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The wiring diagram on the Pages 1-45 and 46 identifies the components and wiring connection
points.
1.11 Troubleshooting Guides
The troubleshooting guides on the following pages are intended to assist service technicians in
quickly isolating the probable causes of equipment malfunctions by following a logical, systematic
process. An additional set of operator troubleshooting guides are contained in Chapter 7 of the
BIGLA30 Series Installation and Operation Manual. It is suggested that service technicians
thoroughly familiarize themselves with both sets.
1.11.1 Troubleshooting the 24 VAC Circuit
Prior to checking for problems associated with the 24 VAC circuit, ensure that the unit is connected
to a power supply, the drain valve is fully closed, and the computer is on and is calling for heat
(green dot appears under heat indicator and display toggles between LOW and TEMP).
NOTE: All voltage measurements must be made within 4 seconds of the unit calling for heat. If unit
does not fire within 4 seconds, ignition modules will lock out and computer must be turned off, then
on to reset.
The following processes will assist you in troubleshooting the 24 VAC circuit and ruling it out as a
probable cause:
• 24 VAC is not present on the interface board J3 pin 9 (LED 5 (GV)).
1. If LED 3 is not continually lit, the probable causes are a failed 24 VAC transformer or failed
wiring between the transformer and interface board.
2. If LED 3 is continually lit, check the right PWR terminal (LED 4) for 24 VAC. Verify that
the F2 fuse is not blown.
a. If 24 VAC is not present, the probable cause is a failed interface board, blown fuse or a
defective heat relay.
b. If 24 VAC is present, check for 24 VAC on V1S.
i. If 24 VAC is not present, check the fuses. If they are good, the probable causes are
failed ignition module(s) or a failed interface board. Replace the questionable
ignition module with one known to be good to isolate the cause.
ii. If 24 VAC is present, the probable cause is a failed interface board.
• 24 VAC is present on interface board J3 pin 9 (LED 5 (GV)).
1. If 24 VAC is not present across the gas valve main coil (MV terminals), probable causes are
an open high-limit thermostat or a failed wire between the interface board and gas valve. It
may also be caused by a failed drain safety switch.
a. Check continuity of high-limit thermostat and drain safety switch. If both are zero,
problem is in wiring.
2. If 24 VAC is present across the gas valve main coil (MV terminals), the 24 VAC circuit is
working and the problem may be with the gas valve.