Lennox ML193UHE Series Unit Information page 12

Primary Limit Location and Heat Exchanger
Install limit face down
FIGURE 7
4. Gas Valve (FIGURE 8)
The ML193UHE uses an internally redundant valve to as-
sure safety shut-off. If the gas valve must be replaced, the
same type valve must be used.
24VAC terminals and gas control switch are located on top
of the valve. All terminals on the gas valve are connected
to wires from the ignition control. 24V applied to the termi-
nals opens the valve.
Inlet and outlet pressure taps are located on the valve. A
manifold adjustment screw is also located on the valve. An
LP/Propane changeover kit is available.
MANIFOLD PRESSURE
ADJUSTMENT SCREW
(under barbed fitting)
INLET
PRESSURE
PORT
GAS VALVE SHOWN IN ON POSITION
FIGURE 8
5. Flame Sensor (FIGURE 6)
A flame sensor is located on the left side of the burner
support. The sensor is mounted on the front burner box
plate and the tip protrudes into the flame envelope of the
leftmost burner. The sensor can be removed for service
(use steel wool only to clean) without removing any part of
the burners. During operation, flame is sensed by current
passed through the flame and sensing electrode. The ig-
nition control allows the gas valve to remain open as long
as flame signal is sensed.
NOTE - The ML193UHE is polarity sensitive. Make sure
that the furnace is wired correctly and is properly ground-
ed.
A microamp DC meter is needed to check the flame signal
on the integrated control.
Flame (microamp) signal is an electrical current which
passes from the integrated control to the sensor during
unit operation.
Current passes from the sensor through the flame to
ground to complete a safety circuit.
To Measure Flame Signal - Integrated Control:
Use a digital readout meter capable of reading DC micro-
amps.
See FIGURE 9 for flame signal check.
1 - Set the meter to the DC amps scale.
2 - Turn off supply voltage to control.
3 - Remove sensor wire from integrated control.
4 - Connect (-) lead to flame sensor wire.
5 - Connect (+) lead to Terminal FS on integrated
control.
6 - Turn supply voltage on and close thermostat
contacts to cycle system.
7 - When main burners are in operation for two minutes,
take reading.
MANIFOLD
6. Ignitor (FIGURE 6)
PRESSURE
OUTLET
ML193UHE units use a nitride ignitor made from a propri-
PORT
etary ceramic material. To check ignitor, measure its resis-
tance and voltage. A value of 39 to 70 ohms indicates a
good ignitor. Voltage to the ignitor should be 102 - 132VAC.
See FIGURE 10 for resistance and voltage checks.
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