I have a double kitchen sink. One side has a disposal. When the dishwasher drains it backs into the disposal side of the sink. The disposal has not been working properly. I have tried resetting it and using the allen wrench to unclog it. Apparently it is just worn out and needs to be replaced. It is 10-15 years old. I was trying to figure out why the back up. I cannot find where the dishwasher drain connects to the sink drain.

Dishwashers drain into the top of the disposal to take care of any large food particle from the dishes. Likely the drain from the disposal is partly clogged. Try cleaning it.

Hi,
I also have a double sink in the kitchen, with the garbage disposal located under the left side, with the drain pipe running into and connected to the drain pipe located under the right side.
The diswasher drains into the top of the garbage disposal.
You said you tried using an allen wrench to check for blockage in the disposal drain. Try using a flexable wire of some type, not a coathanger wire, too hard.
Try something like a larger gauge piece of electrical wiring, that is flexible, running it down through the disposal, and pushing it back and forth in the drain, with the water running from the tap, to flush out clogged stuff.
Also, a piece of old plastic covered wiring from a clothes line, or anything that is flexable will work. If it uncloggs the drain, it is easier than taking the drain itself apart.

Hey Pilot,
Are you saying the disposal side of your sink doesn't drain at all?
Your disposal has a "knockout port" that allows the dish washer to discharge into it and use the sink trap and drain to carry it away. Look under the sink. How many traps do you see? Freds idea of poking stiff wire down the black dischagge elbo is a good way to start. If that don't get it then, if the disposal has its own trap, open up the trap and check the lines leading into and away from it. Let me know what you find. Tom