1. The motor that elevates the fan is turning a tad to much. Just by stepping on the floor near it could cause it to go up/down constantly.
2.Or what i had seen was the wires that go to the switch dont have enough slack, and are just making contact to the switch causing it to think somone is pushing the button constantly.
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This sounds like something has got into the track area and is setting off the safety. they are designed to stop raising or lowering if they meet any resistance and anything in the track area will cause it to happen in both directions
Gail was spot on. That worked for me until the unit would go up and/or down on its own. The problem was traced to the double microswitch called the 'Lower Switch Bracket Assembly' ($24. thru Lowes ). To replace these switches is not for the faint of heart as it is deep within the assembly. A good weekend mechanic can look at the exploded parts diagram and see what / where. Found grease residue on- and slightly within- the microswitchs due to Broan not properly design shielding and protecting these critical switches from grease. Did not find any ghosts, however.
In my case, I pushed the button, the housing goes fully up, then right back down. I removed and disassembled the unit for cleaning. After Assembly I located two microswitches that cause the extension moror to stop at the top and at the bottom of the swing. In my case, the inertia of the motor caused the unit to "Coast" thru the top of the stroke and continue back down. I bent the outer 1/3 of the metal tabs that depress the microswitches so that the unit stopped rising a wisker sooner and no longer goes back down without pushing the button. The same coasting theroy may apply at the bottom of the stroke. The motor that runs the up and down motion is controlled by three switches, the two limit switces plus the momentary push button switch. If you don;t need to completely remove your unit for cleaning, the microswitchs can be located by removing the fan housing and looking up from inside the cabinet (under the stove) on the right side. IN my case, there was 10 years of goop to be removed.
hood should be connected to a fused spur outlet which you havent found. try removing the chimney and you may find it there. chimney only held by 2 screws at top usually.the fuse in the outlet sometimes goes when bulbs blow
The Problem is the wiring. Take the 2 screws retaining the metal plaque off underneath the filters. You will see the wires leaving the motor going into a "molex" plastic connector which connects two another set of wires going to the control board. Check to see if the wiring colors match from side to side of plastic connector. Ive only seen it happen once but it was exactly that. I do service calls for broan & Nutone