26
SERVICING
CHECKING PRIMARY LIMIT CONTROL
All primary limit controls are nonadjustable, automatic
reset, bi-metal type limit control. Refer to the following
drawing for the location of the primary limit.
Primary Limit Control Location
The following drawing illustrates the style of limit switches
used on the 90% furnaces.
ENCLOSED DISK
FRONT VIEW SIDE VIEW
Primary Limit Control Style
(90% Furnaces)
WARNING
HIGH
VO
LTAGE
D
ISCONNECT
ALL
POWER BEFORE SERVICING OR
CHANGING ANY ELECTRICAL WIRING.
M
ULTIPLE POWER
SOURCES MAY BE PRESENT.
F
AILURE TO DO SO MAY CAUSE
PROPERTY DAMAGE, PERSONAL INJURY OR DEATH.
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
temperature is above limit control setting. If not as
above, replace the control.
If the control detects the high limit circuit open, the RED
LED will ash FOUR times and energize the inducer and
indoor blower.
If the limit circuit opens ve times within a single call for
heat, the furnace will go into lockout for one hour. The
RED LED will display a FOUR ash 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 ve min-
utes, the control assumes the blower has failed the RED
LED will ash ELEVEN times.
If the limit circuit opens and does not close within fteen
minutes, the control assumes the manual rollout or fuse
has opened and the RED LED will ash 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 ash 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 ash code will be
stored. If the limit resets in more than 5 but less than 15
minutes, an eleven ash code will be stored. If the limit is
still open after 15 minutes, a ve ash code will be stored.
VOLT / OHM
METER
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.
CHECKING AUXILIARY LIMIT CONTROL
The auxiliary limit control is designed to prevent furnace
operation in case of main blower failure in horizontal instal-
lations. It may also open if the power supply is interrupted
while the furnace is ring.
The auxiliary limit control is suitable for both horizontal
right and horizontal left installations. Regardless of airow
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.
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