The LiftMaster 41A6104 Cable Tension Monitor is a dedicated safety sensor for residential wall‑mount jackshaft garage door openers. It continuously monitors lift‑cable tension; if slack or a broken cable is detected, the sensor signals the operator to stop or inhibit movement, helping prevent the door from traveling under unsafe conditions. This wired safety switch is required for normal operation on compatible side‑mount systems and is designed to work as a low‑voltage input to the opener’s control board.
Commonly referenced as 041A6104, this replacement part is used with LiftMaster residential jackshaft models such as 3800, 8500, and 8500W, and is also installed on comparable Chamberlain RJO wall‑mount units (e.g., RJO20). During installation, the unit mounts near the cable drum/track and the lift cable is routed through the sensor so it can detect loss of tension; the sensor’s lead then connects to the opener’s dedicated cable tension monitor terminals per the opener manual. Intended for torsion‑spring, residential applications, it must be correctly positioned and wired for the operator to run. For safety, do not bypass the sensor, disconnect power before servicing, and follow the specific model instructions for mounting location, wiring, and testing.