Most common problem on these is the control module, look for a burnt heater relay or burnt solder joint. Some of these dishwashers are covered under a recall, I need your serial number to find if this dishwasher is covered under the recall.
Did you test that button switch for continuity with your meter? With the button pressed in and on, it should show continuity and the meter should beep, with the button out and off you should get nothing and no beep. If the switch tests good, your problem is the timer.
2 things to check for: What was the part number of the belt you installed and is the inner wash basket spinning while it's agitating? If the inner wash basket is spinning while it's agitating, the brake pads might be worn from the bad helix. The brake not holding the inner wash basket stationary during agitation could be splashing the water over the tub. The incorrect belt could also be causing the water to splash over the tub during spin.