FAULT ISOLATION OF NON-REPAIRABLE ELECTRONIC CONTROLS

StepExpected 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.

pic1 8 FAULT ISOLATION OF NON REPAIRABLE ELECTRONIC CONTROLS


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