[Jimmer]

Tom,

Welcome to the forum!

It could be a venting problem. If your drain becomes air-locked it will not drain. The disposal may be providing enough vibration to the drain pipe to break that air-lock.

If that is not the problem then since the washing machine drain enter the system below the disposal unit it leaves only the drain pipe as the possible problem. Do you have a p-trap after the washing maching connection? Have you checked to see if it clear? Is your sink a double bowl? If so... if you fill up the sink (without the disposal) with water and pull the plug does it drain?

Please let us know what you find.

Good luck, Jim

'Just a handyman trying to help'