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TM 5-4120-287-15
5-6.
No (or insufficient) Heating
Probable Cause
Probable Remedy
Temperature control variable resistor set below ambient.
Turn variable resistor knob toward WARM.
Refer to probable cause column of paragraph 5-4.
Refer to possible remedy column of paragraph 5.4.
Defective or damaged thermostat (S106).
Substitute new component in place of suspected faulty
thermostat (para. 6-6e).
Heater line contactor relay (K107) defective.
Checkline contactor relay; replace if defective (para. 6-
9c(4)).
Defective temperature control relay (K103).
Substitute new component in place of suspected faulty
thermo relay (para. 6-9c(3)).
Heater temperature switch (S104) defective
With ambient temperature below 125F, check terminal B
and D of plug P112 for continuity. Replace switch S104 if
defective. If heaters cut-out below 125F substitute new
component for S104 (para. 6-6d).
Evaporator blower not functioning, causing S104 to trip.
Check evaporator blower line contactor relay (K106);
replace if defective (para. 6-9c(4)).
Current limiting fixed resistor (R1) shorted, causing K103
coil to energize at excessively low temperatures—cutting off
heaters.
Check resistor R1 for open or short condition. Check for
350 ohms resistance across R1; replace resistor R1 if
defective (para. 6-10, and fig. 1-9).
5-7.
No (or insufficient) Cooling
Probable Cause
Possible Remedy
Condenser discharge door closed, or defective microswitch
(S101).
Open condenser door, make sure microswitch is operative;
replace a defective microswitch (para. 6-6a).
Condenser-subcooler assembly core and/or filter dirty.
Check condenser filter clean indicator; service as required
(para. 3-33 and 3-35).
Evaporator assembly core and/or filter dirty.
Check evaporator filter clean indicator; service as required
(para. 3-32 and 3-34).
Low refrigerant charge.
Check refrigerant charge level (para. 2-8c(1)). If
necessary, add refrigerant-12 to system (para. 7-5).
Condenser or evaporator motor internal overload relay
tripped.
Motor restart automatically when cooled. No action
necessary. If condition recurs frequently, determine cause
and correct. Test motors as necessary (para. 6-13).
Defective evaporator fan line contactor relay (K106)
Check line contactor relay. Replace if necessary (para. 6-
9c(4)).
Defective condenser fan line contactor relay (K106).
Check line contactor relay. Replace if necessary (para. 6-
9c(4)).
Defective compressor motor line contactor relay (K104).
Check line contactor relay. Replace if necessary (para. 6-
9c(4)).
Defective time delay relay (K102).
Check time relay (K102); replace if defective (para. 6-
9c(2)).
Current limiting fixed resistor (R1) open, preventing
temperature control relay (K103) coil from energizing.
Check R1 for open or short condition and for 350 ohm
value. Replace resistor R1 if defective (para. 6-10, and fig.
1-9).
Temperature control variable resistor defective.
Check R3 (fig. 1-9) for open or short condition and
resistance range. Replace if defective (para. 6-3a).
Defective or damaged thermostat (S105).
Replace thermostat (para. 3-39).
Defective motor compressor.
Refer to paragraphs 6-12c, 6-13, and 6-14c.
Defective condenser fan.
Refer to paragraphs 6-12b, and 6-14b.
Defective evaporator blower.
Refer to paragraphs 6-12a, 6-13, and 6-14a.
Defective refrigerant solenoid valve coil (L101) or no voltage
applied to coil.
Check coil (L101). Check for 28vdc across terminals N and
P of jack J108. Replace a defective coil (para. 6-6f).
Defective solenoid valve.
Check sight glass for refrigerant flow (para. 2-8c(1)).
Check to see if tubing near valve outlet is cooler than tubing
adjacent to inlet (indicating partially blocked valve) (para. 7-
8).
Low ambient switch (S102) defective, causing trip relay
K101 to maintain tripped condition.
Check lines 109 and 111 for open condition. Replace S102
if defective (para. 6-6b).
5-2
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