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177820-YIM-A-0506
Unitary Products Group 33
8. If 24 volts is not present at the “FAN” terminal,
check for 24 volts from the room thermostat. If 24
volts are not present from the room thermostat,
check for the following:
a. Proper operation of the room thermostat (contact
between R and G with the fan switch in the ON posi-
tion and in the AUTO position during operation
calls).
b. Proper wiring between the room thermostat and the
UCB.
c. Loose wiring from the room thermostat to the UCB.
9. If 24 volts is present at the room thermostat but not
at the UCB, check for proper wiring between the
thermostat and the UCB, i.e. that the thermostat G
terminal is connected to the G terminal of the UCB,
and for loose wiring.
10. If the thermostat and UCB are properly wired,
replace the UCB.
On a call for cooling, the supply air blower motor is
operating but the compressor is not (the room thermo-
stat fan switch is in the “AUTO” position).
1. If installed, check the position of the economizer
blades. If the blades are open, the economizer is
providing free cooling and the compressors will not
immediately operate. If both stages of cooling are
requested simultaneously and the economizer pro-
vides free cooling, following a short delay the com-
pressor will be energized unless it is locked out,
unless this option has been disabled through com-
puter communications.
2. If no economizer is installed or the economizer is
not opening to provide free cooling and the com-
pressor does not energize on a call for cooling,
check for line voltage at the compressor contactor,
M1, and that the contactor is pulled in. Check for
loose wiring between the contactor and the com-
pressor.
3. If M1 is pulled in and voltage is supplied at M1,
lightly touch the compressor housing. If it is hot, the
compressor may be off on inherent protection.
Cancel any calls for cooling and wait for the inter-
nal overload to reset. Test again when cool.
4. If M1 is not pulled in, check for 24 volts at the M1
coil. If 24 volts are present and M1 is not pulled in,
replace the contactor.
5. Failing the above, if voltage is supplied at M1, M1
is pulled in, and the compressor still does not oper-
ate, replace the compressor.
6. If 24 volts is not present at M1, check for 24 volts at
the UCB terminal, C1. If 24 volts is present, check
for loose wiring between C1 and the compressor
contactor.
7. If 24 volts is not present at the C1 terminal, check
for 24 volts from the room thermostat at the UCB
Y1 terminal. If 24 volts is not present from the room
thermostat, check for the following:
a. 24 volts at the thermostat Y1 terminal
b. Proper wiring between the room thermostat and the
UCB, i.e. Y1 to Y1, Y2 to Y2
c. Loose wiring from the room thermostat to the UCB.
8. If 24 volts is present at the UCB Y1 terminal, the
compressor may be out due to an open high-pres-
sure switch, low-pressure switch, or freezestat.
Check for 24 volts at the HPS1, LPS1, and FS1
terminals of the UCB. If a switch has opened, there
should be a voltage potential between the UCB ter-
minals, e.g. if LPS1 has opened, there will be a 24-
volt potential between the LPS1 terminals.
9. If 24 volts is present at the UCB Y1 terminal and
none of the protection switches have opened, the
UCB may have locked out the compressor for
repeat trips. The UCB should be flashing an alarm
code. If not, press and release the ALARMS button
on the UCB. The UCB will flash the last five alarms
on the LED. If the compressor is locked out, cancel
any call for cooling. This will reset any compressor
lock outs.
NOTE: While the above step will reset any lockouts,
the compressor may be held off for the ASCD.
See the next step.
10. If 24 volts is present at the UCB Y1 terminal and
none of the switches are open and the compressor
is not locked out, the UCB may have the compres-
sor in an ASCD. Check the LED for an indication of
an ASCD cycle. The ASCD should time out within
5 minutes. Press and release the TEST button to
reset all ASCDs.
11. If 24 volts is present at the UCB Y1 terminal and
the compressor is not out due to a protective
switch trip, repeat trip lock out, or ASCD, the econ-
omizer terminals of the UCB may be improperly
wired. Check for 24 volts at the Y1 “OUT” terminal