JD 3720 Air Conditioning line pressures?

[JDneophyte]

I seem to be having a compressor that stops after about 5-7 seconds.......and figured I would start with line pressures (as these things kick off if there is a pressure irregularity). I just bought and received the technical manual and parts catalog.....and no pressures stated (just "do not exceed range of low pressure gauge" when using JT02099 adapter in suction port....low pressure line).

It seems the manuals were written for using only JD testing/repair equipment vs. the do it yourself crowd.

Any thoughts would be much appreciated as I've read through the forums and find little on this specific item. If I can check and verify the pressures, next steps will be a little more involved.

The technical manual covers alot on compressor failure, how to evacuate and recharge system (oil, r134a, etc).....but I thought line pressures would be a good start.

Thank you

 

[Zebrafive]

Depends on ambient temp. For 65* 25-35psi, 75* 35-45psi, 85* 45-55psi, 95* 50-55. This is for the low side. There may be a low pressure switch on the low side. If the system pressure is too low (from loss of freon) it prevents the compressor from running to protect it. Usually a can of 134a will have enough pressure to start filling a low system then the compresser will be able to run and pull in the rest.

 

[JDneophyte]

I initially put a can of r134a on the low side (can had gauge you set to outside temp......so 45psi was what we were looking for).....and the gauge pegged to 80+psi (which I immediately thought I had hooked up to the high side, but the charge valves are different sizes so not to make that mistake). Then my problem might be with the dryer (plugged).

Thank you for the quick response.

 

[Labrat]

Plugged dryer would not cause high suction press. severly over charged, Broken compressor suction valves, condenser plugged or fan not working, non-consibles in the system will.

 

[shortthrow50]

I initially put a can of r134a on the low side (can had gauge you set to outside temp......so 45psi was what we were looking for).....and the gauge pegged to 80+psi (which I immediately thought I had hooked up to the high side, but the charge valves are different sizes so not to make that mistake). Then my problem might be with the dryer (plugged).

Thank you for the quick response.

that will happen if you check the pressure without the unit running. Did you check the pressure with the a/c running on full blast? It needs to be running in order to check pressure or you will get readings like that right after shutdown

 

[pathatten]

The high and low pressure ports are different sizes on all 134a systems. Generally a quick cycling clutch indicates low refidgerant. It seems a lot of manufactures might not be putting a full charge in the systems.

Pat

 

[sunnyside360]

Make sure the condenser in front of the radiator is free of debris. I had similar problem on my 3720 and I thought the condenser looked ok but later was surprised when stuff blew out the front when compressed air was applied from the back side. Makes a huge difference in A/C operation.

 

[jonyyuma]

that will happen if you check the pressure without the unit running. Did you check the pressure with the a/c running on full blast? It needs to be running in order to check pressure or you will get readings like that right after shutdown

While the unit is running full tilt...Does it help, by using a strong fan blowing thru the condenser?? I used a floor fan on my other car when charging it with the old style #12 refrigerant??I am planning to add some 134a to my newer car this week-end..Only puts out air down to 44 degrees...So I am trying to boost my memory on the subject?? Too..

 

[jenkinsph]

While the unit is running full tilt...Does it help, by using a strong fan blowing thru the condenser?? I used a floor fan on my other car when charging it with the old style #12 refrigerant??I am planning to add some 134a to my newer car this week-end..Only puts out air down to 44 degrees...So I am trying to boost my memory on the subject?? Too..

i wouldn't use an extra fan to help cool down the condenser since you won't have it under normal operation. About the new car with 134a, 44 degres is about optimum. You would want the evaporater coil to see a low temperature of 40F. Obviously the air temperature will be a few degrees above that. I think you are good without adding freon. The older r12 works better than the newer 134a so don't expect it to perform as well.

 

[hizoot]

The refrigerant/evaporator temperature is right at 10 degrees colder than the leaving air temperature (output temp). So at 44 degree air temp,the coil temp will be 34 degrees...you dont want that coil temp any colder or it will tend to freeze/ice up and eventually block air flow.

A good way to get close on refrigerant charge on most refrigerants is this: You'll need to hook up both low and high side gauge ,then with the engine running about 1000-1100 rpm,monitor the high side gauge as your adding refrigerant to the low side. There is a temperature scale running parallel to the pressure scale on the gauge. There should be different scales for different refrigerants on the gauge. Find the scale corresponding to the refrigerant your using . Then add refrigerant thru the low side slowly by cracking open the hand valve until the needle on this hi-side gauge corresponds to 30 degrees above the outdoor temperature. So if its 90 degrees outside, then you'll want the needle to be pointing at 120 degrees . But, you need to make absolute sure that the condensor coil is clean as any dirt/blockage will affect it a lot. The most accurate way is to take line temperature readings at the same time....but that gets a lot more involved and will require tools/meters that most people wont have or understand the goals needed . This method I described will get you really close. It will vary on home units as the higher efficiency coils will throw this rule of thumb off some.

edit to add...when working with the high side of a system, its best to use hoses with back flow preventers or fittings on the end of the hose that have check /ball/valves or blow back preventers on them. This way when disconnecting the hose, the high pressure refrigerant wont rush back out of the hose when it separates and frostbites your fingers.

