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Thread: Question about the compressor cycle time

  1. #11
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by mburshtain View Post
    Here is what I see in my static reading at 55 degrees. Both the high and low side of the system have equalized.
    Could it be that on my last service they overcharged my AC?



    According to this table, I should expect to see around 52 PSI at 55 degrees.
    The temperature would have to be what the system is at. If your air temp has been constant for a few hours and the engine is cold then the air temp should be close to what the system temp is. You would see the same pressure even with a system under charged or overcharged. (a few oz to 5 lbs of R12).

    Note the middle red scale (R12 scale) that shows your system must be at 60 deg. F. The gauge accuracy is not perfect so your pretty close. Yes both sides should read the same after the system has bee off a while.
    Last edited by Bitsyncmaster; 04-03-2017 at 02:08 PM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  2. #12
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    Thanks for all your help so far.
    Here is a short video showing my readings when the AC is on, fan is set to 1.

    http://www.youtube.com/watch?v=aQ6_JpOao84


    PSI on the low is between 23 to 44 and on the high between 70 to 123

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    Last edited by mburshtain; 04-03-2017 at 03:07 PM.

  3. #13
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by mburshtain View Post
    Thanks for all your help so far.
    Here is a short video showing my readings when the AC is on, fan is set to 1.

    http://www.youtube.com/watch?v=aQ6_JpOao84


    PSI on the low is between 23 to 44 and on the high between 70 to 123

    Sent from my iPhone using Tapatalk
    Normally you test with the fan on MAX speed, doors open and set to NORM AC. Also good to also see with your engine RPM higher (quick test to read high pressure). But from what I see, your low on R12.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  4. #14
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    Quote Originally Posted by Bitsyncmaster View Post
    Normally you test with the fan on MAX speed, doors open and set to NORM AC. Also good to also see with your engine RPM higher (quick test to read high pressure). But from what I see, your low on R12.
    Thank you Dave.
    My rpm is between 900 to 1100 while the AC is on.
    I run the same test again as you suggested with both doors open and AC is set to max.
    Now I'm getting:
    Low - 23 to 45
    High - 90 to 140

    EDIT - after letting it run a while (10 min), it is stabled at 26 PSI on the low, and 145 and the high. No quick cycles while the doors are open.


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    Last edited by mburshtain; 04-03-2017 at 03:41 PM.

  5. #15
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by mburshtain View Post
    Thank you Dave.
    My rpm is between 900 to 1100 while the AC is on.
    I run the same test again as you suggested with both doors open and AC is set to max.
    Now I'm getting:
    Low - 23 to 45
    High - 90 to 140


    Sent from my iPhone using Tapatalk
    I don't do much testing when the air temp is cold. You should use your chart to compare what pressures your getting at the ambient air temp. R12 runs lower high side pressures. I run R134a so I normally top off AC on a hot day by reading the center vent (fan on speed 4) to get maximum cooling but watching the high side pressure to not exceed the charts.

    The best way to charge is evacuate and use the correct weight of the charge. But you can't do that just topping off.

    You probably don't have a 30 lb. tank of R12 to just try a little more charge to see if your cycle get longer.

    By the way, if you run my Fan Relay, It will keep the fans running even with a short AC cycle. My fan relay extends the fan run times by 20 seconds. I did that to improve the AC and not cause the idle and electrical noise from the fans turning on and off. Each OEM fan has a starting current of 48 amps and quickly drops to normal.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  6. #16
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    I think you're right and I will need to flush it all and start clean. Unfortunately, I only have 12 oz cans so I will need 3 cans + 1/3 to reach 2.5 pounds of gas.
    Sounds about right?
    What type of available oil should I add after flushing the system?
    Do you think it worth adding 12 oz to check if the cycles are decreasing?


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  7. #17
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by mburshtain View Post
    I think you're right and I will need to flush it all and start clean. Unfortunately, I only have 12 oz cans so I will need 3 cans + 1/3 to reach 2.5 pounds of gas.
    Sounds about right?
    What type of available oil should I add after flushing the system?
    Do you think it worth adding 12 oz to check if the cycles are decreasing?


    Sent from my iPhone using Tapatalk
    If it was me, I would add the can but watch your high pressure. With R134 on a hot day the high pressure runs about 250 PSI or more. I think R12 runs about 200 PSI on a hot day.

    Adding the can now will tell you if you have a leak by it returning to the quick cycle again later. Better to loose that one can of R12 than a full charge.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  8. #18
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    Smart advice, I will give it a try. I will also add some AC dye


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  9. #19
    Admins Never Retire Ron's Avatar
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    Quote Originally Posted by mburshtain View Post
    Unfortunately, I only have 12 oz cans so I will need 3 cans + 1/3 to reach 2.5 pounds of gas.
    Sounds about right?
    No -- 3 and 1/2 cans is 3.5 x 12 = 42 oz, or 2.625 lbs.

    Anywho, Service Bulletin ST-29-11.81 changed the R12 charge to 2.2 lbs:
    2.2 lbs = 35.2 oz, or 2.93 12 oz cans.

  10. #20
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    Thank you Ron


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