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Old 04-28-2021, 02:27 PM   #1
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AC compressor for 2006 Thomas Express 3500

Hi,


I have bought a Minotaur and would like to get the AC going.


It barely worked when I got it and I now have replaced most everything, and it still works about the same as when I got it.


I have a feeling that the OEM ac compressor for the Chevy 4.8L engine ( Denso 971-0316 is the biggest available) is just not big enough, and at some point, the previous owner switched the original compressor out with a regular compressor for a plain ol' express.


It has a condenser underneath ( CM-2, 1K 030-040-01) and a big evaporator in the back ( IW2, 1K 77-62167-02).


The single compressor on the engine feeds the one big condensor, which splits to feed both the original evaporator in the dash as well as the big evaporator above the rear door.


Does anyone know the part number for the compressor that should be used here? It is hard to believe that the same compressor for a Tahoe with a little evaporator in the rear would also be able to provide enough liquid refrigerant for the huge evaporator in the back of this thing.


Thanks for any help, I have been wrestling with this for longer than I care to admit.



Doug

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Old 04-28-2021, 02:41 PM   #2
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Join Date: May 2009
Location: Columbus Ohio
Posts: 14,905
Year: 1991
Coachwork: Carpenter
Chassis: International 3800
Engine: DTA360 / MT643
Rated Cap: 7 Row Handicap
sounds like it is a Carrier Mobile Tie - In system..



you said you replaced most everything.. what are tyou seeing for pressures? is it possible the system had a leak and someone has charged so much dye and stop leak into it that you have an excess of non-condensables?



are the heater valves properly closed off? does either system blow cold? (dash or rear?)..



before I throw away compressors I start looking at my pressures and look for signs in the site glass of excessive amounts of dye or stop leak.. or even if the schraeder is extremely wet when you connect and disconnect.. my red bus was so bad that liquid chemical dripped out of my high side gauge hose when i would disconnect..



if you see low head high suction at idle and it tries to meet saturation on engine revving then id be more confident in the compressor being undersized..



I dont know if you can fit a sanden in there or not.. i run the sandem SD7 enhanced compressors and I run them hard and they last and work well.. those little units take nore than the larger TM-21's all the OEMs like to use.. im not a fan of Denso compressors but thats just personal..



Transartic seems to handle a lot of the old carrier information (carrier no longer makes bus A/C).. im not sure how much chassis info they have on the chevies..



first step would be to actually test and deem the compressor defunct before replacing it.. make sure the rest of the system is in spec.. even if it means a full recovery, disassembly, flush, and refill with proper weighted charge and oil charge..
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Old 04-28-2021, 03:24 PM   #3
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my saga...


After much of the troubleshooting you describe, I replaced the compressor, orifice tube, stock ac dryer in the engine compartment, and expansion valve to the rear evap.



I do not think that there was any oil in the system. I did not see any anywhere and no dye at that point. After some investigation, I found that the previous compressor was a brand that does not send the compresor charged with oil, and I think whoever replaced it previously though it had oil, so they did not add any.


After replacing the above stuff...



Upon trying to charge the system, it stopped at about 2.5 pounds, when the sticker under the hood called for 6. I did not have a bottle heater and was strapped for time, so i had to wait. I decided to order a replacement evaporator, cahngee it out and tried again.



I found the carrier procedure of adding liquid to the high pressure line while the system was off, followed it, and got the full 6 pounds in. I let teh bottle warm up before starting the engine, so it would likely take gas if I needed to top it off.



In that procedure, there was a chart to adjust the charge if the high pressure side did not reach a certain number, calculated from the ambient temperature, with the engine at 1200 RPM.


I added gas until the high pressure was in the correct range ( ~145 psi at that ambient temp), but this brought the low pressure up to 60 PSI.


Both front and rear units were cooling 70 degree air to about 60 degrees. The air from the condensor felt a little warmed.


BTW, the reason I had not changed the condensor coil, was I could not find the part number available to order.


I did change the sight glass/filter/dryer under the bus after the condensor. The sight glass showed a solid stream of bubbles. ( really foam) As my assistant would try to maintain 1200 rpm, the stream would sometimes be clear liquid, as the rpm fluctuated up and down.



The compressor is brand new and was not mistreated, so I think it is doing all it can, and that it is just not enough.



The only other theory I have is that the condensor might have a few of the parallel branches plugged, so it is all going through one branch, but i really doubt that.



I certainly do not know for sure, but it seems that the compressor just cannot keep up.



I have not tried to run with just the front ac yet, because after doing the testing above, I had to go to work and the schrader valve on the low pressure side decided to leak horribly, so I put my gauges back on it quickly to keep it from leaking out, and while waiting for another chance to work on it, figured that the compressor probably does not have high enough capacity to pump all the gas that both of these evaportors dump to the low side.


That is where it sits right now. Carrier ( now MCC ) cannot tell me what size compressor should feed the evaporator in this vehicle, or even how many BTU's it can handle.


