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When my dishwasher runs the kitchen sink backs up. I have a two tub sink and only the side nearest the dishwasher with the garbage disposer backs up. Running the disposal for a few seconds causes the sink to drain.
I'm not sure what to look at to address this problem. It doesn't strike me as a clog because if it was the sink would drain poorly.
There is a very simple answer to your question. Your dishwasher is draining into the side drain adapter on your garbage disposer. this was a common practice for many years. All the water draining from your dishwasher is being pumped into the upper basin side of the disposer. Solids from the dishwasher are blocking the drain in the disposer, so when you turn on the disposer, it grinds the waste and drains the water. This is actually the way it is suppose to work. Current codes require a separate trap for the dishwasher and not plumbed into the disposer. If you look at the side of your disposer, you will see the hose from the washer attached there. Only fix is to install a new separate trap and drain for the washer. Good luck.
This might be of interest.
Air gap
Many local building codes require a dishwasher to connect to an air gap before the connection to a garbage disposer. This keeps wastewater from backing up into the appliance. You must purchase the air gap separately. Mount in on top or next to the sink. Connect one flexible hose to the drain of the dishwasher, and connect the other flexible hose to the trap of the sink or to the disposer’s dishwasher inlet. If your local codes do not require an air gap, you can shape the drain hose of the dishwasher into a high arc instead.
As others have pointed out, the diswasher drain line probably connects to the top of the disposal. The diswasher drain line will dump dirty water and food particles into the top of your disposal.
Do you run the disposal prior to starting the dishwasher? If you have crap built up in your disposal, that can be contributing to a backup. The manual for my dishwasher says to make sure you run the disposal at the start of a cycle.
It is possible that your disposal is clogged slightly (constricted, but not completely plugged). You wouldn't notice when you run the sink, because your are putting clean water down the drain. The dishwasher waste water includes lots of solid particles, which would aggravate the clog. Running the disposal grinds the particles and pumps the water.
There are some foaming garbage disposal cleaners, I would try one of them. Look for an actual cleaner, not just a deodorizer. (NOTE: Use something specific for disposals, not general-purpose Draino.)
Another thing to look at: Dishwashers have a built-in mechanism to deal with food particles before draining. Otherwise the hunks of food from your dishes would clog the plumbing. Most dishwashers in the US have little garbage disposal type blades that grind anything in the waste water. Some dishwashers (notably Bosch) have a little strainer basket to catch the particles. The strainer basket needs periodic cleaning.
If your dishwasher has a built-in disposal, it may not be running properly, allowing large particles to slip by.
If your dishwasher has a strainer basket, make sure the basket isn't missing and doesn't have holes in it, which would allow particles to enter the drain line.
Your disposal is clogged. When you turn on the disposal, the water slowly drains because the motor slowly sucks the water out but otherwise is not operating properly. There should be a clear hose that goes from the dishwasher to your disposal. That is the drain line for the dishwasher. You can either use the hex key provided with your disposal to manually turn the disposal to see if you can clear the clog and also hit the reset button. If that does not work, then you will have to replace your disposal - it is likely shot.
An easy fix: Pre-rinse large gunk off your dishes before loading them into the dishwaher. DOn't expect the dishwaher to deal with lasagna and mom's meatloaf.
