GE PROFILE PDE9900 Technical Service Manual

– 23 –Turbidity SensorThe turbidity sensor is located on the side of thesump.The turbidity sensor measures the amount ofsuspended particles in the wash water in the sump.The control sends the turbidity sensor a pulse widthmodulated 5-volt signal for calibration and usageduring operation.The control then receives an analog signal ofthe sensed turbidity, which is processed by themicroprocessor.The baseline reading is taken during the first fillwhen the sump water level is between the 1/ 4- to3/ 8-in. gap between the LED transmitter and thereceptor. Successive turbidity measurementsare supplied to the control module and used tode ter mine whether any prewash or rinse cycles canbe skipped. The sensing LED is on during all prewashcycles and during the final rinse.Decisions are based on a compar i son of cleanwater measurements at the beginning of the firstfill, measurements taken at selected fills, and watertemper ature. By measuring the turbidity level, thecontrol module can conserve energy on lightlysoiled loads by skipping unnecessary cycles.Note: If the turbidity sensor circuit fails to open oris shorted, the sensing LED on the control panel willnot light, and the unit will operate for the maximumamount of time, using the maximum number ofwash and rinse fills for the selected cycle.The turbidity sensor also contains the thermistorfor automatic temperature controlTurbidity SensorTestTurbidity SensorKeyThe thermistor’s resistance has a negativetemperature coefficient. As the temperatureincreases, the resistance goes down. At 75°F, theresistance is approximately 9.9K Ω. At 140°F, theresistance is approximately 2.8K Ω.Note: When installing the turbidity sensor, align thekey on the sensor with the keyway on the sump.The Service Mode is the most accurate way to testthe turbidity sensor circuit. The turbidity sensorcircuit contains the control module, wiring, and thetur bid i ty sensor.Turbidity Sensor Strip CircuitCalibrating the Turbidity SensorWhen replacing the turbidity sensor, or if it has beendisconnected, always calibrate the turbidity sensorto the control board.Note: The calibration cycle MUST be entered within2 minutes of power up.Press the Normal pad (the up arrow pad on TopControl Models) and the Cookware pad (the downarrow pad on Top Control Models) for 3 seconds oruntil all LEDs turn on.The dishwasher will then perform a series of steps.Once the drain cycle starts, calibration is completeand the Start/Reset pad can be pressed to end thecalibration cycle. The dishwasher will continue topump out remaining water and cycle of.NTC1234YELLOW/BLKDK. BLUEGRAYORANGE