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.