Trying to troubleshoot control wiring is sometimes confusing and frustrating. But, if you have a step-by-step procedure it becomes easy and accurate. Below you find a procedure for troubleshooting a 24 volt open in an HVAC system.
Symptoms - With 24 volts present one or more control circuits (relay, contactor, blower motor relay, defrost control board, reversing valve)is not operational.
Troubleshooting Steps Step 1 - Inspection
● Remove thermostat from the sub-base
○ Inspect for broken, loose, corroded, or disconnected wires.
● Remove air handler cover
○ Inspect for broken, loose, corroded, or disconnected wires.
● Remove condenser cover
○ Inspect for broken, loose, corroded, or disconnected wires.
○ Note - weed eater, lawn mower, and dog chewing damage is common where the control wiring exits the chase at the ground level at the condensing unit.
○ Note - rusted and corroded wire nut connections are common on previous repair or spliced control wiring.
Step 2 - With the thermostat still removed from the sub-base● Disconnect and separate all control wiring from condenser.
● Disconnect and separate all control wiring at the air handler
● At the air handler, wire nut together all control wiring to the thermostat.
● At the air handler, wire nut together all control wiring to the condenser.
Step 3 - From Thermostat Sub-base to Air Handler● Set your meter to the audible ohms (beep) setting.
● Alligator clip one lead to the common wire coming from the thermostat.
● One-by-one check:
○ Common to red.
○ Common to orange.
○ Common to yellow.
○ Common to green.
○ Common to any other wire that may be connected (2-stage system)
● Remove alligator clip from common and connect to the red wire.
● One-by-one check:
○ Red to orange.
○ Red to yellow.
○ Red to green.
○ Red to any other wire that may be connected (2-stage system)
● Remove alligator clip from red and connect to the orange wire.
● One-by-one check:
○ Orange to yellow.
○ Orange to green.
○ Orange to any other wire that may be connected (2-stage system)
● Remove alligator clip from orange and connect to the yellow wire.
● One-by-one check:
○ Yellow to green.
○ Yellow to any other wire that may be connected (2-stage system)
● Remove alligator clip from yellow and connect to the green wire.
● One-by-one check:
○ Green to any other wire that may be connected (2-stage system)
Step 4 - From the Condenser to the Air Handler● Make the exact same check as listed in Step 3 with the wires going out to the condensing unit.
Step 5 - Diagnosis● If your meter DID NOT beep in Step 4, the open or broken wire is between the air handler and the thermostat.
● If your meter DID NOT beep in Step 5, the open or broken wire is between the air
handler and the condenser.
● If your meter beeped on all wires, the field installed control wiring is good.
● The open or broken wire can be determined by noting on which wires your meter
did not beep.
○ Locate open or broken wire - repair or replace wiring.
○ Locate open or broken wire - repair or replace wiring.
○ Check for factory wiring opens in the air handler and condensing unit.!