I have a bathroom ceiling fan with a light. The fan is controlled by a mechanical dial timer switch and the light is controlled by an on-off switch in the same wall box. The mechanical dial has been sticking, so I decided to replace it with this digital timer switch with minute buttons:
Inside the wall box, the in/out wires are both black, but the old timer switch was marked Line and Load, so I used those to determine which wire was which and connected Line to black and Load to red on the new timer switch. I also connected the ground wire in the box to both the green and white on the new timer switch, according to its instructions.
Now the new timer works as expected when the light switch is off. But when I turn the light on, the timer switch seems to power down -- its LEDs slowly dim and turn off, and its buttons don't do anything. When I turn the light back off, the timer switch goes through a restart sequence of blinking LEDs, then it works normally again.
In case the original timer was wired backwards, I reversed the wires connected to the new timer's red and black, but then it didn't turn on at all. I also tried reversing the wires connected to the light switch and that reversed the function of the timer, so it only worked when the light was on instead of only working when the light was off.
I don't understand why the light switch would affect the fan timer since they are independent functions on the fan unit and were working independently with the old timer. Maybe there is some kind of interference with the wiring inside the box? There is a lot of extra wire in there and it's packed in pretty tightly, but even with both switches pulled out and dangling by their wires, the same behavior occurs.
Inside the wall box, packed into the back, is also a lot of white wire with various twist cap connections between them. I hoped this was other room wiring that is being joined here inside the box and doesn't affect the fan and light; I thought I could just swap out one timer switch for another and not mess with the rest. But maybe this is more complicated than I thought.
Does anyone have an idea of what to do, before I give up and call an electrician?
Here's the wiring diagram from the instructions; I used option 2:
Since some of the comments have questioned the instructions regarding the neutral wire, I searched the product reviews and found this: "It uses the ground as neutral (!) to power the electronics inside the switch. That's pretty bizarre even if the current involved is minuscule." Given that, it does make sense that you would connect a neutral wire to the timer if available, or connect the ground wire to both the neutral and ground on the timer if not. Since the timer does function correctly with the light off, that part seems correct, but what happens when the light is turned on that apparently removes power from the timer?
Update and solution
As suggested by manassehkatz, I pulled everything out to see what was happening. At that point I remembered that the light switch not only controls the fan light but also a light above the sink. The switch box also seems to be acting as a junction box for an outgoing romex wire, probably leading to a half bathroom directly above this room. So that explains all the wires packed into there: one pair coming in from the basement, and four pairs going out to the fan (via the timer switch), the fan light (via the light switch), the sink light (also via the light switch) and the upstairs room (connected directly to the incoming wires).
Anyway, this also made it clear that the neutrals were indeed in there, all bundled together as manassehkatz described. I connected the timer switch to that bundle instead of the ground and then everything worked as expected.
So the problem is solved, but I'm still curious why this change made a difference, since the timer switch is supposed to work when the neutral wire isn't available in the box, and turning on the light shouldn't affect it. 🤷
I did take some pictures, but I couldn't untangle the black wires enough to make them easy to trace without running a finger along them.