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Wiring up a new bathroom vent fan. We're trying to pull power from the line coming into the bathroom light switch, and just adding a second switch that will control the fan. My house is from the 1940's and the current light switch only has two wires coming into it - black and white. I can't figure out why the wiring setup in this picture isn't working. Hooked up as such, the light still works as it used to, but the fan isn't coming on at all. What am I missing?

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    Please edit your question to show us clear, focused, well lit pictures of the actual wiring behind the light switch. Turn off the breaker, pull the cover plate and unscrew the switch from the wall. DO NOT disconnect any existing wiring. The question is whether the always hot power comes into the switch, or if it comes into the light. On a house built in the 40's there's a very good chance that the power goes directly to the light and you cannot do what you want with the existing wiring. We can walk you through the process but need to know which way your house is actually wired.
    – FreeMan
    Feb 25 at 16:28
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    When talking about cabling a house you cannot draw diagrams like electronic schematics. You have to show cable pairs (black/white) or with /3 or /4 cables, show whatever they are, all running together from one junction box to another (light, fan, switch, etc) and in each box show what's connected to what. Usually in an old house the challenge is to do what you want without breaking walls and ceilings. You haven't addressed that issue in your question or diagram. Are you cutting open walls and ceiling to pull cable wherever needed?
    – jay613
    Feb 25 at 17:11
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    OP STOP and learn how to make these connections before you cause damage or injury. The line hot (always hot) and its accompanying neutral will be in the same cable if the wiring is at all modern. The line hot and its accompanying neutral can enter the circuit in the switch box or in the (ceiling?) box for the light. It will help (and might be necessary) for you to determine which case you have. Feb 25 at 17:49

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It looks like you made the novice mistake of splicing like colors together without identifying the wires.

Importantly, all switched wires are hot wires. So by taking the 2 hot wires from one switch and connecting them both to the fan, you've got exactly zero voltage across the fan.

This simply won't work.

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    OP may think all W wires are neutrals. And he may not realize that W wires connected to ordinary switches are either always hot or are switched hot. So far he has not made the mistake of connecting a hot W with a neutral W. He should STOP trying things and understand before he causes damage to wires or his new fan. Turn off the switch and disconnect the W wire he connected to the fan module. Feb 25 at 17:37
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    Be a good idea for him would be to get a non contact voltage tester. Since the light worked before and still works, there is a neutral W in the box with the light. THAT is what he need to connect to the neutral side of the fan. The hot side of the fan connection must be a switched hot from the new fan switch. Feb 25 at 17:41

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