York DM120 Installation Manual download pdf (Page 77)

264973-YIM-A-0806

Unitary Products Group 77

COOLING TROUBLESHOOTING GUIDE

On calls for cooling, if the compressors are operating but the

supply air blower motor does not energize after a short delay

(the room thermostat fan switch is in the “AUTO” position):

1. Turn the thermostat fan switch to the ON position. If the

supply air blower motor does not energize, go to Step 3.

2. If the blower motor runs with the fan switch in the ON

position but will not run after the first compressor has

energized when the fan switch is in the AUTO position,

check the room thermostat for contact between R and G

in the AUTO position during calls for cooling.

3. If the supply air blower motor does not energize when

the fan switch is set to ON, check that line voltage is

being supplied to the contacts of the M3, contactor, and

that the contactor is pulled in. Check for loose wiring

between the contactor and the supply air blower motor.

4. If M3 is pulled in and voltage is supplied to M3, lightly

touch the supply air blower motor housing. If it is hot, the

motor may be off on internal protection. Cancel any

thermostat calls and set the fan switch to AUTO. Wait for

the internal overload to reset. Test again when cool.

5. If M3 is not pulled in, check for 24 volts at the M3 coil. If

24 volts are present at M3 but M3 is not pulled in,

replace the contactor.

6. Failing the above, if there is line voltage supplied at M3,

M3 is pulled in, and the supply air blower motor still does

not operate, replace the motor.

7. If 24 volts is not present at M3, check that 24 volts is

present at the UCB supply air blower motor terminal,

“FAN”. If 24 volts is present at the FAN, check for loose

wiring between the UCB and M3.

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 follow-

ing:

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, and

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 con-

nected to the G terminal of the UCB, and for loose wir-

ing.

10. If the thermostat and UCB are properly wired, replace

the UCB.

On calls for cooling, the supply air blower motor is operating

but compressor #1 is not (the room thermostat 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 oper-

ate. If both stages of cooling are requested simulta-

neously and the economizer provides free cooling,

following a short delay compressor #1 will be energized

unless it is locked out. If compressor #1 is locked out,

compressor #2 is energized. Compressor #2 is always

energized in place of compressor #1 when compressor

#1 is requested but locked out.

2. If no economizer is installed or the economizer is not

opening to provide free cooling and compressor #1 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 contac-

tor and the compressor.

3. If M1 is pulled in and voltage is supplied at M1, lightly

touch the compressor housing. If it is hot, the compres-

sor may be off on inherent protection. Cancel any calls

for cooling and wait for the internal 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 operate,

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, and

c. Loose wiring from the room thermostat to the UCB

8. If 24 volts is present at the UCB Y1 terminal, the com-

pressor may be out due to an open high-pressure 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 terminals, e.g. if LPS1 has opened,

there will be a 24-volt potential between the LPS1 termi-

nals.

9. If 24 volts is present at the UCB Y1 terminal and none of

the protection switches have opened, the UCB may have

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York DM120 Installation Manual Page 77

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