17
MSEV Series and USHX Hardware Installation Manual Rev 1.7
4 Troubleshooting
The following section describes troubleshooting procedures for the USHX and MSEV series valve. If
the system is running abnormally, first check the wiring for broken or shorted connections. Repair broken
wires and remove short circuits between touching wires or between wires and any metal surfaces. If
there are no wiring problems, then use the following table to further diagnose the problem (assuming
everything else in the system such as the compressor, evaporator, filter drier, etc. is working properly).
Problem
Possible Cause
Action
High Superheat:
This may indicate
that the MSEV
series valve is not
fully opening.
Common symptoms
include compressor
short cycling.
Inadequate Power to
the Valve
Check the power source voltage. The voltage leaving the
transformer and entering the USHX should be close to the
intended supply voltage (24V or 12V). If the transformer
voltage is too low, there is a problem with the transformer or
its wiring. If the voltage entering the USHX is low and the
transformer voltage is normal, there is a problem with the
terminal connections or interconnecting wiring to the USHX.
Temperature Sensor
Incorrectly Mounted
Check the mounting of the USHC temperature sensor. The
sensor should be firmly mounted to the outlet of the
evaporator at a 10 o’clock or 2 o’clock position. Check that the
temperature sensor is wrapped with insulated tape.
Over Voltage to the
MSEV Series Valve
Check the resistance across the valve terminals. Remove both
power connections from the MSEV series valve and measure its
resistance with a multimeter. The resistance reading should be
between 26-34Ω for a 24V valve and between 6-13Ω for a 12V
valve. If the resistance is significantly out of this range (or zero),
the MSEV series valve is damaged and should be replaced.
MSEV Series Valve
Slow to Open
Check that the USHX Gain settings meet the system
requirements. (Also applicable if the MSEV series valve opens
too quickly.)
Connect and disconnect the power to the valve several times
to manually actuate the valve.
Low Superheat:
This may indicate
that the MSEV
series valve is
staying open.
Common
symptoms include
compressor
frosting.
MSEV Series Valve
Slow to Close
Check that the USHX Gain settings meet the system
requirements. (Also applicable if the MSEV series valve closes
too quickly.)
Connect and disconnect the power to the valve several times
to manually actuate the valve.
Temperature Sensor
Incorrectly Mounted
Check the mounting of the USHC temperature sensor. The
sensor should be firmly mounted to the outlet of the
evaporator at a 10 o’clock or 2 o’clock position. Check that the
temperature sensor is wrapped with insulated tape.
Severely Oversized
Valve
Determine the capacity of the evaporator and check the valve
model number to confirm that the two are compatible
regarding their capacities.
