Carrier 48MPD Adjustments, Pressure Switch Settings Psig kPag, Belt Installation and Tensioning

Replace the ®lter cores whenever the moisture/liquid in- dicator shows moisture in the system.

Follow instructions on the packaging for the replacement cores for disassembling and reassembling the ®lter assem- blies and housings.

Liquid Line Service Valve Ð Located immediately ahead of the ®lter drier, this valve has a 1¤4-in. ¯are connec- tion for ®eld charging. With the liquid circuit shut, the com- pressor can be used to pump the refrigerant down into the high side. The refrigerant can then be stored there by closing the compressor discharge valve.

Compressor Suction and Discharge Service Valves Ð Each compressor is provided with suction and discharge service valves. With these valves, the compressor can be isolated from the refrigerant system for servicing or replacing.

High-Pressure Switch Ð This switch has nonadjust- able settings. Figure 60 shows connection on a cylinder head. See Table 31 for pressure switch settings.

Table 31 Ð Pressure Switch Settings,

Psig (kPag)

SWITCH	CUTOUT		CUT-IN
High	426	± 7	320	± 20
	(2935	± 48)	(2205	± 138)
Low	27	± 4	67 ± 7
	(186 ± 28)		(462	± 48)

NOTE: High-pressure switch must be removed from cylin- der head before removing compressor from the unit.

TO CHECK Ð Slowly close the discharge shutoff valve until the compressor shuts down. This should be at approx- imately 426 psig (2935 kPag). Slowly open the valve. When the pressure drops to approximately 320 psig (2205 kPag), the pressure switch resets. To reenergize the control circuit, manually switch the fan circuit breaker off and then on. The compressor starts again under Time Guard controls.

Low-Pressure Switch Ð The low-pressure switch (LPS) has ®xed nonadjustable settings. It is located at the pump end of the compressor above the bearing head. See Table 31 for pressure switch settings.

TO CHECK Ð Slowly close the suction cut-off valve and allow the compressor to shut down. This should occur at approximately 27 psig (186 kPag). Slowly open the valve. The compressor restarts under Time Guard control when the pressure builds to approximately 67 psig (462 kPag).

Pressure Relief Ð High-side pressure relief is provided by a fusible plug in the liquid line at the serv- ice valve. For low-side pressure relief, a fusible plug is inserted in the side of the accumulator. See Fig. 60. A pres- sure relief valve installed on the compressor relieves at 450 psig (3102 kPag).

Adjustments

EVAPORATOR FAN AND POWER EXHAUST MOTOR PLATE Ð Adjust using a 15¤16-in. wrench on the adjusting bolts:

1.Loosen holddown bolts. (See Fig. 61.)

2.Turn the adjusting bolts to move the motor mounting plate toward or away from the fan to loosen or tighten the belts. Make the same number of turns to each bolt.

3.Retighten holddown bolts.

BELT INSTALLATION AND TENSIONING

IMPORTANT: When installing or replacing belts, al- ways use a complete set of new, matched belts to pre- vent potential vibration problems. Mixing belts often results in premature breakage of the new belts.

1.Turn off unit power.

2.Adjust motor plate so belts can be installed without stretch- ing over the grooves of the pulley. (Forcing the belts can result in uneven belt stretching and a mismatched set of belts.)

3.Before tensioning the belts, equalize belt slack so that it is on the same side of the belt for all belts. Failure to do so may result in uneven belt stretching.

4.Tighten belts using the motor plate adjusting bolts.

5.Adjust until proper belt tension (1/2-in. de¯ection with one ®nger) is obtained. Be sure to adjust both adjusting bolts the same number of turns.

NOTE: Check the tension at least twice during the ®rst day of operation, as there is normally a rapid decrease in tension until the belts have run in. Check tension periodically thereafter and keep it at the recommended tension.

With the correct belt tension, belts may slip and squeal momentarily on start-up. This slippage is normal and dis- appears after wheel reaches operating speed. Excessive belt tension shortens belt life and may cause bearing and shaft damage.

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