Bradford White U-TW4-75S-76R-3N - Manual (Page 42)

Page 42
Safety Circuit Voltage Trace
Note: This procedure assumes a cold tank.
Step 1. Remove three screws (Phillips screw driver) from
control access cover on blower and remove
cover.
Safety Circuit Voltage Trace
WARNING
115 volt potential exposure. Use caution making voltage checks to avoid
personal injury.
Control access
cover
Position gas control switch to the “ON”
position and adjust the thermostat dial
to call for heat. Is there 10 to 13VAC
between the red wire leading to the
blower temperature switch and the
green ground wire? Blower must be
running during this voltage check (see
photo 15).
Is LED on gas valve flash-
ing the “heartbeat” code
(alternating bright/dim)?
No voltage from gas control.
Call for technical support.
Verify 115VAC to gas con-
trol (see page 41).
Is there 10 to 13VAC between
the yellow wire leading from the
pressure switch and the green
ground wire (see photo 27)?
Is there continuity through
the blower temperature
switch (see page 31)?
Check for loose or broken
wire connection at switch
terminals.
Do you hear or can you
see the igniter sparking?
Check igniter/sensor (see page
22). Is igniter/sensor OK?
Correct igniter/
sensor problem
Determine cause of blower
temperature switch activa-
tion and correct.
Faulty cord set
or gas control
Safety circuit
voltage is OK.
If burner does not light, ob-
serve LED flash code on gas
control and refer to trouble-
shooting section on page 17.
Green ground wire
Red sire leading to
blower temp. switch
Yellow wire leading
from pressure switch
15
14
N
N
Y
Y
N
Y
N
Y
N Y
Y
42