Service Manual

SERVICING
79
VOLT / OHM
METER
RED
RESET
BUTTON
COLOR
IDENTIFYING
TAB
Testing Auxiliary Limit Control
(80% & 90% Furnaces)
WARNING
T
O AVOID POSSIBLE FIRE, ONLY RESET THE AUXILIARY LIMIT CONTROL
ONCE.
I
F IT SHOULD OPEN A SECOND TIME, A QUALIFIED SERVICER
MUST DETERMINE WHY THE AUXILIARY LIMIT OPENED BEFORE
RESETTING AGAIN.
S-302 CHECKING FLAME ROLLOUT CONTROL
A temperature activated manual reset control is mounted to
the manifold assembly on 80% furnaces, as shown in the
following illustrations.
ROLLOUT
SWITCH
Flame Rollout Switch Location
(80% Upflow Furnace Shown, Downflow Similar)
The control is designed to open should a flame roll out oc-
cur. An over firing condition or flame impingement on the
heat shield may also cause the control to open. If the rollout
control opens, the air circulation blower will run continuously.
On two-stage models the ignition control the diagnostic light
will flash (4) four times indicating a trip of the rollout switch.
These symptoms are identical to a trip of the primary limit
control.
To aid in identifying these controls, color coded labels have
been affixed to the back of these controls. Refer to the
Rollout Limit Charts in furnace Technical Manual for tem-
perature settings and color codes.
The circuit between the ignition control and gas valve will be
interrupted when the rollout switch opens up.
LINE VOLTAGE NOW PRESENT
WARNING
1. Remove the burner compartment door to gain access to the
rollout switch(es) mounted to burner bracket.
The servicer should reset the ignition control by opening and
closing the thermostat circuit. Then look for the ignitor glowing
which indicates there is power to the ignition control. Measure
the voltage between each side of the rollout control and ground
while the ignition control tries to power the gas valve.
2. Measure the voltage between each side of the rollout
control and ground during the ignition attempt. Refer to
the following figure.
VOLT / OHM
METER
GN
D
RED
RESET
BUTTON
COLOR
IDENTIFYING
TAB
Checking Flame Rollout Switch
a. If no voltage is measured on either side of control it
indicates ignition control or wiring to control problem.
b. If voltage is measured on one side of the control and
not the other it indicates the control is open.
c. If voltage is measured on both sides of the control
the wiring to gas valve or valve is at fault.
3. After check and/or replacement of rollout switch, rein-
stall burner compartment door and verify proper unit op-
eration.