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Old 01-04-2021, 05:57 PM   #1
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engine driven a/c freon capacity. repair advice needed.

I am trying to repair the ac system on our 2003 Thomas bus. It has two separate systems. I replaced the compressor and drier on one system evacuated system and added the amount of r134a hand wrote on a decal using a scale and from a 30lb bottle. 5lb 8oz. Pressures on manifold guages were very low, 15hg vacuum in low side and 70psi on high side and little to no cooling. Im not very experienced with charging ac systems so I thought something may be clogged. I asked my cousin who is an experienced ac tech with home systems and he suggested I keep adding freon unless I get high pressure on the high side. I wound up adding another 4+ lbs till the vents were blowing about 58į. Low side was about 28psi, high side was about 250. The outside air temp was about 68 but I had the heat on and brought the inside temp up to 80 before i started and ran the heat the whole time.
Why would I need so much more than the sticker says? Did someone goof up and write the wrong amount down on the decal maybe?
If thats the case how much oil should the system hold? the sticker listed 11oz of pag 150. I added 8oz based on recommend calculations for the parts I replaced.

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Old 01-04-2021, 06:24 PM   #2
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I have the Excel file from Daimler that has the AC charge info. I canít upload it
To the site because itís an XLS file. If you want it, DM your email address and Iíll email a copy to you.
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Old 01-04-2021, 06:32 PM   #3
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Quote:
Originally Posted by aridgedell View Post
I am trying to repair the ac system on our 2003 Thomas bus. It has two separate systems. I replaced the compressor and drier on one system evacuated system and added the amount of r134a hand wrote on a decal using a scale and from a 30lb bottle. 5lb 8oz. Pressures on manifold guages were very low, 15hg vacuum in low side and 70psi on high side and little to no cooling. Im not very experienced with charging ac systems so I thought something may be clogged. I asked my cousin who is an experienced ac tech with home systems and he suggested I keep adding freon unless I get high pressure on the high side. I wound up adding another 4+ lbs till the vents were blowing about 58į. Low side was about 28psi, high side was about 250. The outside air temp was about 68 but I had the heat on and brought the inside temp up to 80 before i started and ran the heat the whole time.
Why would I need so much more than the sticker says? Did someone goof up and write the wrong amount down on the decal maybe?
If thats the case how much oil should the system hold? the sticker listed 11oz of pag 150. I added 8oz based on recommend calculations for the parts I replaced.

250 over 28 yowsers! either a condenser that has no fans running or a blockage in the system.. what kind of compressor and evaporator and approximately how long are the lines?



with those kind of pressure differentials id expect there to be ice on something or at least part of the high side is cold.



things to check.
1. in an overcharged state its tough to troubleshoot.. so i would recover to a vacuum and recharge with stamped amount back in.. and then first place id look is if it is seriously icing across the TxV expansion valve at the evaporator inlet..



2. look for cold or icing pipes right after the the condenser and before the evaporator.. a clogged up filter dryer would most likely be icing up or be much colder at its outlet than its inlet. you mentioned replacing the dryer so i dont suspect it but doesnt hurt to check.


im assuming you flushed the system out? a broken compressor often results in particles being circulated through the system, you hope that the filter dryer catches them all.. however on disassembly if some dirt ended up past it.. the TxV is the next stop. and a partially blocked TxV will act just like what you are seeing. if the discharge line from the compressor to the condenser and the condenser werent flushed then the new filter dryer might clog right back up again.. thus why looking for cold lines after it can be telling.



for reference, on my bluebird short bus.. it is a 6 window bus, TM-16 / Sanden SD7 style compressor underhood (dognose bus).. condenser is on lower left corner of bus, evaporator is in rear of that 6 window bus.. and it holds 7 lbs of freon.. so the 5 lb 8 oz for a front system. esp if its condenser is near front and also is its evaporator. (and front compressor). then it is a reasonable charge.. a rear system would use a couple pounds more depending on how far back it is..



