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I'm almost ashamed to ask for a/c help but...

5K views 20 replies 6 participants last post by  Jim L 
#1 · (Edited)
Ok so some back story first I have repaired dozens of car air conditioners with fantastic results and I find myself working on a/c in class 8 trucks almost weekly at work, so I am no A/C novice. The problem is this stupid pickup of mine has me stumped! So in the nearly 7 years since I purchased the pickup and redid the a/c I have had a couple of bad expansion valves that caused poor performance. Ok that was no issue to fix, a new X valve and dryer got me back in business. The last time I had to swap the X valve I sprung for a genuine Mopar/Denso valve PN#55036079AF thinking cheap parts store junk was my issue with Xvalves going bad and all was working well (as in 38* vent temps). Well not now! Lately my A/C will freeze you to death idling or driving through town but on the highway the compressor cycles alot. So back to the gauges for diagnostics I went. At idle in 95*-100* ambient temps with 134A and the sight glass clear I was seeing 225-250psi high side and 22psi on the low side. So I did what any rational Mechanic would do and wasted a bunch of refrigerant trying to adjust the charge level thinking I had a leak. Well in my futile effort I found I could add freon to it to the point of the high side pushing 400psi and the low side would stay at 22psi and with the slightest rev of the engine the low side would dip into the low teens and cycle the compressor off. So in my baffled state once again I did what any competent mechanic would do and threw another brand new genuine Mopar Xvalve at it. Yea no change... So I am baffled, I cant find any apparent blockage in the system, It's obvious the compressor will make pressure, At idle or in town the A/C blows super cold, fan clutch is good,and the evaporator is squeaky clean. No matter what I do the low side sets at 22psi at idle and quickly drops with added RPM. So short of tearing the whole thing apart, flushing and installing a new compressor, condenser, dryer, Xvalve and just starting over I dont know. So what say? Anybody want to ponder this mystery with me? Or better yet give me a simple cheap fix that will make me feel stupid for not already knowing...
 
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#2 ·
I don't have an answer for you, or pressure readings when past idle, but mine blows a heck of a lot colder at idle and through town than the highway also. I just did a complete overhaul on mine a week or so ago with no leaks or blockages. I am interested in this too. But I do know that my pressures where at 23psi and around 220psi also at idle also. Sorry for no help, but wanted to stay informed with the thread. The only thing that I could think of is that maybe my HVAC tube at the bottom of the dash is plugged up with leaves....have yet to cut mine open.
 
#4 ·
As many times as you have had your system open replacing parts I wouldn't be surprised if it wasn't' full of non-condensibles. (air, moisture). If no leaks on system, have you tried a triple evacuation? ( dropping the charge and evacuating the system to 500 microns, purge system with dry nitrogen and evacuate again to 500 microns, perform procedure three time.)
Moisture ( non-condensibles in a system) doesn't start to boil off until below 27" vacuum. If you can pull a system down to 500 microns and it holds it there for an extended period of time you have no leaks and the moisture has been removed.( At least 1 hour.) After triple evacuated then weigh in a factory charge.
Also an oil soaked system will also produce your symptoms if all other components are functioning properly. Oil will gather at the bottom of an evaporator and/or condenser or both and reduce the systems capacity therefore a proper oil charge is crucial.
Make sure condenser and evaporator are clean and blower wheel vanes are clean. These systems are critically charged to a point so don't over charge it or you risk slugging the compressor with liquid refrigerant. You do know that 134A is an blend if Freon's so charging with virgin Freon as a liquid is also required to maintain a proper blend. Weigh in the factory charge. Also any restrictions in a system are indicated by a frosting point.
Keep us posted of your findings.
I once way back when have repaired and maintained Sanden International's large chiller systems for many years in my younger days at their Wiley Texas plant. I'm no small refrigerant application person but do Industrial/Commercial Refrigeration/Air Conditioning.
Upon buying my new 2015 Ram 3500 DRW my air wasn't performing properly and a quick check determined my system to be undercharged from the factory, easily remedied but my system was new and untampered with so easily diagnosed of a automation charging system fault from the Mexico factory.
 
