Scotsman MVP456 Service Manual page 26

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Check starting components.
Potential relay. If the compressor will not start,
check the amp draw of the starting circuit. If very
low, the potential relay contacts or start capacitor
may be open. Measure the resistance of the
potential relay contacts and the start capacitor. If
either is open it should be replaced. If the
compressor starts but draws very high amps
from the starting circuit, the potential relay may
not switch off. In that case the relay should be
replaced.
Measure resistance from Common to ground. It
should be infinite. Measure resistance from
Common to Run - compare to the chart. Measure
resistance from Common to Start - compare to
the chart.
Compressor check for high amp draw. Measure
amp draw of starting circuit. If it does not drop off
immediately after start up, the starting relay
should be replaced.
If the compressor is drawing excessive amps but
is operating, the run capacitor may be open.
Disconnect electrical power, discharge the
capacitor and measure its resistance. If open,
replace it. If shorted to ground, replace it.
Any time the compressor is replaced, the potential
relay, start capacitor and run capacitor should be
replaced with the compressor.
Fan motor
1. Test using the controller's indicator lights.
Note: Fan pressure control connection must
be jumped to perform this test.
Put the controller into test mode (depress Off for
3 seconds then depress Clean for 3 seconds).
At the end of the test cycle, the fan motor will be
powered and the Condenser Fan motor indicator
light will be on. The fan motor should start and run
at that time. If it does not, repeat the test but
check the voltage to the fan motor, it must receive
full voltage at the fan motor lead connection at
the end of the test. If there is voltage and the
motor does not operate, replace the motor. If
there is no voltage, check the controller high
voltage harness connection.
The fan motor lead is the top wire. Check voltage
from it to ground, at the end of the test, when the
fan motor indicator light is On, there must be
voltage from this pin to ground. Note: high voltage
power is supplied to the bottom pin from the
contactor line. Refer to the machine wiring
diagram as needed.
2. Test using an ohmmeter. Disconnect
electrical power. Unplug fan motor from harness.
Measure fan motor winding resistance. If open,
replace the fan motor.
Water Pump
1. Test using the controller's indicator lights.
Check the indicator light during the freeze cycle.
The light will be On for all but the 30 second anti-
slush period, so observe the light for one minute.
When it is On, check the water pump, it should be
operating. If not, check voltage to the pump. If low
check the voltage from the controller to ground.
The water pump pin is number 6. If there is
voltage at that pin to ground, but very low voltage
at the pump motor, there must be a broken wire
in the harness. If the voltage is low at pin 6, the
controller should be replaced.
2. Test using an ohmmeter. Disconnect
electrical power. Unplug the water pump motor
leads from the harness. Measure the resistance
of the motor windings. If open, replace the pump.
Measure resistance to ground. If there is any,
replace the pump.
Purge valve
1. Test using the controller's indicator lights. Shut
unit off by holding the Off button for 3 seconds.
Wait four minutes. Push and release the On
button, observe the Purge Valve indicator light.
As the unit drains the reservoir, the purge valve
will be powered. When it gets power, the indicator
light will be ON. If the purge valve does not open
to drain the reservoir when its indicator light is on,
do a voltage check. Shut the unit down by holding
the Off button in for 3 seconds. Unplug the
harness connection from the purge valve. Wait
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