If the timer on a dishwasher fails, the machine can get stuck in a cycle, the cycle times can run too long or the dishwasher may not start. The timer governs the amount of power that goes into each component, and in what order, to control the washing cycles.
There are two types of dishwasher timers, mechanical and solid state. The mechanical style has a rotating cylinder with contact "fingers" protruding from the surface. As the cylinder, or cam, turns, the fingers brush against electrical contacts, triggering a certain part of the dishwasher to run. The cylinder constantly moves, very slowly. As the fingers move from one electrical contact to another, it times the cycles.
The solid state timer has an electronic control board that does the same thing. The user selects the settings using the push buttons on the dishwasher to program the wash cycles. For example, if a dishwasher has a heavy, normal and light load button and the normal load is set, the control board uses the proper wash, rinse and dry cycles. The mechanical timer has the same choices, but it is controlled by setting a dial that makes the cylinder move.
Most timers have a reset button. Pushing this may solve the problem. If the timer motor is defective, it may be possible, and less expensive, to just change the motor instead of the entire assembly.