LiftMaster 3840 3840 Manual - Page 32

Safety Reversing Sensor Diagnostic Chart 1 FLASH Safety reversing sensors wire open (broken or disconnected). OR 2 FLASHES Safety reversing sensors wire shorted or black/ white wire reversed. Diagnostics Located On Motor Unit "Learn" Button LED or Diagnostic LED "Learn" Button Installed Safety Reversing Sensor Your garage door opener is programmed with self-diagnostic capabilities. The "Learn" button/diagnostic LED will flash a number of times then pause signifying it has found a potential issue. Consult Diagnostic Chart below. Symptom: One or both of the Indicator lights on the safety reversing sensors do not glow steady. • Inspect sensor wires for a short (staple in wire), correct wiring polarity (black/white wires reversed), broken or disconnected wires, replace/attach as needed. • Disconnect all wires from back of motor unit. • Remove sensors from brackets and shorten sensor wires to 1-2 ft (30-60 cm) from back each of sensor. • Reattach sending eye to motor unit using shortened wires. If sending eye indicator light glows steadily, attach the receiving eye. • Align sensors, if the indicator lights glow replace the wires for the sensors. If the sensor indicator lights do not light, replace the safety reversing sensors. 3 FLASHES Door control or wire shorted. 4 FLASHES Safety reversing sensors slightly misaligned (dim or flashing LED). 5 FLASHES Possible RPM sensor failure. Unplug to reset. Symptom: LED is not lit on door control. • Inspect door control/wires for a short (staple in wire), replace as needed. • Disconnect wires at door control, touch wires together. If motor unit activates, replace door control. • If motor unit does not activate, disconnect door control wires from motor unit. Momentarily short across red and white terminals with jumper wire. If motor unit activates, replace door control wires. Symptom: Sending indicator light glows steadily, receiving indicator light is dim or flashing. • Realign receiving eye sensor, clean lens and secure brackets. • Verify door track is firmly secured to wall and does not move. Symptom: RPM Sensor = Short travel 6-8" (15-20 cm). • Unplug opener to reset. Try to operate motor unit, check diagnostic code. • If it is still flashing 5 times and motor unit moves 6-8" (15-20 cm), replace RPM sensor. 32