THERMOSTAT CHECKS AND MEASUREMENT PROCEDURE

StepExpected Result/Action
At the thermostat, set the FAN switch to the ON position.If the indoor fan motor runs, the control transformer is operating and the fan
switch section of the thermostat is good. Troubleshoot the related load and control
circuits per SP-8.
If the indoor fan motor does not start, proceed to step 11.
11. Turn off power to the equipment. Set up the VOM/DMM to measure AC
control voltage on a range that is higher than the highest voltage expected.
(Refer to the unit schematic.) Connect the VOM/DMM between test point 1
(C) and terminal G as shown in Figure SP-7-4, View A. Turn power on, then
observe the VOM/DMM voltage indication.
Turn power off and disconnect VOM/DMM.
If the VOM/DMM indicates control voltage (24 volts), the fan switch section of the
thermostat is good. Troubleshoot the related indoor fan motor control and load
circuits per SP-8.
If the VOM/DMM indicates zero volts, check voltage at the control transformer
secondary winding per step 16. If voltage is good, replace the thermostat or
repair the associated open wiring.
NOTE: Steps 12 and 13 check the cooling circuit section of the thermostat.
12. At the thermostat, set the FAN switch to the AUTO position and the function
switch to the COOL position. Set the thermostat to call for cooling.
If the compressor, outdoor fan, and/or indoor fan motor start, the cooling switch
section of the thermostat is operating. Troubleshoot the related load and control
circuits per SP-8.
If the compressor, outdoor fan, and/or indoor fan motor do not start, proceed to
step 13.
13. Turn off power to the equipment. Set up the VOM/DMM to measure AC
control voltage on a range that is higher than the highest voltage expected.
(Refer to the unit schematic.) Connect the VOM/DMM between test point 1
(C) and terminal Y as shown in Figure SP-7-4, View B. Turn power on, then
observe the VOM/DMM voltage indication.
Turn power off and disconnect VOM/DMM.
If the VOM/DMM indicates control voltage (24 volts), the cooling switch section of
the thermostat is good. Troubleshoot the related cooling control and load circuits
per SP-8.
If the VOM/DMM indicates zero volts, replace the thermostat or repair the
associated open cooling control circuit wiring.
NOTE: Steps 14 and 15 check the heating circuit section of the thermostat.
14. At the thermostat, set the function switch to the HEAT position. Set the
thermostat to call for heat.
If the pilot ignites, the heating switch section of the thermostat is working.
Troubleshoot the related load and control circuits per SP-8.
If the pilot does not ignite, proceed to step 115.
15. Turn off power to the equipment. Set up the VOM/DMM to measure AC
control voltage on a range that is higher than the highest voltage expected.
(Refer to the unit schematic.) Connect the VOM/DMM between test point 1
(C) aind terminal W as shown in Figure SP-7-4, View C. Turn power on, then
observe the VOM/DMM voltage indication.
Turn power off and disconnect VOM/DMM.
If the VOM/DMM indicates control voltage (24 volts), the heating switch section
of the thermostat is good. Troubleshoot the related heating control and load
circuits per SP-8.
If the VOM/DMM indicates zero volts, replace the thermostat or repair the
associated open heating control circuit wiring.

pic1 91 THERMOSTAT CHECKS AND MEASUREMENT PROCEDURE


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