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!