44
McQuay OM 751
Diagnostics and Service
Space Humidity Sensor Failure (optional) (
FA
)
The Space Humidity Sensor Failure fault occurs when the UVC detects open or short
conditions from the sensor.
Effect:
•
IA/OA Enthalpy comparison economizer (if used) is disabled.
•
Dehumidification function (optional) is disabled.
•
Fault is indicated.
Outdoor Humidity Sensor Failure (optional) (
Fb
)
The Outdoor Humidity Sensor Failure fault occurs when the UVC detects open or short
conditions from the sensor.
Effect:
•
IA/OA Enthalpy comparison or OA Enthalpy economizer (if used) is disabled.
•
Fault is indicated.
Space CO
2
Sensor Failure (optional) (
FC
)
The Space CO
2
Sensor Failure fault occurs when the UVC detects open or short conditions
from the sensor.
Effect:
•
CO
2
Demand Controlled Ventilation function is disabled.
•
Fault is indicated.
Change Filter Indication (
FF
)
The Change Filter Indication fault occurs when the UVC calculates that the total fan run time
has exceeded the allowed number of hours since the last filter change.
Effect:
•
Fault is indicated.
EPROM Memory Indicator (
EE
)
The EPROM Memory Indicator occurs when an unusual electrical event has scrambled the
EPROM memory within the controller board. In the event that this happens, the controller
board must be replaced.
Configuration Display (
--
)
The Configuration Display occurs when the display file “**.cfg” is incorrect or has not been
downloaded with the appropriate file from service tools.
Troubleshooting Temperature Sensors
The UVC is configured to use passive positive temperature coefficient (PTC) sensor whose
resistance increases with increasing temperature. The element has a reference resistance of
1035 ohms at 77°F (25°C). Each element is calibrated according to the tables shown.
Use the following procedure to troubleshoot a suspect sensor.
1
Disconnect both sensor leads from the UVC.
2
Using some other calibrated temperature sensing device, take a temperature reading at the
sensor location.
