Carrier 48FK Controls Operation And Troubleshooting

Carrier 48FK Controls Operation And Troubleshooting Manual pdf 5 page image

The potentiometer locations and functions are as follows:P1 — SUPPLY-AIR SET POINT — This potentiometer islocated on the display board. The supply-air set point is thecooling mode control temperature which the VAV controlsystem will attempt to maintain at Thermistor T1 by con-trol of economizer position and/or cycling unloaders andcompressors.P2 — ECONOMIZER POSITION — Economizer feedbackpotentiometer is located on the economizer motor. The micro-processor is programmed to indicate an alarm if the travelduring initialization is less than 10% of the total potentiom-eter’s resistance. An alarm condition will also be signaled ifthe potentiometer fails during operation, indicating that thedamper blades are stuck. If either situation occurs, the pro-cessor will try to drive the economizer dampers closed.P3 — RESET LIMIT — This potentiometer is located onthe accessory board (provided standard from the factory) inthe unit main control box and establishes the maximum amountof reset that can be applied to the supply-air set point (P1).Reset is limited by the P1 default of 70 F. This potentiom-eter is used only when accessory, field-installed temperaturereset is used. If temperature reset is used, DIP (dual, in-linepackage) switch 2 must be in the ON position.P4 — DEMAND LIMIT — This potentiometer is locatednear TRAN4 in the unit control box. The demand limit po-tentiometer is used only if accessory, field-installed demandlimit is used, and if DIP switch 5 is in the ON position. Forsingle-step demand limit, a field-installed 5 to 20 Kohm po-tentiometer and switch must be used.P5 — ECONOMIZER MINIMUM POSITION — This po-tentiometer is on the accessory board (provided standard fromthe factory) located in the unit main control box. This po-tentiometer specifies the minimum opening position for theoptional economizer. If a fault condition is detected by theprocessor, an alarm condition will be signaled and the econo-mizer dampers will close.P6 — WARM-UP SET POINT — This potentiometer is onthe accessory board (provided standard from the factory)located in the unit main control box. This potentiometerestablishes the set point temperature for the MorningWarm-Up function. When the temperature is reached, Morn-ing Warm-Up is terminated and VAV operation begins. DIPswitch 4 must be in the ON position if morning warm-upheat is to be used.P7 — SASP (SUPPLY AIR SET POINT) RESET TEM-PERATURE — This 10 Kohm potentiometer is used only ifthe accessory, field-installed temperature reset package is in-stalled. This potentiometer determines the temperature at whichreset will begin. It is located on the accessory temperaturereset board. DIP switch 2 must be in the ON position to en-able SASP reset.PROCESSOR BOARD OUTPUTS — The processor boardalso controls outputs through the relay board. The relay boardplugs into the processor board using a ribbon cable.In addition, the processor board controls the display board.The display board is connected to the processor board by aribbon cable, and has an LED (light-emitting diode) displayshowing the status of the unit and diagnostic information.CONFIGURATION HEADER AND DIP SWITCHASSEM-BLY — The processor board is programmed to control a va-riety of air conditioning units. To tailor the processor to theparticular unit being controlled, 2 devices are used. One isthe configuration header, and the other is the DIP switchassembly.The configuration header (part no. 30GB660001) is aseries of 8 small wires that are broken or unbroken in apattern to indicate several unique characteristics of the unit.The configuration header is factory set and should not bechanged. Changing the factory setting may cause the unit tomalfunction.The DIP switches configure the unit for several field-installed options, as well as for several other options thatmay be unique to the unit. The DIP switches are located un-der a plastic enclosure which must be removed for access.The switches can be field adjusted, but must be adjusted onlywhen the unit control circuit breaker is off.Relay Board — The relay board is used to control 24-vand 115-v loads. See Fig. 5. The relay board is connected tothe processor board by a ribbon cable at pin J9. Electricalconnections to the relay board are made through pins J5(115 v) and J6 (24 v). The relay board has eight 24-v relaysand five 115-v relays. See Table 3.Display Board — The display board is located in themain unit control box and is connected to the J10 port of theprocessor board through a ribbon cable. The display boardcontains the supply-air set point potentiometer P1; a 2-digit,LED display; and the display button (see Fig. 6). The LEDdisplay is used to convey the operating information and op-erational error codes.Thermistors — The processor uses up to 5 thermistorsto sense the temperatures at various points in the system.See Table 1 and Fig. 7-14. All the thermistors have identicaltemperature versus resistance and voltage drop characteris-tics, and are monitored by the processor for a short or opencircuit. The valid range for a thermistor is 362,640 to219 ohms. Thermistor details and locations are as follows:T1 — SUPPLY-AIR TEMPERATURE THERMISTOR —This thermistor is located in the unit supply fan discharge.It provides information for the processor to stage the num-ber of capacity steps required to maintain a desired supply-air temperature.T2 — RETURN-AIR TEMPERATURE THERMISTOR —This thermistor is located in the mixed-air portion of the unitcabinet. The thermistor’s primary function is to provide morn-ing warm-up information. This sensor will also provide dif-ferential information for the processor during cooling opera-tion (such as the rate of change for a capacity step).T3 — SATURATED CONDENSING TEMPERATURE, CIR-CUIT 1 — This thermistor is located on the condenser coilreturn bend. See Fig. 13 and 14. It controls the staging of theunit condenser fans based on the condensing temperature ofthe refrigerant at the designated position on the condensercoil.T4 — SATURATED CONDENSING TEMPERATURE, CIR-CUIT 2 — This thermistor is located on the condenser coilreturn bend. See Fig. 13 and 14. It controls the staging of theunit condenser fans based on the condensing temperature ofthe refrigerant at the designated position on the condensercoil.T10 — RESET TEMPERATURE — This thermistor is usedonly if the accessory temperature reset package is used. Itprovides occupied space temperature information to the pro-cessor, which determines whether or not reset is required.The thermistor is remotely mounted outside the unit in theconditioned space.5