I really am hoping that it just needs a bigger compressor, and someone out there has that answer. I am prepared to try other things. I never really pretend to have anything figured out until it has been working for a month or two.



Proper high pressure and elevated low pressure, it just seems that either the resistance from high side to low side is too small, or the pump is too small.



Doug
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Old 04-28-2021, 09:14 PM   #4
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Finally got some meaningful responses from people who work on these a lot.


Seems the systems that use one compressor for two evaporators are just sub-par. The consensus is that the best thing to to is to get a separate compressor for the rear evaporator, and totally separate the systems.


I am not even sure if there is a kit to mount a second ac compressor on a 2006 Chevy 4.8 liter in ah Express.



Second best would be to disable either the front or rear and let the compressor just run one system. Not sure if I could get something to block the orifice tube to test this out.



Doug
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Old 04-29-2021, 06:34 AM   #5
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Chassis: International 3800
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Rated Cap: 7 Row Handicap
ive driven plenty of these Tie in systems with one comopressor that blow a lot colder air than 65 or 70.. ive got 2 larger evaporators in mine running off one compressor.. you just need to get the right compressor and balance the evaporators properly.. one being an orifice tube is a disadvantage but the txV on the back can be set for the constant flow of the orifice tube.. one of the issues comes in that the front unit cant vary its flow and the use of an accumulator to catch any liquid refrigerant causes the rear evap to be starved. . installing a variable orifice tube in the front system can help with getting more flow to the rear..



if you dont mind running a flammable refrigerant you can get huge gains in cooling efficiency going with envirosafe 134a replacement..



you definitely can disable your rear system and run the dash air only.. thats a common mod done around here esp for people who dont carry passengers in back and want the ceiling space for something else.



I seriously though would see if there is a sanden SD7 enhanced that would fit.. im running a CS-3 condenser and 2 evaporators(a dash unit rated at 20k and a mid-ship unit rated at 40-50k (ive got the TxV opened a little wider right now to flood the distributor).. im still able to pull my suction pressure down low and masintain a solid column of liquid at each evap with a compressor RPM around 3k (engine RPM 1800-1900).. 35+ degree drop across my evaps on high fan is oretty normal for me..
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Old 04-29-2021, 06:43 AM   #6
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Join Date: May 2009
Location: Columbus Ohio
Posts: 14,905
Year: 1991
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Chassis: International 3800
Engine: DTA360 / MT643
Rated Cap: 7 Row Handicap
the biggest issue with any orifice tube system is the wasted refrigerant.. the design is such that you flow a lot of refrigerant at RPMs but can still maintain enouigh flow at idle for the system to cool.. when you over saturate the evaporator the excess liquid goes in the accumulator to evaporate instead of slugging the compressor..



when the evaporator temperature approaches freezing the pressure drops down to the point where a low pressure switch kicks off the compressor (more modern systems will use an evap inlet temp sensor)..



a TxV system is designed for a constant flow from the compressor.. the valve slows down the flow as the ewvaporator temperature drops.. a backup freeze-guard sensor will cycle the compressor is the system load is so low that even the minimum flow at the TxV is still enough to freeze the coil..



this adjusted flow model is perfect for dual evaporator systems as each one runs independent of its counterpart esp if heat load varies.. you can turn the fans off on one and still have the other cool..



in your case you technically should be able to shut off the fan to your rear evap and get ice cold air at your dash system.. the rear TxV will slow down to minimum flow as soon as the coil gets cold which allows more refrigerant flow to the front dash system.



if this occurs then you can know that comporessor capacity is an issue as you essentially reduced the rear load to next to nothing so its not wasting flow.. now if that coil freezes instantly into a block of ice then either the TxV is mis-adjusted or its got a minimum flow thats too high for your design..



id be interested to see what kind of results you get with the rear fan off..
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Old 04-30-2021, 07:58 AM   #7
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Thank you. I will do some more experimenting, once the battery is charged back up.

I will look for a Sanden, but don't remember seeing one that will bolt up to the chevy 4.8.

Thanks again.

Doug
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Old 04-30-2021, 10:03 AM   #8
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Looks like Dirty Dingo makes some brackets to put a Sanden on the Chevy 4.8. I am going to have to do some measuring to see if it would fit in the Express engine compartment.



What refirgerant were you thinking as an alternative to R134a? I remember reading that R134yf is basically rendered non-flammable if it is mixed 90-10 with R134a.


With 6 pounds of refirigerant, I am less likely to go with a flammable refrigerant, but we do drive around with 30 gallons of gasoline...



Battery is not just discharged, but dead, so any testing will have to wait until I get a new one.


Thanks again,
Doug
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Old 05-26-2021, 02:46 AM   #9
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When replacing the compressor of an air conditioning system, there are 4 main parameters that you should know exactly to select the best match one for your vehicle AC:
1. Nr. Of Cyl.
2. Swept Volume(cm)
3. Displ. Volume
4. refrigerant
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