SERVICING
34
The following drawing illustrates the style of limit switches
used on the 90% furnaces.
ENCLOSED DISK
FRONT VIEW
SIDE VIEW
Primary Limit Control Style
WARNING
H
I
GH
VOLTAGE
D
I
SCONNECT
ALL
P
O
W
E
R
BEFO
R
E SE
R
V
I
C
I
NG O
R
CHANG
I
NG ANY ELECT
RI
CAL
WIRI
NG.
M
ULT
IP
LE
P
O
W
E
R
SOU
R
CES MAY BE
PR
ESENT.
F
A
I
LU
R
E TO
D
O SO MAY CAUSE
PR
O
P
E
R
TY
D
AMAGE,
P
E
R
SONAL
I
N
J
U
R
Y O
R
D
EATH.
1. Remove burner compartment door to gain access to
the primary limit.
2. Remove low voltage wires at limit control terminals.
3. With an ohmmeter, test between these two terminals
as shown in the following drawing. The ohmmeter
should read continuous unless heat exchanger tem-
perature is above limit control setting. If not as above,
replace the control.
Limit Fault Codes
If the control detects the high limit circuit open, the RED LED
will flash FOUR times and energize the inducer and indoor
blower.
If the limit circuit opens five times within a single call for heat,
the furnace will go into lockout for one hour. The RED LED
will display a FOUR flash code during this time. The control
board can be reset by cycling 115 volt power to the furnace.
If the limit circuit opens and does not close within five minutes,
the control assumes the blower has failed the RED LED will
flash ELEVEN times.
If the limit circuit opens and does not close within fifteen
minutes, the control assumes the manual rollout or fuse
has opened and the RED LED will flash FIVE times. The
control will enter a one hour lockout and the inducer will run
continuously.
In an open limit condition the red LED will flash as described
above but the error code will not be stored in memory until
the limit resets or until 15 minutes has elapsed. So if the limit
resets in less than 5 minutes a four flash code will be stored.
If the limit resets in more than 5 but less than 15 minutes, an
eleven flash code will be stored. If the limit is still open after
15 minutes, a five flash code will be stored.
COLOR
IDENTIFYING
TAB
Testing Primary Limit Control
4. After completing check and/or replacement of primary
limit control, reinstall burner compartment door.
5. Turn on electrical power and verify proper unit operation.
S-301 Checking Auxiliary Limit Control
The auxiliary limit control is designed to prevent furnace
operation in case of main blower failure in horizontal
installations. It may also open if the power supply is interrupted
while the furnace is firing.
The auxiliary limit control is suitable for both horizontal
right and horizontal left installations. Regardless of airflow
direction, it does not need to be relocated. The (2) two auxiliary
limits are located on the blower housing (one on each side),
as shown in the following illustration.