 

[Dr Dave]

Most R 134a systems use a low pressure cut off switch and a high pressure cutoff switch. If you are low on freon, they will tend to cycle on and off. So if it is shutting down, may be over charged, or condensor/ radiator dirty. Blow out and wash out, condensor and radiator cores. Also check evaporator core and cab filter, blow out if dirty. You can check an auto repair manual, for guage readings. R134a works best at exact charge, too much or too little makes a big difference. If you are low it is best to reclaim, evacuate air and moisture from system and add the exact charge. Look for a decal on or near a/c with charge amount, or check your manual.HF has cheap air powered vacuum pumps, evacuate for atleast 15 min.

 

[jenkinsph]

hizoot and Dr Dave both make good points imo. One of the common problems I see is overcharged systems because lots of junior technicians seem to misdiagnose and add freon. I agree with using guages to watch the low and high sides and use clamp on Fluke thermometers but not everyone has the tools and know how to accurately diagnose their ac system. So unless you have the tools and skills needed and have 44F air at the ducts I would leave it alone.

 

[JDneophyte]

I bought a set of gauges, hooked them up, ran the tractor up to 1500 RPM. The low side held at 50 PSI......and the high side climbed, climbed......and then the compressor cut off.

Plugged system? I bought a new drier and expansion valve. Hoping its not the compressor (tractor is a 2007).

Am I headed down the right path?

Thank you

 

[ljohnson778]

I bought a set of gauges, hooked them up, ran the tractor up to 1500 RPM. The low side held at 50 PSI......and the high side climbed, climbed......and then the compressor cut off.

Plugged system? I bought a new drier and expansion valve. Hoping its not the compressor (tractor is a 2007).

Am I headed down the right path?

Thank you

Generally, if the compressor on a 134a system is cycling on and off rapidly, I've almost always found it to be an overcharge condition... Usually, undercharge will prevent the compressor from cycling on at all, though not always. Bleed off some pressure(environmentalists scream here) you might be surprised.

 

[chasenmojo]

you low side pressure is good, but as far as the head pressure rising could be a good number of things. be sure your condenser is completely free of dirt and debris,if its plugged you can give it an acid wash to clean it out. sounds to me like its all in your condenser, whether it be a bad fan or dirty condenser. the good thing is there is nothing wrong with your compressor if that were the case your pressures would be equlized. good luck bud! P.S. dont dump any refrigerant thats not a good idea unless you know you overcharged it but it doesnt sound like that to me with 50lbs suction. If idid that it would cost me 10k and jail time and lose my career.

 

[JDneophyte]

I thought about that, though the low side is holding right at 50psi....about where it needs to be. The high side is where the pressure is ever increasing until the sensor cuts out the compressor ( takes about 15 seconds until cut out from turning AC on)...that leads me to believe there is a plug in the system somewhere. The gauges read 50 PSI with the tractor shut off (about where the A/C charge should be).

Any thoughts?

 

[shortthrow50]

what is the temperature coming out of the vents? did you take a temp reading yet. Unless i missed it somewhere in this thread. Just saying becuase obviously we dont have windows that open and close wondering if this cycling is becuase of the temp reading and you are just now noticing the cycling. Maybe a normal condition. I have blacked out windows so even last week in 100 degree temperatures, my machine was on fan setting number 1 and freezing cold. Doesnt take much with these machines with small cabs and cooling capacity to get to temperature and start cycling a/c compressor

 

[chasenmojo]

with your system running what does your head pressure climb to before tripping out? when the unit is not running your pressures should eqallize and be the same on both sides of the system. i dont believe its a plug in the system, but not able to get rid of the heat in the condenser. not sure how your system is set up but do you have an independant condenser fan

 

[Tractor Seabee]

AC is all about thermodynamics and pressure/temperature relationships. If you do not understand the principles involved you are just stabbing in the dark and changing parts hoping for a solution; gets expensive. Refrigeration/AC is a complex trade that does not fit into the DIY realm. Yoy have already spent as much as a professional diagnosis would have cost and not done yet. In my younger days refrigeration was my trade. Been away from the nuts and bolsts for 30 years and it has not gotten any simpler. I would even be leery of a tractor mechanic as a diagnostition unless he has extensive training/experience on AC. We always hated following the part changer mechanics as it was hard to collect any more money from the customer when he had already broke the bank and still no fix. Bite the bullet and take it to an expert.

Ron

 

[JDneophyte]

If this small fix doesn't do it.......I'll take it in to an AC shop. Any recommendations in the Milwaukee area/Elkhorn Area?

Thank you for the excellent input