PTC START THERMISTOR MEASUREMENT PROCEDURE

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StepExpected Result/Action
29. Turn power
off. Use the capacitor discharge tool (Figure SP-8-6) to discharge
citor and any other capacitors in the unit.
Power is turned off and the unit capacitors are discharged.
30. Locate the wiring between the compressor start thermistor and the run
capacitor, then disconnect and remove the start thermistor from the circuit.
(Refer to the unit wiring diagram.)
The start thermistor is disconnected and removed from the unit in preparation for
the resistance measurement.
31. Set up the VOM/DMM to measure resistance on the R x 100 ohm scale.
NOTE: Before attempting to measure the resistance, wait at least 10 minutes
to allow the PTC thermistor to cool to ambient temperature.
If measuring a 25-ohm PTC thermistor, a factory-supplied jumper is across
the two end terminals. Connect the VOM/DMM across the center tab and end
tab as shown in Figure SP-9-23. Measure the resistance.
If measuring a 50-ohm PTC thermistor, connect the VOM/DMM across the
end tabs as shown in Figure SP-9-23. Measure the resistance.
The cold (ambient temperature) resistance of a good PTC thermistor should be
about 100% to 180% of the stated resistance (25 ohms or 50 ohms). For
example:
A 25-ohm thermistor should measure between 25 and 45 ohms
A 50-ohm thermistor should measure between 50 and 90 ohms
If the resistance of the PTC thermistor is more than 180% higher than its rating,
replace the thermistor.
If the thermistor resistance is within the acceptable range and the compressor
does not start, leave the thermistor disconnected from the circuit. Start the
compressor by giving it a temporary capacitance boost as directed in steps 21
through 23. Run the compressor for about 10 minutes, then turn off the power to
the compressor. Let the compressor sit idle for about 5 minutes to allow time for
the system pressures to equalize. Use the capacitor discharge fool (Figure SP-8-6)
to discharge the run and temporary start capacitors. Make sure the temporary
start capacitor is disconnected from circuit and reconnect the start thermistor in
the circuit. Proceed to step 32.
32. Turn on power to start the compressor.Compressor starts.
If the compressor does not start after two attempts, turn off the power to the
unit. Use the capacitor discharge tool (Figure SP-8-6) to discharge the run and
any other capacitors in the unit. Remove the start thermistor. Install a permanent
start capacitor and start relay supplied in a manufacturer's recommended start
kit.

pic1 15 PTC START THERMISTOR MEASUREMENT PROCEDURE

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