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9.0 DIAGNOSTICS & TROUBLESHOOTING
9.1 Checking Transducers & Temperature Sensors
TRANSDUCERS:
Checking transducers for accuracy can be tricky.
A technician will be required to do some voltage
reading and math to validate the transducer is
functioning properly. Additionally, comparing it to a
reliable source can also be a challenge. Unless the
gauges have been calibrated recently, the question
remains, which one is right?
Knowing where to take the measurement, and getting
solid reading is crucial. Measurements are done in
the 0-5 volt DC range. One confusing point may
be that we will take our measurements OUT and
IN to the control, which are reversed when we talk
formulas because we want the IN and OUT of the
transducer. For instance, 5VDC out of the control
board, translates to 5VDC into the transducer. And
the lower variable voltage output from the transducer
will be the input to the control board.
The transducer is not removed or disconnected to
make these checks. The technician's meter leads
need to be the smaller needle type, or actual
needles can be used to gain access to the points of
measurements.
Formulas for calculating gauge pressure are shown
below.
SUCTION LINE TRANSDUCER:
PSIG = 375 * (VDC out / VDC in) - 51.956
LIQUID LINE TRANSDUCER:
PSIG = 812.5 * (VDC out / VDC in) - 105.358
IMPORTANT: Do the division inside
the parenthesis first, muliplication second, and
subtraction last.
SUCTION LINE TRANSDUCER:
VDC out = 2.4
VDC in = 5.1
So...
PSIG = 375 * (2.4 / 5.1) - 51.956
PSIG = 375 * (0.47) - 51.956
PSIG = 176.25 - 51.956
PSIG = 124.294
LIQUID LINE TRANSDUCER:
VDC out = 3.4
VDC in = 5.1
So...
PSIG = 812.50 * (3.4 / 5.1) - 105.358
PSIG = 812.50 * (0.667) - 105.358
PSIG = 541.67 - 105.358
PSIG = 436.312
The pressures are best measured when the system is
off since they will be the most stable.
The indoor transducer may be removed so the
pressure can be checked with gauges at the actual
port the transducer is connected to. Pressures for
outdoor transducers can be checked at the outdoor
unit service ports with gauges since they are close
enough in proximity to the transducers.
With the system powered, use the smaller meter tips
to measure the voltage at the back of the transducer
harness where it plugs into the control board. Voltage
In will be measured from the Red and Black wires.
(Red is +, Black is -). This should be very close to
5VDC, but may vary by a few 1/10ths. Voltage Out
will be measured from Green to Black. (Green is the
variable +, Black remains -).
Diagnostics
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