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Can't diagnose 2 1/2 ton residential AC

I have been working on a Ruud 10 seer  residential split system.  The customer said that the AC would run but wasn't cooling.  Right away I noticed that the condenser was dirty.  After cleaning it I hooked up my gauges and started it up.  When it was stabilized I had a 150# head and 8# suction.  I checked the filter - clean.  Pulled the evap coil - clean.  So assumed the charge was low.  Using the liquid line charging chart attached to the cover, I brought the head pressure up to 180#.  The suction stayed at 8#.  Then I hear the internal relief start to bleed.  Shortly after that the thermal overload trips.   My high side gauge was only at 180#.  The liquid line was ambient temp, like nothing was going through it.  I let it cool down and restarted it so I could check amp draw on the compressor.  The compressor was drawing  only 8 amps - a little over half what the rating plate said.  A coworker suggested that maybe there was a restriction in the condenser coil.  But I figured if that was the case the compressor amps would be high.

Any suggestions?


  • Wayne_16
    Wayne_16 Member Posts: 130
    You are not undercharged!!

    First off, what kind of training have you had? Not to be critical but just having gauges does not make you a a/c service guy. 

    You have described the classic case of a severe restriction.  The problem is where?  Is this a txv system or a fixed orifice or cap tube coil?  When the internal pressure bypass valve opened creating the high pitched squeal and then the overload opened, confirms you created a overcharge situation. 

    Other questions; has this system had other problems?, compressor replaced,? poor maintenance?.  Excessive moisture in the system?  How long has the system operated in the restricted state? 

    I am going to go out on a limb and suspect this is your first time at the job and the system has operated OK or fair in the past.  You will have to recover all the refrigerant, check the filter dryer, txv, metering orifice, kinked or bent liquid or suction line.  Eliminate the easy ones and then go after the hard ones.  That's all there is, most likely the compressor is OK if the unit has not run for a long time in a vacuum or near vacuum state.  Many of the RUUD units had low pressure switches to protect from loss of charge damage.

    Once you have corrected the problem, replace the filter dryer, evacuate the system to 500 microns and weigh in the new charge of refrigerant.  Recheck subcooling or superheat depending on manufacturers specs.

    Good Luck,

    Minnesota Wayne
  • Techman
    Techman Member Posts: 2,144

    .You can put on [temporarily]a piercing valve to check the compressor discharge pressure to see if the condensor coil is internally restricted.If it is blocked,then blocked with what?Possibly a blown suction/discharge reed if it is a piston compressor.Is there a liquid line filter/drier in this system?
  • Empire_2
    Empire_2 Member Posts: 2,343
    Good Morning Terry:

    I think Wayne is on the right track.  Recover the charge and check that fixed restrictor, and or TXV.  Some come with a screen which may be clogged.  Your system is basically going into pump down..

    Mike T.
  • Techman
    Techman Member Posts: 2,144
    edited June 2010
    Hi Empire

    With the low low side press, the low hi side press , with adding freon,with the low amps,with the internal relief lifting,withthe thermal overload opening, I dont see a restricted orifice/metering device, or crimped liquid line.Some of those units came with a factory installed filter drier inside of the cond unit,before the king valve. Help us out here dsu1
  • Techman
    Techman Member Posts: 2,144
    Revision 1-A

    OK I'll give you PART of the problem is a restricted metering device ,but ,there is something in that cond coil and something wrong with the compressor because of the low amp draw .My humble revision .
  • Wayne_16
    Wayne_16 Member Posts: 130
    Sure it is plugged,

    The condenser is full, no more room for refrigerant, the compressor can only pump what it has available on the suction side. hence the low amp draw, low suction pressure and low high side pressure.  There is NO REFRIGERANT FLOW.  The condenser is so full of refrigerant there is no room for any more causing the internal overload to open.


    Find the restriction it may only be a partial restriction. 

    Minnesota Wayne
  • Techman
    Techman Member Posts: 2,144
    edited June 2010
    hi high side

    the discharge pressure of the compressor has to be 375-450  psi when the comp's internal relief valve opens ,and,the higher the head pressure the higher the amp draw.But here in this case the amp draw is low.
  • JStar
    JStar Member Posts: 2,752
    Maybe...'s restricted and undercharged enough not to put a full strain on the compressor. I would also verify correct suction line size, and see that there are no kinks in the suction line, as well as check the metering device and drier for issues.

    If it's worth it, you can add service ports before and after the evaporator to check each part of the system.

    Did you notice what the standing pressure was? Did it seem low, high, normal before the unit was even running?
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