AAON RQ SERIES, Manual

Page: 32 / 36

32
Troubleshooting
The following section describes troubleshooting procedures for the USHX and MSEV. 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 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
MSEV
Check the resistance across the valve terminals. Remove both
power connections from the MSEV and measure its resistance
with a multimeter. The resistance reading should be between 26-
34 Ω for a 24 V valve and between 6 - 13 Ω fo r a 12 V valve. If
the resistance is significantly out of this range (or zero), the
MSEV is damaged and should be replaced.
MSEV Slow to
Open
Check that the USHX Gain settings meet the system
requirements. (Also applicable if the MSEV 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 is staying
open.
Common symptoms
include compressor
frosting.
MSEV Slow to
Close
Check that the USHX Gain settings meet the system
requirements. (Also applicable if the MSEV 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.