24VNA6 / 25VNA4: Service Manual
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
14
Fan
If verification of proper operation is required for the ECM motor used in
this unit, refer to
and follow these steps:
1. Remove fan harness at VFD
2. Check the resistance between connector pins according to the
expected resistances provided in
.
A200184
Fig. 12 – Fan Motor Connections
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.
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
A200045
Fig. 13 – 25VNA4 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
.
Table 4 – Variable Speed Compressor Resistances (ohm)
(winding resistance at 68°F /20°C)
WINDING
24
36
48
60
Between terminals
T1, T2, and T3
(U,V,W)
.74
0.453
0.424
0.424
Between terminal &
ground
>1 mega OHM
CAUTION
!
EQUIPMENT DAMAGE HAZARD
Failure to follow this caution may result in equipment damage or
improper operation.
Do not use Meggar for measuring the winding resistance.
Table 5 – 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 6 – 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