This will be an academic question as I worked on this circuit and failed and won't be going back to the customer, but I want to know what I could have done next time (actually I may be willing to go back at no charge if I can fix it easily):

1. Garbage and dishwasher outlet under the sink; upper (switched) garbage disposal works fine with the switch. Lower dishwasher one does not (reversed cords/tried other appliances: the lower outlet simply doesn't work).
2. BOTH outlets show it's correctly wired (with tester); BOTH outlets show 120v with multimeter as well; Disposal outlet works perfectly with the switch on and off.
3. I changed the tandem 20amp breaker as customer wanted it changed (as it was really old). But this did not fix it...
4. I also changed the outlet even though I suspected it was not the problem...
5. Outlet, of course, was split; the one neutral wire from the junction box was wired to the common neutral screws; one black and one red (red=disposal switch) was wired to the hot sides of the outlet and SPLIT as it should be.
6. Ground wire correctly attached to the outlet.
7. I opened the disposal switch junction both and the red disposal wire was attached to a neutral wire; Odd. Nonetheless I checked the pigtail and it was fine.
8. I also checked all the neutral connections for tightness at the breaker panel.
9. If there was a problem with the one and only NEUTRAL wire for the outlet junction box, I would suspect the disposal half of the outlet wouldn't work either, but it does.
10. If there was a problem with dishwasher HOT wire, I would suspect I wouldn't get the 120 I was getting.

So I am so confused. ???? Help!!

 

Be careful checking AC power connections with an electronic multi-meter/high impedance tester on unloaded circuits. The high impedance voltage reading will sometimes fool you into thinking you have a good circuit where there is a series high resistance somewhere in the circuit that drops the voltage to almost nothing under load.

 

Good advice above.
To check mains power, use an incandescent lamp/bulb 100watts on extended wires (Like a volt meter).
Take care.
Good luck..

 

I'll second nsaspook's suggestion that it might be high impedance somewhere along the wiring that appears to show the correct voltage when you're connecting the voltmeter but won't allow sufficient current to flow when you've got an appliance connected. Could be a break in the wire inside the insulation. Could be a bad splice.

How was the HOT wire connected to the dishwasher half of the receptacle? Was there a wire nut connecting the HOT to the switch or were two wires, one the incoming HOT line and the other the outgoing switch HOT? If there was a wire nut, there's a possibility of failure inside it. Was the neutral split as well? Maybe the neutral for the dishwasher half was not connected.