evaporator but a condenser of over 30,000 BTU capacity! I get very low head pressures and very good cooling. The refrigerant is as close to ambient as possible, and the refrigerant oil stays cool. The difference in price between a condenser that was "just big enough" and the one I installed was less than $35, the difference in operation, is staggering. Yeah it was overkill, but is works well and components last, head pressure is low and the energy required is less (not that it is really a factor on a semi tractor). I fought marginal condenser capacity for years, and when new would just work, but once a little corrosion reduced heat transfer it was a battle to keep cool, head pressures rose and things started failing.