035-18940-000-B-1003
46
Unitary Products Group
4. If M2 is not pulled in, check for 24 volts at the M2
coil. If 24 volts is present and M2 is not pulled in,
replace the contactor.
5. Failing the above, if voltage is supplied at M2, M2
is pulled in, and the compressor still does not oper-
ate, replace the compressor.
6. If 24 volts is not present at M2, check for 24 volts at
the UCB terminal, C2. If 24 volts are present,
check for loose wiring between C2 and the com-
pressor contactor.
7. If 24 volts is not present at the C2 terminal, check
for 24 volts from the room thermostat at the UCB
Y2 terminal. If 24 volts is not present from the room
thermostat, check for the following:
a. 24 volts at the thermostat Y2 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 Y2 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 HPS2, LPS2, and FS2
terminals of the UCB. If a switch has opened, there
should be a voltage potential between the UCB ter-
minals, e.g. if LPS2 has opened, there will be 24
volts of potential between the LPS2 terminals.
9. If 24 volts is present at the UCB Y2 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 a 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, remove any
call for cooling at the thermostat or by disconnect-
ing the thermostat wiring at the Y2 UCB terminal.
This will reset any compressor lock outs.
NOTE: While the above step will reset any lock outs,
compressor #1 will be held off for the ASCD,
and compressor #2 may be held off for a por-
tion of the ASCD. See the next step.
10. If 24 volts is present at the UCB Y2 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. The UCB can be programmed to lock out compres-
sor operation during free cooling and in low ambi-
ent conditions. These options are not enabled by
default. Local distributors can test the UCB for this
programming.
12. If none of the above corrected the error, test the
integrity of the UGB. Disconnect the C2 terminal
wire and jumper it to the Y2 terminal. DO NOT
jump the Y2 to C2 terminals. If the compressor
engages, the UCB has faulted.
13. If none of the above correct the error, replace the
UCB.
On a call for cooling, the supply air blower motor and
compressor #2 are operating but compressor #1 is not
(the room thermostat fan switch is in the “AUTO” posi-
tion).
1. Compressor #2 is energized in place of compres-
sor #1 when compressor #1 is unavailable for cool-
ing calls. Check the UCB for alarms indicating that
compressor #1 is locked out. Press and release
the ALARMS button if the LED is not flashing an
alarm.
2. 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 is 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.