250 head pressure on a 68 degree day is WAY high.. low suction pressure is expected on such a day.. even with the heat on in the bus, that doesnt account for the outside ambient where you are sending up very cool liquid due to the condenser being extra efficient in 68 degree weather.. at 70 degress a high side of 140-150 isnt uncommon at all.. and depending how aggressive the Txv is on the evaporator a suction of 25-30 would be pretty common.. and dropping as the evaporator cooled down.. the Txv should seriously slow down the flow if the unit gets cold enough and low pressure enough to freeze (teens or 20 PSI range)..





when you had the presribed charge in it did you happen to shut it off and watch for how long it took the system to equalize out to standing pressure?
was it super slow-mo or did it equalize over a minute or so?
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Old 01-04-2021, 08:21 PM   #4
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A 2004 CE200 I recently moved for another member had twin inboard ceiling units and individual compressors, and it called for approximately 6.5 lbs PER SYSTEM.

When I picked this bus up for transport, I was informed that one unit worked, the other didn't. As I had told the new owner I would take a look at it for them and figured it was a simple fix, I bought enough refrigerant for a full charge of 6.5 lbs. But if memory serves, it began cycling after putting in 24 oz, and another 18 oz had it pumping so hard I was getting goosebumps at 55-60 mph on a 72 degree day.

That's with approximately 1700 sq ft to cool, mind you. The single unit was struggling to keep up before. Hell of a difference, and that's one of several reasons I tell people not to remove factory A/C units.

There should be a label somewhere on the firewall with the correct charge capacity for any installed units, if not, call the manufacturer of the units with the unit serial #'s.

Too much refrigerant can cause even worse problems than not enough. You might check any pressure cut-out switches for proper function (mostly only need the connector jumped out to see if it is cutting out prematurely). If the lines ice over, it is undercharged.

Also, do not use refrigerant with oil added for just a simple charge, oil is only necessary if the system has had major service and been flushed. Refrigerant ONLY otherwise, if you add refrigerant with oil mixed to a system that already has oil, you can destroy it, as the compressor will become flooded with oil and unable to function. It's possible someone tried to charge this thing with oil-mixed refrigerant prior to purchase and hydro-locked the system.
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Old 01-04-2021, 09:16 PM   #5
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Quote:
Originally Posted by cadillackid View Post
250 over 28 yowsers! either a condenser that has no fans running or a blockage in the system.. what kind of compressor and evaporator and approximately how long are the lines?



with those kind of pressure differentials id expect there to be ice on something or at least part of the high side is cold.



things to check.
1. in an overcharged state its tough to troubleshoot.. so i would recover to a vacuum and recharge with stamped amount back in.. and then first place id look is if it is seriously icing across the TxV expansion valve at the evaporator inlet..



2. look for cold or icing pipes right after the the condenser and before the evaporator.. a clogged up filter dryer would most likely be icing up or be much colder at its outlet than its inlet. you mentioned replacing the dryer so i dont suspect it but doesnt hurt to check.


im assuming you flushed the system out? a broken compressor often results in particles being circulated through the system, you hope that the filter dryer catches them all.. however on disassembly if some dirt ended up past it.. the TxV is the next stop. and a partially blocked TxV will act just like what you are seeing. if the discharge line from the compressor to the condenser and the condenser werent flushed then the new filter dryer might clog right back up again.. thus why looking for cold lines after it can be telling.



for reference, on my bluebird short bus.. it is a 6 window bus, TM-16 / Sanden SD7 style compressor underhood (dognose bus).. condenser is on lower left corner of bus, evaporator is in rear of that 6 window bus.. and it holds 7 lbs of freon.. so the 5 lb 8 oz for a front system. esp if its condenser is near front and also is its evaporator. (and front compressor). then it is a reasonable charge.. a rear system would use a couple pounds more depending on how far back it is..



