Chosen Solution
After using trimmer head came into contact with water. It came on and will not turn off.
Because the design is bad. Likely a floating input pin to the microcontroller. I disassembled it and found out that just by touching the switch solder pads, it can turn itself on or off. I went through two of these things and both had the same behavior. I don’t want to reverse engineer it, so I will likely just redesign the whole electronics.
thank you gerry bp. that works, and i agree, i have a wahl 9818L. experienced the exact issue. will not turn off after shaving with water. i removed the trianglular ON/OFF pad gently, ( if you remove on side and gently lift it up, you will not break the plastic clear switch away), then wipe the inside with a q-tip. press the on/off switch with a firm pointed object. it should turn off and then repeat for on. i didn’t want to seal the switch but, i might have to place a breathable water resistant cover over the switch to prevent this from re-occuring. oh, i meant over the top of - not under the switch.
I haven’t the slightest clue why. Must be something to do with the moisture on the switch and electrical conductivity. plugging it in and taking it out may override the connection. However, this is iffy, disassembling and re-assembling unit did not help either. seems that it will correct itself after drying out.
