186CNV / 284ANV: Service Manual
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
13
Thermistors
The equipment uses four negative temperature coefficient (NTC)
thermistors. As temperature of the sensor increases, its resistance
decreases. The OAT, OCT and OST thermistors are nominally
10k-ohms at 77°F (25°C). The ODT thermistor is nominally 50k-ohms
at 77°F (25°C). The PCM provides separate Open and Shorted
diagnostics for each thermistor to indicate if the detected resistance goes
outside of the normal sensing range.
Table 5 provides a reference of resistance vs. temperature as a
troubleshooting reference. Note that some of the thermistors may be at a
temperature significantly different than ambient temperature if the
equipment was recently operating. This is particularly true for ODT
which could be much warmer than ambient and have a resistance reading
lower than those shown in the table. The ODT thermistor will have a
resistance as low as 2.2k-ohms if the discharge tube is near the
compressor discharge operating limit at 240°F.
Outdoor Ambient Temperature (OAT) Thermistor
The OAT thermistor is located at the bottom of the control box with the
sensing element protruding through the bottom of the control box to be
in contact with the outside air. The OAT thermistor location is illustrated
in Fig. 12.
A200045
Fig. 12 – Outdoor Air Thermistor (OAT) Attachment
(on control panel)
If the OAT sensor fails, compressor operating speed may be limited to a
reduced range of operation. When OAT is faulted and the equipment is
running in heating mode, the reported value of OAT will be the same as
OST. In all other conditions when OAT is faulted, the reported value of
OAT will be the same as OCT.
Outdoor Coil Temperature (OCT) Thermistor
The OCT thermistor provides the coil/liquid line temperature to the
PCM where it is used primarily for defrost control. The OCT thermistor
is located on an extension tube on the outdoor coil. The sensor must be
securely mounted to the location illustrated in Fig. 13.
A200046
Fig. 13 – Outdoor Coil Thermistor (OCT) Attachment
(On Coil Tube Extension)
If the OCT sensor fails, defrost will occur at a fixed 60-minute interval if
operating in Auto mode, and the defrost cycle will run for the maximum
allowable time. When OCT is faulted and the equipment is running in
heating mode but defrost is not running, the reported value of OCT will
be the saturated suction temperature. In all other conditions when OCT
is faulted, the reported value of OCT will be the saturated discharge
temperature.
Outdoor Discharge Temperature (ODT) Thermistor
The ODT thermistor is used for determining discharge superheat and
protecting the compressor from over-temperature operation. It must be
secured tightly to the discharge tube and aligned parallel to the tube axis
(see Fig. 14).
NOTE: ODT must be mounted inside of the sound blanket to reduce the
influence of ambient temperature and provide accurate tube temperature.
Table 4 – DC Voltage and PWM Measurement
Vdc-GND Vcc-GND Vsp-GND FG-GND
Pin 1 (3 pin) &
Pin 1 (4 pin)
Pin 2 (4 pin) &
Pin 1 (4 pin)
Pin 3(4 pin) &
Pin 1 (4 pin)
Pin 4 (4 pin) &
Pin 1 (4 pin)
1.36 meg OHM 45.4 meg OHM 115k OHM Open
Table 5 – Thermistor Resistance (k-ohms) vs. Temperature
Temp (F) OAT-OCT-OST ODT
0 85 430
10 62 310
20 46 230
30 35 170
40 26 130
50 20 99
60 15 77
70 11.9 59
80 9.3 46
90 7.3 37
100 5.8 29
110 4.7 23
120 3.8 19
EQUIPMENT DAMAGE HAZARD
Failure to follow this caution may result in equipment damage or
improper operation.
In order to minimize the ambient influence, make sure the discharge
thermistor curved surface hugs the pipe surface and is secured tight
using the wire tie fished through the original slot insulating polymer
body.
To Next Page
Loading...