250 head pressure on a 68 degree day is WAY high.. low suction pressure is expected on such a day.. even with the heat on in the bus, that doesnt account for the outside ambient where you are sending up very cool liquid due to the condenser being extra efficient in 68 degree weather.. at 70 degress a high side of 140-150 isnt uncommon at all.. and depending how aggressive the Txv is on the evaporator a suction of 25-30 would be pretty common.. and dropping as the evaporator cooled down.. the Txv should seriously slow down the flow if the unit gets cold enough and low pressure enough to freeze (teens or 20 PSI range)..





when you had the presribed charge in it did you happen to shut it off and watch for how long it took the system to equalize out to standing pressure?
was it super slow-mo or did it equalize over a minute or so?
Thanks for the reply!
A6 compressor
em-9 evaporator
front engine flat nose 38ft
pretty long runs from compressor back to condenser then back up to the evaporator.
fans are all blowing great.

The compressor was just leaking around seals and did not send metal through the system. I did flush the line from the compressor to the condenser and the condenser. the lines had some corrosion in it. The bottom of the old drier had quite a bit of corrosion dust maybe a couple tablespoons. I breifly tried to blow through the lines to the evaporator but I never disconnected the txv so I really couldn't get any air through there even though they looked way cleaner than the other lines. I would have to cut the bands to disconnect the lines to the txv.

when I originally added exactly the prescribed amount I was getting a few bubbles and mostly liquid through the sight glass of the drier. I did happen to pull the cover off the evaporator and at one point the txv was icing a little even though the rest of the evaporator was not particularly cold. At this point I was reading way negative on low side and about 75 on high side.

Today when I fired it up to further diagnose, before i added any more refrigerant, I did look at the drier again, this time the glass was full foam and the exit of the drier was very cold, going into the drier was cool, going into the condenser was warm. I did not look at the txv this time. This is when my cousin advised I "add more refrigerant and just watch the high side to make sure it doesnt go way up to 400". Once I got to a total of about 10 lbs in, the sight glass on drier was clear, drier was cool on both sides (no real cold spots like before). vent temp had come down to 58 ( the lowest it had been yet ), the high side would fluctuate between 245 and 255 ( i was thinking this was the txv doing its thing). It had been running for close to an hour at this point so the engine bay and compressor was pretty heat soaked.
I did not pay attention to how fast it equalized untill after I added more refrigerant. But when I was done I did pay attention. Immediately after shutting everything down both sides started moving but after 10 minutes there was still a difference of 30 to 50 psi, probably took 20 to 30 minutes to really even out. Later on after supper and everything cooled down, I ran it again and my high side was under 200 i think, and low side was a little lower also, and vent temp was around 55 maybe.

I still have the other system to do and the txv is a little easier to get to, so I may go ahead and replace it so I can flush everything out good and see how it runs with the prescribed amount of refrigerant for comparison.

My thought now (in regards to the first system) is either:
1) There is a blockage and overcharging it is allowing it to work better.
OR
2) It was undercharged with 5lbs 8oz and that was causing my symptoms and all is well enough with how much is in it now.

This is as deep as Ive ever gotten into ac so I am relying on advise from those more experienced than me.

Here is the tag, a little hard to read but I assume it is 5 lb 8 oz wrote on it.20201221_140016.jpg20201221_135946.jpg
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Old 01-04-2021, 09:32 PM   #6
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Quote:
Originally Posted by CHEESE_WAGON View Post
A 2004 CE200 I recently moved for another member had twin inboard ceiling units and individual compressors, and it called for approximately 6.5 lbs PER SYSTEM.

When I picked this bus up for transport, I was informed that one unit worked, the other didn't. As I had told the new owner I would take a look at it for them and figured it was a simple fix, I bought enough refrigerant for a full charge of 6.5 lbs. But if memory serves, it began cycling after putting in 24 oz, and another 18 oz had it pumping so hard I was getting goosebumps at 55-60 mph on a 72 degree day.

That's with approximately 1700 sq ft to cool, mind you. The single unit was struggling to keep up before. Hell of a difference, and that's one of several reasons I tell people not to remove factory A/C units.

