Cobra cf4, Service Manual

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5
Fault Finding
11
Moffat Cobra Series - CF4 © Moffat Ltd, August 2012
Amendment 4
To test the thermocouple connected to the gas
control valve. Light the pilot burners, and while still
holding the flame failure button depressed,
measure the voltage between the line to the over-
temp and earth using a multimeter. (See
Figure 5.2.1c below) The reading should be
approximately 30mV.
If the reading is less than 10mV and there is
satisfactory flame impingement on the
thermocouple the thermocouple is faulty -
replace
the interrupted thermocouple .
If the voltage is over 10mV then there is a fault with
the flame failure valve electromagnet -
replace the
flame failure valve .
With the pilot burners lit and while still holding the
flame failure button depressed, measure the
voltage between the line from the overtemp and to
earth using a multimeter. (See Figure 5.2.1d.).
If the reading is over 5mV, the gas control valve is
faulty -
replace the gas control valve .
If the reading is less than 5mV and there is
satisfactory flame impingement on the
thermocouple the overtemp is faulty -
replace the
overtemp .
Or there are faulty connections between the
interrupter block and the overtemp -
check, clean
and re-tighten all connections and re-test.
5.2 Fault Diagnosis
5.2.1 Pilot Drops Out
Pilot Flame too Small
If the pilot can be lit but the flame is too small to
impinge on the thermocouple, then check the gas
pressure. If the gas pressure is correct, check the
adjustment of pilot flow by adjusting the screw
below the control knob on the gas control. If the
pilot flame is still too small, remove the pilot injector
from the pilot burner and check for correct pilot
injector size and blockage of the pilot injector.
Thermocouple Faulty
Check thermocouple connections are secure and
that the over-temperature lead connection to the
interrupter block on the thermocouple is secure
(loose connections will cause high resistance in the
millivolt circuit and result in pilot outage).
If all connections are satis, light the pilot burners.
Press and hold the ignition button on the control
panel depressed and at the same time hold the
flame failure button depressed. Manually light the
pilot burner until the pilot ignites. Continue holding
the ignition button and the flame failure buttons
depressed until the second pilot cross lights from the
first pilot.
Continue holding the ignition button and flame
failure button depressed for approx. 10 to 15
seconds after the second pilot burner has ignited.
Release the ignition button, both pilot burners
should remain alight.
If the pilot flames don't go out when the flame
failure button is released then release the ignition
button.
If the pilot flames go out when the ignition button is
released, there is a problem with the thermocouple
circuit connected to the main gas control valve.
Figure 5.2.1a.
Pilot Screw
Adjustment
Low Fire
Blanking Screw
Gas Control
Valve
Flame Failure
Button
Overtemp
Cut-Out
Ignition Button
'Off' Button
Figure 5.2.1b.
Figure 5.2.1c.
Gas Control
Valve
Overtemp
Interrupter
Block
Test between
here and Earth
Figure 5.2.1d.
Interrupter
Block
Test between
here and Earth
Overtemp
Gas Control
Valve
Thermocouple
Thermocouple