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Units with the factory installed low ambient option are equipped
with a solid-state head pressure control which regulates fan speed.
A temperature sensor, mounted on either circuit 1 (38AUZ units)
or both circuits 1 and 2 (38AUD units) of the outdoor coil controls
the speed of approved outdoor fan motors in order to maintain a
constant head pressure in the outdoor coil (see Fig. P-R). The con-
trol maintains the appropriate head pressure at low ambient tem-
peratures down to –20°F (–28°C).
Wind baffles are required to prevent wind cross currents from
causing abnormally low condensing temperatures.
IMPORTANT: Wind baffles are field-fabricated; they are
not included with the factory-installed Motormaster control
option.
Use 20-gauge sheet metal to fabricate wind baffles (see Fig. S and
Table A). See Fig. T-V for finished installation.
OPERATION
Fan on/off control in cooling-only units (38AUZ, 38AUD) is pro-
vided by an outdoor fan relay (OFR) or two outdoor fan relays
(OFR1 and OFR2) for 38AUD 28. In cooling mode, fan motor
speed of outdoor motors OFM1 and OFM3 is regulated by the
speed control temperature sensor on outdoor coil 1 for a minimum
coil condensing temperature of approximately 100°F (38°C) at
higher outdoor ambient temperature and 80°F (27°C) at lower am-
bient. Additionally, outdoor fan motor OFM2 and OFM4 are
turned on/off by the low ambient temperature switch (LAS), oper-
ating the low ambient relay (LAR). The LAS control temperatures
are open 55°F ±2.16°F, close 60°F ±2.16°F (open 12.8°C ±1.2°C,
close 15.6°C ±1.2°C).
To override the speed control for full fan speed operation during
service or maintenance, either:
a. remove sensor and place in hot water >120°F (>49°C).
or
b. rewire to bypass control by connecting speed control
input and output power wires.
APPENDIX C — LOW AMBIENT OPTION
55
Troubleshooting
OBSERVATION
Fans won't start.
Cooling — Center outdoor
fans (OFM2, OFM4) off
below approximately 55°F
(16°C) outdoor ambient.
Cooling — Center outdoor
fans (OFM2, OFM4) not on
above approximately 60°F
(12.8°C) outdoor ambient.
Cooling — Slow fan speed
for outer fans (OFM1,
OFM3) at start or during low
outdoor ambient
Cooling — Slow fan speed
for outer fans (OFM1,
OFM3) above 85°F (29°F)
outdoor ambient (should be
full speed)
Cooling — motor current into
speed control is greater than
motor nameplate FLA
Speed Control Sensor Resistance
TEMPERATURE
°F ± –2°F
°C ± –1°C
–22
–4
14
32
50
68
77
86
104
122
140
158
POSSIBLE REMEDY
All fans:
Check power and wiring
Check outdoor fan relay (OFR)
OFM1, OFM3 only:
Check speed control sensor location
Check speed sensor resistance
OFM2, OFM4 only:
Check low ambient switch (LAS)
Check low ambient relay (LAR)
Normal operation
Check low ambient switch (LAS)
Check low ambient relay (LAR)
Normal operation
Check speed control sensor location
Check speed control sensor resistance
Check fan motor capacitor
Normal operation
Up to 30% higher A at partial speed at
low ambient
RESISTANCE
Ohms, nominal
–30
88,350
–20
48,485
–10
27,650
0
16,325
12
9,950
20
6,245
25
5,000
30
4,028
40
2,663
50
1,801
60
1,244
70
876
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