Question about 2002 Mercury Grand Marquis

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Seems the AC is freezing up after about 30 or 40 minutes. After it unfreezes, it will work fine for another 30 or 40 minutes. I believe that the freon levels are correct.

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  • Travis Mcilree
    Travis Mcilree May 11, 2010

    Two things that can cause this are low refrigerant charge and insufficient air flow over the evaporator.


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  • Mercury Master
  • 4,369 Answers

If you have ice accumulating on the evaporator, there should be no air blowing out the registers because the air will be stopped at the evaporator.
If this is happening, the a/c is not cycling on and off as often as it should. At 70 degrees F, it should cycle on and off about 4 times/minute. Less when hotter.

If not cycling off at all, replace the screw-in pressure switch on top of the accumulator under the hood near the fire wall on passenger side.

report results of test.

The A/C cycling switch is mounted on a Schrader valve-type fitting on the suction accumulator/driver.

  • A valve depressor, located inside the threaded end of the A/C cycling switch, presses in on the Schrader valve stem.
  • This allows the suction or low side pressure to control the operation of the A/C cycling switch.
  • The electrical switch contacts open when the suction pressure drops. The contacts close when the suction pressure rises. For specifications regarding operating pressure(s), refer to Section 412-00 .
  • When the A/C cycling switch contacts close, the A/C clutch field coil is energized.
  • When the A/C cycling switch contacts open, the A/C clutch field coil is de-energized and compressor operation stops.
  • The A/C cycling switch will control the A/C evaporator core pressure at a point where the plate/fin surface temperature will be maintained slightly above freezing.
  • This prevents icing of the A/C evaporator core and blockage of airflow.
  • It is not necessary to discharge the refrigerant system to remove the A/C cycling switch.

Posted on Jul 30, 2009

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