There should be a label somewhere on the firewall with the correct charge capacity for any installed units, if not, call the manufacturer of the units with the unit serial #'s.

Too much refrigerant can cause even worse problems than not enough. You might check any pressure cut-out switches for proper function (mostly only need the connector jumped out to see if it is cutting out prematurely). If the lines ice over, it is undercharged.

Also, do not use refrigerant with oil added for just a simple charge, oil is only necessary if the system has had major service and been flushed. Refrigerant ONLY otherwise, if you add refrigerant with oil mixed to a system that already has oil, you can destroy it, as the compressor will become flooded with oil and unable to function. It's possible someone tried to charge this thing with oil-mixed refrigerant prior to purchase and hydro-locked the system.
Thank you, my system does not have pressure switches. It could be possible that there is too much oil in it. I have not added any refrigerant that contains oil. I used this chart to approximate how much oil to add though. The compressor came with 3oz in it. I added about 2 oz to the compressor and poured about 2 oz into the oulet side of the drier and another 1 oz into the high side hose. so i added 7 or 8 oz. When i replaced compressor, flushed one line, flushed condenser, replaced drier. Screenshot_20201222-190823_Chrome.jpg
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Old 01-04-2021, 10:47 PM   #7
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Iím going to say crap/stuck txv and now overcharged.
Your not getting hardly any condensing happening. If you check your coil
Temps with a infrared thermometer it will have a cold spot but be quite a bit warmer on the outlet end because it doesnít have enough liquid to evaporate. The low ambient temps and helping it limp along and give you some cooling.

Cadillac kid and others are way smarter than me though. Just thought I would chime in with my ďguessĒ
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Old 01-04-2021, 11:14 PM   #8
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One other weird thing I noticed when I was charging the system on the '04 I mentioned...

Granted, the 'gauge' you get with parts store retrofit/charge kits is hardly known for accuracy, but I checked the reading on both compressors to see whether the system being charged showed about the same pressure. Strangely, the one that was operating in the first place actually showed LOWER pressure at the charge port than the one I charged -- go figure.

It could be that it still needs a shot or two of the smaller cans, but the one that needed to be charged actually cools better than the one that didn't, and showed higher pressure. Perhaps the other one was undercharged/ overcharged to start with, or perhaps it is on its way out from running non-stop to keep up. *shrugs* I caution anyone with A/C to never assume that all is well just because it works. Just because it works, doesn't mean it's happy.
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Old 01-04-2021, 11:50 PM   #9
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If it doesn't equalize pressure within a few minutes there is a restriction and that also explains the problem of poor cooling. How to find it, can't help much on that, txv valve might be clogged or not working properly.
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Old 01-05-2021, 09:36 AM   #10
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Blockage would be expansion valve or orifice tube (in-line filter that sort of serves the same function). Could be an orifice tube clogged up with dirt or excessive oil.
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Old 04-07-2021, 07:05 AM   #11
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Update/ repair complete

I wanted to give an update. I finally got the second system finished up, I replaced compressor, expansion valve, and drier. Flushed all the lines real good and charged with the amount on the sticker. Pressures seemed normal and it is blowing nice and cold. So I went back to the first system and replaced the expansion valve, another new drier, replaced a few fittings and charged with the right amount of freon and it is blowing nice and cold with normal pressures. So as long as things keep working good it seems the expansion valve was causing an issue originally. If someone is repairing a similar system I would definitely recommend going ahead and replacing everything, expansion/ TXV, drier, compressor, and low and high side fittings. It really was pretty easy to do everything, not any more difficult than say the plumbing on the bus.
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Old 04-07-2021, 09:09 AM   #12
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I rewplace or adjust a lot of TxV's.. I find many of them misadjusted from the factory which causdes issues in the field.. usually low suction pressure which leads to techs over-chargong the system to bring the suction pressure up when the engine speed is higher..



worked on a scout-II this spring already where a guy added A/C to it.. said it wont cool right.. added more freon and cooled worse yet his can with a gauge kept sayings its low when it showed full he said it cooled worse..



