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Diagnostics
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) - 8.1
LIQUID LINE TRANSDUCER:
PSIG = 812.5 * (VDC out / VDC in) - 51.85
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) - 8.1
PSIG = 375 * (0.47) - 8.1
PSIG = 176.25 - 8.1
PSIG = 138.75
LIQUID LINE TRANSDUCER:
VDC out = 3.4
VDC in = 5.1
So...
PSIG = 812.50 * (3.4 / 5.1) - 51.85
PSIG = 812.50 * (0.667) - 51.85
PSIG = 541.67 - 51.85
PSIG = 460.40
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 -).
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