Section 2 – SR-2 Hardware Description
UNIT SENSORS
The sensors monitor air temperatures at various
points in the system, as well as the engine oil
pressure, engine oil level, engine coolant level,
engine coolant temperature, engine speed,
alternator frequency and fuel level. Refrigerant
pressures are also monitored on ETV units.
Sensors are connected to the microprocessor via
the J3 Sensor Connector.
AIR TEMPERATURE
SENSORS
Graded and Un-graded Air Temperature
Sensors
The dual Return Air and Discharge Air sensors are
graded sensors. Sensor grading allows maximum
accuracy without the need for ice water calibration.
The sensor grade (from 1L through 9H) is stamped
on the sensor and must be entered into the
microprocessor when a sensor is changed, in order
to properly calibrate the sensor for accurate
temperature readings. If the grade is not changed
from the factory setting of 5H, then Alarm Code 92
Sensor Grades Not Set will occur. Always update
the sensor grade nameplate when graded sensors
are changed. The nameplate is mounted on the
side of the control box.
Un-graded sensors are used to measure the
evaporator coil temperature and ambient
temperature, since these temperatures are not as
critical as the return and discharge air temperatures.
Do not replace a graded sensor with an un-graded
sensor.
Dual Sensors
Dual temperature sensors are provided for both
return air temperature and discharge air
temperature. The sensors are located next to each
other to insure common readings. One sensor is the
control sensor and the other sensor is the display
sensor. The return and discharge control sensors
are used for unit control. The return and discharge
display sensors are used by the HMI control panel to
display the temperature.
Dual Sensor Changes
Manufacturing changes were made by the sensor
manufacturer during the first quarter of 2004. Due to
these changes, the sensor readings of the new style
and old style sensors change at a different rate over
time. Since the sensors are compared to each
other, using one old style sensor and one new style
sensor can create nuisance alarm codes when the
temperature is changing rapidly, such as during a
pull-down or pull-up condition. For this reason, both
sensors installed in a dual sensor position (dual
return air sensors or dual discharge air sensors)
must both be either new style or old style sensors.
See Service Procedure D01A for complete details.
2-21
31 January 2005
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