My Dishwasher Power Has No Positive or Negative

AC and DC

• Alternating current, the electrical power in municipal systems, shifts between positive and negative peaks 60 times each second. Circuitry in a voltmeter set to measure AC converts the waveform to a value called root-mean-square voltage, or RMS. This measurement represents actual power, not peak voltage, and reads as 120 volts on the hot lead of household systems. Voltmeters set to DC detect no voltage on an AC power line. The average DC value of the positive and negative cycles of the AC input equals zero. Most voltmeters include input protection to prevent damage if users choose the wrong mode.

Dishwasher Power

• Dishwashers with electronic displays and control systems convert some of the AC input to DC power. Step-down transformers reduce the amplitude of the AC power signal, and rectifier circuits convert the AC waveform to a steady polarized voltage. Storage capacitors hold the DC charge that powers the dishwasher's electronics. Measuring voltages across parts of the control circuitry reveals varying levels of DC voltage. Either positive or negative charges might run the circuit, depending on the types of transistors and integrated circuits used. Measuring no voltage across the supply points in a DC system indicates serious trouble, such as a shorted part or a damaged DC supply.

AC System

• The motors in the dishwasher do the actual work by converting AC power to mechanical force. Part of the AC motor's rotational energy powers a pump that discharges rinse water. AC heating elements raise the temperature of hot water used in the sanitation cycle, and AC runs the blower that dries dishes at the end of the wash. Relays, triggered either mechanically or electronically, activate different parts of the AC-driven machinery. When the motors aren't turning, a voltage check at their power connections shows no AC. Controls apply power to this part of the system only when it operates.

Common Problems

• Dishwashers draw so much current that the machines require a dedicated power circuit supplying at least 15 amps. Most dishwashers connect to the AC supply through a junction box in the dishwasher base. If the dishwasher blows a fuse in the house electrical panel, an AC voltmeter measures no power at the junction box. Mistakes in the dishwasher wiring might blow fuses repeatedly and put the home at risk for electrical fires. In older homes, built before three-wire systems became standard, hot and neutral wires might be reversed from today's color code or use different coding systems. Never rewire connections without disconnecting power first, and call a professional if problems continue.