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Thread: dead short!

  1. #1
    Join Date
    Sep 2002
    Posts
    607
    im working on a York sunline 15 ton rooftop unit (circa 1994). It has an intermittent low voltage short somewhere. ive been here twice and am going a third time tommorrow. Here is what ive done so far:

    1st call- economizer is open first stage mechanical cooling on , power exhaust on. Find enthalpy outdoor sensor shorted. No problem order new sensor. complaint was not enough cooling.

    2nd call- return 2 days later to install new sensor. When i get there entire unit is shut down including ID fan. Find low voltage circuit breaker tripped. What the f***? i say to myself. I install sensor and reset breaker. Fan starts ,first stage cooling starts, second stage cooling starts then control breaker trips again. i Proceed to eliminate loads from control circuit.Have determined problem is NOT, contactors,low and high pressure switches, economizer module or sensor. I then disconnect thermostat wires from terminal strip in unit and jumper fan and both stages of cooling. Everything works fine. So i figure ive narrowed it down to thermostat wire and/or thermostat. Thermostat is a t7300 with automatic Q7300 subbase.I replace T7300(i happened to have one in my truck)reconnect wires start unit.Bingo! everything works.That was on tuesday.Tonight(thursday) get call from customer ,unit down thermostat screen is blank! I know breaker has tripped.Im at wits end right now. Im going down tomorrow and change the subbase(which i didnt have on last call)Any suggestions where this short could be happening.I figure if its some bare wire rubbing on steel somewhere in the unit im in for a long day tomorrow.Biggest problem is how can i charge customer for the time?Please help ,someone!


  2. #2
    Join Date
    Feb 2003
    Location
    Wichita Ks
    Posts
    1,473
    check the stat wire from the unit to the stat. it may have a bare spot from rubbing on soething metal. check it close.

  3. #3
    Join Date
    Jun 2004
    Location
    Richmond Virginia
    Posts
    1,078
    Be patient. Put the meter down and start using your hands and eyes and carefully look at the wiring. If you went through many of the "short" threads on this site you'd find that most of the time a wire rubbed or melted is the usual cause.

  4. #4
    Join Date
    Nov 2003
    Posts
    125

    Arrow

    I've found it to be in the economizer in a lot of different brand rooftops.

    Have you unplugged the economizer yet trying to isolate the problem?

  5. #5
    Join Date
    Apr 2005
    Location
    Richmond, Virginia
    Posts
    365
    Just went through a similiar issue with a York roof top. What kinda reistance are you getting on your t-stat wire? Any type of low 1-1.9 resistance would indicate a wire is rubbing or you've got a bad stat(you've elimated this).

    Try taking a short piece of t-stat wire from the unit and hook up the t-stat to that and see if you can duplicate the problem. If the unit works fine this way, re-run the t-stat wire.
    Sean Cantrell

  6. #6
    Join Date
    Sep 2002
    Posts
    607
    I found it!. Myself and one of other techs went to the job today.BTW, its a Mcdonalds with aerco hvac control system. Anyway, what we found is that Aerco has hvac units for Mcdonalds modified before installation. I was getting a feedback through a lockout relay when 2nd stage came on.We had to eliminate the freeze stat for stage 2 to keep it running.We just started to service this McD's and the owner told us he has had many problems with this unit in the past. ITs hard to explain the exact problem on this forum without showing you a wiring diagram

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