TM 5-4520-240-14
CHAPTER 7
REPAIR OF ELECTRICAL CONTROL COMPONENTS
7-1. Description.
cessive temperature is reached, the fuel solenoid valve
will deenergize to stop combustion, As the thermostat
a. The electrical components which control automat-
cools to 210 12F (92 to 105C), the circuit will
ic heater operation, and which are not mounted on the
control panel, include the control relay K2 thermocou-
again be energized and the fuel solenoid will again
ple relay (flame switch) S3, and overheat thermostat
open the fuel valve to reestablish combustion.
S5. A terminal block TB1 is provided to facilitate mak-
e. A terminal block (11, fig. 4-7) is mounted on the
ing electrical connections.
bottom of the heater housing to help facilitate electri-
cal connections to the various components of the
b. The control relay (10, fig. 4-7) is mounted on the
heater. Connector tabs are installed on the terminal
heater housing next to the control panel. This double-
block to permit making multiple slip-on type connec-
pole, double-throw relay is energized through the ther-
tions to the terminals of the block.
mocouple relay to switch heater operation from the
start mode to the run mode when the presence of flame
7-2. Removal.
is sensed by the thermocouple.
WARNING
c. The thermocouple relay (9, fig. 4-8) is connected to
T h e ignition system of this space
the thermocouple by means of the thermocouple lead
heater contains dangerous voltages
(13) which is part of the thermocouple. The thermocou-
w h i c h can cause severe electrical
ple extends into the heat exchanger and, when the fuel
shock. Be sure to disconnect the power
is ignited, the flame heats the thermocouple and gener-
plug before servicing or repairing the
ates a small current. This current trips the thermocou -
heater.
ple relay which, in turn, energizes the control relay to
a. Disconnect the power plug and remove the top and
change heater operation from the start mode to run
side cover panels to provide access to the electrical con-
mode.
trol components. Remove the warm air louver.
d. The overheat thermostat is mounted on a bracket
b. Tag and unsolder the electrical leads to the control
attached to the heat exchanger. The thermostat is nor-
really closed. At a temperature of 250 6F (118 to
relay (3, fig. 7-1). Remove the two screws (1) and lock
124C), the thermostat opens. The thermostat is con-
washers (2) that secure the control relay to the housing,
remove the control relay.
nected into the fuel solenoid circuit so that when an ex-
7-1
