Light switch replacement: switch wall box has many wires - how should I wire it to both work and be safe?

Question

I’m trying to replace the light switch in my bedroom and fix the outlets in the living room, but there are 5 separate lines and I don’t know what goes to what. I only have 1 light in my bedroom which is directly above the only switch in the room, and on the other side of this wall is my living room, which has only one switch and one light. The outlets on that side don’t work but they do in the bedroom. I took a few pictures of the light switch in the bedroom before I undid all the wires, and in the process of taking the switch out I broke the wall box it was in so I’m doing a quick temporary replacement of that as well. I’m afraid to turn the power back on to the house until it’s all put back together but again, I’m not sure how to wire it. Please help

Answer 1

Somebody bodged it up, not terribly, but not good.

1. two wires under one screw on the switch, not good and not code legal. The correction for this is to remove the wire going into the cable (top middle), and connect with the other black wires in the wire nut in the lower middle of the pic since it's already pig tailed (a very short wire from the wire nut to the switch).
2. The cable coming in from the lower right is coming thru a hole that was obviously drilled in the box, altering a UL listed product. Again, not code legal.
3. The ground wire from that cable was lazily wrapped around the existing ground without a proper crimp sleeve. Obviously this cable was an "add on".
4. Having all the neutrals (white wires) tied together is common and OK. The only issue could be if one or more of them have a poor or non-existent connection.
5. I disagree that you can solve this with a non-contact voltage detector. They can be very miss-leading.

Lastly, I would suggest a multi-meter and start tracing circuits. You can do that with the power off (and must), but with a non-contact voltage detector, the power needs to be on. My recommendation, EDIT (Thanks Jason) Test resistances first, being sure the power is off, record the readings per wire/cable pair, then plug in a something like an incandescent light bulb, space heater, anything that uses real power (not LED or LCD bulbs) Then test again for resistance (ohms) between each black and white wire on the same cable. Record and mark them, maybe using a sharpie and masking tape. If some are not near infinite resistance, those cables are going to lights. At any rate... After testing for voltage again, If one of them drops to near zero resistance, you found that circuit. Repeat until you've tested all circuits.

EDIT: Forgot to mention to get access for the continuity checks, you'll need to undo the white wires under the red wire nut, but that shouldn't be a problem to reconnect, since it seems as if all the white wires are already connected. Next, the sloppy ground should also be corrected with a proper crimp sleeve.