Service manual

SERVICING
44
4. If limit test open allow unit to cool and retest.
5. If still open, replace the control.
S-301 TESTING AUXILIARY LIMIT
CONTROL This control is preset nonadjustable control
mounted in the blower compartment area.
It is connected in series with the limit control wiring to the
ignition control. If its temperature should be exceeded, it
will open, interrupting the voltage to the gas valve causing
it to open.
An additional limit (primary limit) control is required for safety
control of high temperature within the furnace or duct work.
AUX. LIMIT
WARNING
Disconnect Electrical Power Supply:
1. Remove the wires from the auxiliary limit control termi-
nals.
2. Using an ohmmeter, test for continuity across the two
terminals. No reading indicates the control is open.
Push the red reset button, test again - if still open, re-
place the control.
OHMMETER
LIMIT OR AUXILIARY LIMIT CONTROL
S-302 CHECKING FLAME ROLLOUT SWITCH
Package Gas Units with serial numbers beginning 9001 and
later use a temperature activated manual reset control
mounted to the manifold assembly.
This control is wired in series with the primary limit and in-
tegrated control. The control is designed to open should a
flame roll out occur. An over firing condition or flame im-
pingement on the heat shield may also cause the control to
open.
If the rollout control has opened, the circuit between the
ignition control and gas valve will be interrupted and the
ignition control module will go into lockout. The servicer
should reset the ignition control by opening and closing the
thermostat circuit. The servicer should look for the ignitor
glowing which indicates there is power to the ignition con-
trol. The servicer should measure the voltage between each
side of the rollout control and ground while the ignition con-
trol is try to power the gas valve.
Limit Switch Operation (Applies to Primary, Auxiliary, and
Roll Out Limits) DSI systems.
If a limit switch opens, the indoor blower is energized on
heat speed and the induced draft blower is energized. The
LED on the control flashes "4" to indicate an open limit
switch. The blower and inducer remain on while the limit
switch is open. The gas valve is de-energized. Power to
the thermostat "R" is removed while the limit switch is open.
When the limit switch re-closes, the induced draft motor
runs through its post purge and the indoor blower goes
through the heat off delay.
If a call for heat exists when the limit switch re-closes, the
control goes through a pre-purge period and then makes
an ignition attempt. The indoor blower remains on (for the
delay off time) during the re-ignition attempt.
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.
Servicing procedure with furnace not firing.
1. Confirm that the outer door was in place and all screws
tightened. (No leaks under the door.)
2. Check to see if any damage was done to the furnace
especially the wiring.
3. Confirm that heat exchanger is not obstructed by feel-
ing for discharge air from the flue hood when the com-
bustion blower is running but the unit is not firing.
If the above steps do not suggest the reason the control
has tripped the furnace should be fired.
1. Remove the heating compartment door.
2. Turn of the power or open the thermostat circuit.