I recovered the freon out, and proceeded to charge it.. and right away knew the TxV was culprit.. the head pressure and suction pressure were low.. Superheat was way high, yet the site glass was clear showing that I had a solid column of liquid at the TxV..



the reason the head pressure goes low on a blocked or misadjusted TxV is the fact you are performing less work on the refrigerant therefore its temperature drops in the codnenser quite quickly as it condenses much faster than design.. in a larger A/C you may see the head pressure go off the scale if you completely fill the condenser with liquid but rarely happens in a mobile system.



I recovered the freon again, took 1/2 turn on the TxV adjustment (had to adjust inside the pipe so it requires teardown each time).. put it all back together, charged and ice cold.. less refrigerant, higher head pressure (I adjusted his fan clutch a little cooler to engage his fan a little faster).. about 10 oz less freon in it total than when I got it..



almost all Bus air conditioners use TxV's as opposed to fixed orifices like many factory car A/C's were.. most all newer cars have gone to TxV (full circle since the cars of the 50s, and 60s into the mid 70s were all TxV.. GMC used a closed loop EEVIR system in their 70s vehicles.. that was by far the coldest system ever.. however it was prone to failure as even the least little bit of dirt caused the valves to stick, not to mention without re-adjustment these older systems..



in the new vehiclkes it was found that 134a didnt like the cold temperatures of operating near freezing so the manufacturers installed much larger blowers and run them with warmer coil temperatures.. more modern vehicles have much stronger vent air but dont taske the humidity out quite as well as the old R-12.. the units move heat but move less moisture so the net gain is a "slight" gain in fuel efficiency running the compressor less.. and for those of us that remember the ice cold A/C of old cars the comfort level in new cars is a bit lower in humid conditions.. (luckily thus far in every new car ive bought or leased ive been able to hack the A/C to blow out that ice cold air I was always used to)
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Old 04-15-2021, 03:52 PM   #13
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Thanks for the wealth of information! I am happy I was ablen to get it fixed with yalls help!
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Old 09-05-2021, 12:01 AM   #14
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Quote:
Originally Posted by Danjo View Post
I have the Excel file from Daimler that has the AC charge info. I canít upload it
To the site because itís an XLS file. If you want it, DM your email address and Iíll email a copy to you.
I tried private messaging you Danjo but it said I can't until I'm a trusted member. How can I pm you my email? I left you a profile comment in the mean time
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Old 09-05-2021, 12:05 AM   #15
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I have a2007 Thomas c2 freightliner with mbe906 72 passenger with factory air. The school district had emptied the ac system and I need to recharge. You think I should add compressor oil? The serpentine belt had bypassed the ac completely and presumed it was for gas economy as the school district wasn't aware the busses for this year had factory ac. They promptly made sure all future busses did not: have ac after a driver thanked the school board for the factory AC
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Old 09-05-2021, 09:57 AM   #16
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are you sure they bypassed it for MPG? or did it break and they bypassed it? more often than not when I find a broken , missing or bypassed belt its because the system suffered a major failure and the school just bypassed it and ran without.. it often happened a year or 2 before a bus was scheduled to get sold .. ive never seen a school empty out and bypass the belt on a factory A/C.. they usually just unplug the wire and pull the fuses if they dont want one used.



I revived the A/C on a delivery Van that was 18 years old that had been disabled.. a system thats sat that long, brand new or not should be completely gone over esp the compressor.. I rotated it by hand a few revolutions (the inner part). and sprayed brakleen on the clutch area.. it had lost its freon since none of the seals were lubriucated so I pulled the connections apart , replaced the O-rings and the accumulator / dryer.. put it back toigether and worked like a champ. the inside part in that case was fine since the driver was allowed to use the heater just not the A/C




if the school truly sucked out a full charge of freon chances are you dont need to add oil... rarely does the oil get sucked out wit ha recovery machine.. the freon evaporates in the pipes and leaves the oil behind.
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