Incorrect Potentiometer Setting Ð A potentiometer turned fully clockwise or counterclockwise is outside the valid range and will result in a failure.
Faulty Wiring Ð If the wiring between the potentiometer and the processor board is incorrect, a failure will result.
Potentiometer Failure Ð If potentiometer is shorted or open, a failure will occur.
CODE 85: DEMAND LIMIT POTENTIOMETER (P4) FAIL- URE Ð Used only if demand limit is being used. If demand limit is used, DIP switch 5 must be in the ON position.
Two types of demand limit are available: a
For both types of demand limit, the control uses only 80% of the total potentiometer resistance. If resistance of poten- tiometer is less than 10% or greater than 90%, alarm light
will be energized, a diagnostic code of 85 will be displayed
when the display button is pushed, and demand limit will be terminated. If a failure occurs, it is probably due to one of the following:
Potentiometer Failure Ð If a potentiometer is shorted or open, a failure will occur.
Incorrect Potentiometer Setting Ð A potentiometer turned fully clockwise or counterclockwise will put potentiometer out of range resulting in an error.
Faulty Wiring Ð If wiring between the potentiometer and the processor board is incorrect, an error will occur.
DIP Switch 5 Ð If DIP switch 5 is in the ON position and potentiometer is not installed, an error will occur.
CODE 86: MINIMUM POSITION ECONOMIZER PO- TENTIOMETER FAILURE Ð If potentiometer P5 (on ac- cessory board) setting is less than 0% or greater than 100%,
alarm light will be energized, a code of 86 will be dis-
played when display button is pushed and economizer out- door air dampers will move to the fully closed position.
The potentiometer
This failure will automatically reset when potentiometer returns to normal.
If a failure occurs, one of the following is the probable cause:
DIP Switch 3 Ð If this switch is in the ON position and the accessory board is not installed (accessory board is standard on these units, so it should always be on the unit).
Incorrect Potentiometer Setting Ð If potentiometer is turned fully clockwise or counterclockwise, potentiometer will be out of the allowable range, and an error will result.
Faulty Wiring Ð If wiring between the potentiometer and the processor board is incorrect, an error will occur.
Potentiometer Failure Ð If potentiometer is shorted or open, potentiometer will be out of range and an error will result.
CODE 87:
potentiometer (P6) is located on the accessory board. If po- tentiometer is set at less than 0° F or more than 95 F, alarm
light will be energized, a diagnostic code of 87 will ap- pear on the display when display button is pushed, and con- trol will use a default value of 40 F. If setting is between 0° F and 40 F, control will use a value of 40 F but no di- agnostic code will be displayed; if setting is between 80 F and 95 F, control will use a value of 80 F but no diagnostic code will be displayed.
The potentiometer
The failure will automatically reset once potentiometer re- turns to normal. If a failure occurs, one of the following is the probable cause:
DIP Switch 4 Ð If this switch is in the ON position and the accessory board is not installed (accessory board is standard on these units, so it should always be on the unit).
Incorrect Potentiometer Setting Ð If potentiometer is turned fully clockwise or counterclockwise, potentiometer will be out of the allowable range, resulting in an error.
Faulty Wiring Ð If the wiring between the potentiometer and the processor board is incorrect, an error will occur.
Potentiometer Failure Ð If potentiometer is shorted or open, potentiometer will be out of range, resulting in an error.
Thermistor Troubleshooting Ð The VAV control sys- tem uses thermistors to measure temperatures of the enter- ing and supply air, as well as the saturated condensing tem- peratures of the refrigerant circuits. The resistance versus temperature and electrical characteristics for all thermistors in the system are identical. To obtain an accurate reading, a
Thermistors in the VAV control system have a 5 vdc sig- nal applied across them any time the unit control circuit is energized. The voltage drop across the thermistor is di- rectly proportional to the temperature and resistance of the thermistor.
To determine temperatures at the various thermistor lo- cations, disconnect the thermistor from the processor board and measure the resistance across the appropriate thermistor using a
The microprocessor has been programmed to check the operation of the thermistors. If the measured temperature is outside of the range of −24 to 225 F or 98,010 to 282 ohms, then it will be treated as a sensor failure and a diagnostic code will be displayed. See Table 17 for sensor temperatures versus resistance drop. It is also possible to check the op- eration of the thermistors using the quick test routine.
If a thermistor has failed or the wire is damaged, replace the complete assembly. Do not attempt to spice the wires or repair the assembly.
Electronic Controls Checkout Ð The following will help determine whether a processor board, a relay board, dis- play set point board, accessory board, or
Before checking out any board, do the following:
1.At initial
2.If system has been operating and a malfunction occurs, check display for diagnostic codes. Use diagnostic chart located on inner panel of access door to control box sec- tion of unit; this chart will help determine probable cause of failure.
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