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#5 ·
Holy smokes! Great information. With that being said I'm going to check the HVAC tube and if it's not that I'm going to run it how it is. I'm not sweating...you guys are on a whole different level than me haha
 
#6 ·
I don't really have access to the stuff to purge with nitrogen. We have it at work but that doesn't help me. I have a solid robin air vacuum pump that will peg my compound gauge while vacuuming. In my ing with this I have vacuumed for as long as 2 hours. Always always charge with liquid freon. Yea the temp drop is how I checked for blockages. I'll keep updated on this. I will beat it or burn it one of the two... Lol


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#7 ·
I also have worked on commercial systems for 40+ years and a lot of auto a/c's , sounds like the refrigerant side is probably doing what it should , however cycling on low pressure switch with plenty of load does sound unusual , and would drive me nuts also, could it be an airflow problem ?, possibly too much air recirculating thru evaporator , is it possible to accurately check supply temp v/s return across evaporator ? just some food for thought ,
 
#8 ·
Yes sorry you will need an electronic micron gauge to monitor the vacuum of 500 microns or less, I evacuate to 250 microns on the large system I work on and it takes days to achieve it not hours. If you know for sure you don't have any leaks just change the vacuum pump oil so its fresh and evacuate that small system for like 4 to 6 hours and that should boil out any non-condensibles in the system. If you do have a leak it will just suck more non-condensibles into the system. Non-condensibles stack up in the top of the condenser causing high head pressure just as a dirty condenser coil will do the same. You can also evacuate the system and if no dry nitrogen is available to purge the system with then you can purge the system with refrigerant and evacuate again to 500 microns. Bleed all hoses before purging so you don't allow any more non condensibles into the system just as you would bleed them before charging a system.
I think there is a strainer in the system close to the expansion valve or at least manufacturers used to use one and the strainers can get clogged up with debris making you think the expansion valve it bad and sticking. Cleaning the strainer is highly recommended if doing any repairs to the system like a compressor replacement and may require more than one cleaning to get all the debris removed from the refrigerant system. Clogged strainer indicated by lower than normal suction pressure for the running conditions of the surrounding environment and can also fool a person into believing the system is undercharged (low on Freon) or thinking the expansion valve is faulty. An expansion valve is a controlled restriction (metering device) used to maintain proper charging of the evaporator coil with the proper amount of superheat returning to the compressor and subcooling leaving the condenser feeding the expansion valve then to the evaporator. Metering devices are very reliable and don't go bad often in a refrigeration circuit 98% of the time you will find the fault in the system to be elsewhere like a strainer or line restriction. Line restrictions are also not stopped up but possibly a kink in a line or a bad weld that stopped up the line. In a automotive systems your not likely to have a fault line restriction caused by a weld.
Good luck with it and keep us posted.
 
#9 · (Edited)
I'm looking at a large Chilled water system now hang on I'll post you a pick of it. Enough cooling to cool 4000 large houses on a 100 degree day and still not strain the system. LOL Shot of my Ram to go with it couldn't resist.

Factory Building Machine Industry Interior design


Land vehicle Vehicle Car Motor vehicle Tire


Transport Building Factory Warehouse Machine
 
#13 ·
Yes Sir they surely are. Hard to stop a Trane. LOL
 
#16 ·
No problem on the derail. The closest thing to a strainer in my system is the filter dryer. after some more thought I am convinced that I must have a blockage and it has to be in the evaporator, I turned my pressure switch down to allow the system to freeze and only about half of the evaporator is freezing up. Tearing it down now to check and will report.
 
#18 · (Edited)
Got it all fixed now. Turned out to be a blockage in the evaporator. It took two quarts of solvent and 4 tries at back flushing to get it flowing good. I cut open the dryer to inspect it and all the desiccant and filter mesh looked good, so I don't have any idea where the mystery substance that created the block came from. Anyway I guess we will see if the problem comes back... Thanks to everybody for helping get the squirrels running in the right direction.


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#19 ·
I had a hose on my system delaminate and fill the condenser with bits of the hose.

I replaced the old condenser with a new parallel flow style, replaced the evaporator after finding I had a pinhole leak in it, adapted a newer Sanden SD-709 compressor, then I made all new lines, welding on flare fittings and using aeroquip hose, I now have a system that is custom and completly repairable.
Should last a lifetime.

Used to work on transit coaches that had a 5 cylinder Trane compressor driven by a Kiki or Perkins.

Jim
 
#20 ·
Yea I read about your sanden conversion. I have always wondered why you don't like the C171? As best I remember it has a larger displacement than the sanden does.


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#21 ·
True, C-171's were very good compressors if they were new but I was never able to a find a decent remanufactured C-171 compressor they were all junk from every place I had tried including NAPA (A1-Cardone) I got them seized right of the shrink wrapped box, they were also about twice the price of the SD-709

I can get a new SD-709 if I want one locally and if I need more cubic inches per revs I can get one that will fit the same footprint.
 
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