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Thread: Question about the compressor cycle time

  1. #1
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    Question about the compressor cycle time

    Hi Guys,
    What should be a normal cycle time for the A/C compressor? my car is blowing cold air, but the compressor re-cycle every 5-8 sec.
    Due to that quick cycles, my fan relay is getting really hot.
    I know that normally it could mean that I'm low on refrigerant, but I was wondering if there is any way to Troubleshoot before adding more R12.

  2. #2
    Senior Member Bitsyncmaster's Avatar
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    With a gauge set on the system you can usually tell if your low on charge by seeing lower than normal high pressure side. The low pressure side is controlled by the low pressure switch so it always runs about 20 to 40 PSI.

    The high pressure is dependent on ambient air temp, and the amount of air the cooling fans are drawing.

    Since your still running R12, my guess is you are low on the charge. If it had been converted often the problem is contamination of the system which plugs up the orifice and condenser.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #3
    Delorean Guru
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    If the low pressure switch has not been adjusted, you probably need a small amount (less than 16 oz) of R-12. Of course that also depends on the ambient temperature that you were testing at. If it was very hot you are definitely low.
    David Teitelbaum

  4. #4
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    My A/C is still on R-12 and my low pressure switch was never adjusted.
    I will check the low and high reading tomorrow.

    BTW, can anyone direct me to the spec showing what should be the correct pressure?


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    Last edited by mburshtain; 04-02-2017 at 11:17 PM.

  5. #5
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    Quote Originally Posted by mburshtain View Post
    My A/C is still on R-12 and my low pressure switch was never adjusted.
    I will check the low and high reading tomorrow.

    BTW, can anyone direct me to the spec showing what should be the correct pressure?


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    That's the one, right?

    AC.JPG

  6. #6
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    As a rule of thumb psi of 35 on the low side should be accurate enough. If it's cycling it's the low pressure switch protecting the compressor. Why not convert to R134 while you're at it? It really is hard to over fill but at 35 psi it's at it's top end.

  7. #7
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by mburshtain View Post
    That's the one, right?

    AC.JPG
    Yes that should give you a reference of what to expect. What makes if cycle faster is the low side pulls down faster because there is not enough liquid Freon feeding the orifice.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  8. #8
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    measuring while A/C is running

    It might be a trivial question, but I had to ask.
    I should measure the pressure, on both high and low side while the A/C is running, right? the important thing would be to capture the PSI on before the compressor clutch is engaged and once it is disengaged. Correct?

  9. #9
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by mburshtain View Post
    It might be a trivial question, but I had to ask.
    I should measure the pressure, on both high and low side while the A/C is running, right? the important thing would be to capture the PSI on before the compressor clutch is engaged and once it is disengaged. Correct?
    Static pressure only shows you if your system has some charge. You can find the static pressure via a temperature table of R12. As long as you have any liquid Freon, that pressure is accurate.

    You want to look at both sides. If you see the low pressure going to low that would indicate a bad low pressure switch (I found one on a friends car). The high side is where you can get more data to determine what is wrong.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  10. #10
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    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.
    Last edited by mburshtain; 04-03-2017 at 02:48 PM.

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