I have an old Trane Central Air unit (model BLU0682L942A) which recently went out on us. Nothing worked … no AC … no heat … no fan. I replaced the transformer and then the relay (S87R-1079-1). I believe everything is correctly wired. I don't have a wiring diagram for the unit. I tested the heater and the unit ignites and flames on, the blower comes on and everything appears to work. However, when I test the AC, I get a definite “click” from the relay and then a loud buzzing sound. The outside condenser does not come on and the condenser fan does not spin. I can manually turn the fan blades of the outside unit, so the motor is free. I do not hear any noise from the condenser. I disconnected the 24V wires (in my case red and white) from the condenser wires (blue and yellow). Seems as long as I do not have a complete 24V circuit to the condenser I get no buzz (blower fan still works). I am uncertain as to what is causing the buzz at the relay. I am thinking motor start capacitor or short at the condenser? I am looking for help in “next steps” to isolate the problem.

Check the contactor in the outdoor unit it may not be wired correctly. Check to see if the coil on the contactor is 24 volt and ensure that 24 volts is being fed to it.

A low voltage to a relay will cause chattering and or buzzing of a relay or contactor