| Step | Expected Result/Action |
|---|---|
| 1. With power applied to the furnace, use any built-in component test feature (if so equipped) to help isolate the cause of the problem. Refer to the manufacturer's service instructions. | Many units are equipped with some form of indicator on the electronic control that displays the unit's operating status and/or fault codes. Normally, a label is attached to the unit that defines the operating status or fault associated for each of the fault indication codes. Many units also are equipped with a built-in component test feature to aid the service technician in isolating problems. When the unit being serviced is so equipped, follow the manufacturer's instructions for the use of the built-in diagnostic features and for troubleshooting the unit. For our example assume: You begin troubleshooting the unit based on your observation that the inducer motor failed to run during the component test. |
| 2. Set the VOM/DMM to measure AC voltage. Note that the voltage level expected is 24 volts. Connect the VOM/DMM meter leads across the thermostat connection R and COM terminals as shown in Figure SP-12-4. | If the VOM/DMM indicates 24 VAC, this indicates that the 24 VAC control voltage input to the electronic control is good. Proceed to step 3. If the VOM/DMM reads 0 VAC, use the hopscotch method to isolate any open or failed component or wiring. Check for the following: • • • • Is power applied to the furnace? Is the interlock (ILK) switch closed? Is the fuse (FU1} on the control board good? Is the flame rollout switch (FRS) or main limit switch (LS) open? For our example assume: The VOM/DMM indicates 24 VAC |
| 3. Connect the VOM/DMM meter leads across the thermostat connection W and COM terminals as shown in Figure SP-12-4. | If the VOM/DMM reads 24 VAC, this indicates that the 24 VAC "call-for-heat" input to the electronic control is good. Proceed to step 4. If the VOM/DMM reads 0 VAC, use the hopscotch method to isolate any open or failed component or wiring. Check for the following: • • • Is the thermostat function switch set in the heating mode? Is the thermostat temperature set high enough? Is the thermostat defective? For our example assume: The VOM/DMM indicates 24 VAC. |
| 4. Set the VOM/DMM to measure 115 VAC. Connect the VOM/DMM meter leads across connector PL3 terminals 1 and 3 as shown in Figure SP-12-4. | The VOM/DMM reads 115 VAC but the inducer motor does not run. This indicates that the 115 VAC output from the electronic control is good; therefore, the control board is good. Use the hopscotch method to isolate any problem external to the electronic control. Check for the following: • • • • Are the electrical connections tight? Is the inducer motor capacitor defective? Is there some mechanical defect such as a jammed inducer wheel preventing the motor from turning? Is the motor hot, indicating the internal overload may be open? If the VOM/DMM reads 0 VAC, replace the control. For our exam ple assume: The VOM/DMM indicates J 15 VAC, indicating that the electronic control is good Further, assume the problem is found to be a loose connection between the electronic control and the inducer motor. |
