Automotive Air Conditioning Information Forum

I know this is an Auto AC forum but.... I have been working on an MT745 CAT Challenger and am at my wits end with it... a little background.. I have replaced compressor, accumulator, orifice tube - flushed out complete system - new thermostatic switch and low pressure cycling switch... ended up changing condenser and evaporator core... Evacuated and charged up to specs with Virgin R134a... what i am running into is this: at the shop i can run it @ hi idle and get awesome cooling 40 - 45 F - thermostat cycles and all is great... however... take it out for a drive and 2 miles down the road it starts cycling fast - the low side is hitting 5 psi - i believe the switch cuts out at 6 psi... my high side is 150-200 roughly - i have tried adjusting refrigerant levels anywhere from 3.5 lbs all the way up to 5 lbs... the label on the machine says 3.5 lbs - Agco told me 4.25 lbs.. Ap Air Says 4.5 lbs... I have even tried putting in a bigger orifice tube... I can breathe through all my lines so i don't believe there are any restrictions.... please give me some pointers... What to look for...

Ok, added tractor to your title - otherwise the Dodge Challenger takes over any search.

If it is a CCOT system, and it is going to 6 psi the switch is either misadjusted or bad.

Pull the connector off of the cycling switch, and look between the prongs on the switch. If there is a screw it is adjustable for pressure . It should open around 26psi & close around 34.

Don't think AGCO made any changes in CAT design spec's.
Maybe thermostat is not operating and clutch cycles on low ambient 6 psi switch, causing evaporator icing.
System is unique in that accumulator is a counter measure against liquid slugging on a thermostat clutch cycling system.
Saw a yellow AGCO Challenger plowing a dry field in Henrietta TX ( where Quanah parker married 2 of his wives) yesterday on trip from Dillon.
Low suction or high discharge pressure is not caused by the Sanden compressor.
hotrodac

I've never worked on a CAT Challenger. Nevertheless, mobile A/C theory is essentially the same regardless of vehicle, so I'll jump in with some questions.

HeavyS wrote:I have replaced compressor, accumulator, orifice tube - flushed out complete system - new thermostatic switch and low pressure cycling switch...

HeavyS wrote:...ended up changing condenser and evaporator core...

At what point did you flush the system, what components were flushed, and what kind of flush did you use? I assume that you flushed the old components and lines with some sort of solvent before changing everything out with new. I don't think you mentioned the lines, though, so I assume that you reinstalled the originals. Did any of the original lines have mufflers in them? If so, it's possible that some of the flushing solvent remained in the muffler(s) and subsequently contaminated the new refrigerant and oil, creating havoc with proper system operation.

HeavyS wrote:Evacuated and charged up to specs with Virgin R134a... what i am running into is this: at the shop i can run it @ hi idle and get awesome cooling 40 - 45 F - thermostat cycles and all is great... however... take it out for a drive and 2 miles down the road it starts cycling fast - the low side is hitting 5 psi - i believe the switch cuts out at 6 psi...

Sounds like the low-pressure cutout (LPC) is doing its job. But the thermostatic switch (TS) could still be suspect, even though it's new. If the TS isn't shutting off the compressor before the evaporator's temp gets into freeze-up territory, the low pressure will continue dropping until the LPC shuts off the compressor or cycles it rapidly.

HeavyS wrote:my high side is 150-200 roughly

Your high-side pressures point to an ambient temperature of around 70 to 80 degrees. Is that correct?

HeavyS wrote:i have tried adjusting refrigerant levels anywhere from 3.5 lbs all the way up to 5 lbs... the label on the machine says 3.5 lbs - Agco told me 4.25 lbs.. Ap Air Says 4.5 lbs

In this case, I recommend charging only according to what the label says assuming this is not an R-12 to R-134a conversion.

HeavyS wrote:I have even tried putting in a bigger orifice tube...

In theory, that should raise the low pressure some, but maybe not if there are other issues going on at the same time.

HeavyS wrote:I can breathe through all my lines so i don't believe there are any restrictions

Have you double-checked to ensure that there isn't some kind of debris partially blocking the high-pressure line up to the orifice tube?

Originally the compressor was faulty(why i can't remember) but we changed out compressor and accumulator and orifice tube and flushed complete system(lines and cores with zep ac flush) - i recall the orifice was plugged - off and on over the last few years we have done some servicing on it... lines, accumulator and orifice tube - It always worked at the shop here but customer would take it out and it would work for a while... we ended up replacing the condenser - the old one had close to 60 degrees temp drop - definitely improved our cooling but again on the field it would then would cycle fast and not cool properly - I suspected a restriction in the low side somewhere... so ended up replacing the evaporator core and orifice... (thankfully this customer gave me confidence and said replace everything if that't what it takes) - and yes double checked lines for mufflers or restrictions - we even recovered refrigerant and put in new virgin R134A just to eliminate that... So here is what I ended up doing... I put in the biggest orifice tube i could find(ended up being a delco black/white) and just kept adding refrigerant regardless of what the specs say.... at about 6 lbs of R134a - the thermostatic switch started cycling our compressor - the low side still would go down to about 10-15 at high rpms... but now it is cooling...would always like to see it in the lower 40s but we were around 48 give or take... ambient temp was 90-95 and hi side was between 200-225.... I am up in Manitoba, Canada so our daily temps can vary quite a bit as does the humidity.... Am I going to damage the compressor like this? the reason I did this is I found a post online about a situation similar to what this is and AGCO recommend a larger orifice and more refrigerant...

Your compressor will be fine at those modest high side pressures. As a rule of thumb, when the high side P/T ratio goes much past 3:1 things are not well.

Here in the desert when the ambient goes to 125f, you can see 375 psi on the high side. Sounds like you have enough condenser & airflow to keep the high side at moderate pressures for 95f. As long as ice formation is not an issue with the evaporator, then things are working.

Ag applications can cause liquid slugging and oil wash out of the compressor.
Additional R134a without appropriate oil charge increase would magnify "Dry Lock" of compressor.
Cool mornings (and cold compressor) with sun on the cab and warn evaporator will cause liquid migration to suction.
Some OEMs engage the compressor during starting to gently rotate the compresor rather than slugging the compresor at 2500 rpm.

I would charge 2 to 4 oz of 5GS mineral oil to lub a compressor that has been exposed to several cool engine bay/warm cab cycles and pulse the compressor clutch during initial engagement.hotrodac

Hog Killer - Good point about additional refrigerant & needing more lube. But why 525 Mineral in a 134a system? What am I missing?

Just a theory: after daily crankwashout of PAG from thermal migration of R-134a w/o daily operation, the bearing surfaces are squeeky clean.
Mineral oil is not solubable with R-134a so some should remain on bearings to survive 2500 crpm startup. One of the lube enhancers may work better than 5GS as a countermeasure for "DRY LOCK".

Sounds lie a great place for Militec-1 . That is the only snake oil I have tested that actually does what it says. I have been running it in systems for years without any downside.

Remember that I refrigerant flush with straight R290 so I know how well refrigerant removes lube. This stuff adores aluminum oxide, & hangs on hard. Any aluminum bore or piston will acquire a nearly permanent coating. We used it in the screw compressors & switched to ISO32 oil from ISO100.

Are you still involved in testing compressors? I wonder how a quality PAO would work in that role?