Midco J121A-3 - Manual (Page 9)

■ The switch mechanism is a bi-metallic disc that snaps
over when it is heated to the set temperature. The switch is
mounted so that the disc bears against the side plate of the
blower housing.
■ Testing the switch to prove that it is functional can be
done by removing the ignitor assembly and blowing hot air
through the opening toward the blower inlet. If hot air is not
available, remove the switch and check for continuity. With
the switch cool, there should be continuity between
terminals 1 (black wire) and 3 (red/white). To simulate the
“hot” condition, place the switch on a firm surface and apply
finger pressure to the switch disc. When the disc “snaps”,
there should be continuity between terminals 1 and 2
(yellow/black wire).
FIGURE 5 Type “3” Ignitor and Regulator Assembly
XII MAIN AUTOMATIC AND IGNITOR GAS
VALVES
Both valves are single function on-off type with automatic
closing on current failure. Replace entire valve if valve
fails to open when power is applied or if leakage is
detected on standby.
XIII BLOWER ASSEMBLY
Model J81A-3 uses a 1550 RPM shaded pole motor with
auto-reset overload protection. It contains a centrifugal
Interlock Switch which is connected in the control circuit to
prevent burner operation if the motor fails. Model J121A-3
is equipped with 1⁄6 HP split phase 3450 RPM motor. It may
contain either a manual or auto-reset overload protector. It
has no interlock, this function being performed by an Air
Pressure Switch on the burner.
Both motors feature permanently lubricated ball bearings
that require no routine oiling maintenance.
■ Cleaning of the blower wheel is usually the only service
required. Need for cleaning is required if the inlet screen or
blower wheel shows an accumulation of dust and lint. The
motor air cooling vents should also be cleaned at this time.
■ If the motor must be replaced, disconnect the motor wires
from the burner terminal strip and thermal switch. Remove
the blower inlet screen and blower wheel and remove the
motor case bolt nuts and lockwashers.
■ When remounting the blower wheel, the distance from
the wheel and the outside of the blower housing air inlet
side plate should be 5⁄16".
XIV THERMAL SWITCH
The Thermal Switch is a heat sensitive single pole double
throw snap switch that energizes the blower motor to
provide cooling air if, because of a heat back up through the
burner when the motor is off, the temperature of the blower
housing reaches 160° F. The thermal switch is not wired
into the gas valve circuit and does not turn valve off when
activated by heat back up. FLAME
SENSOR
ELECTRODE
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