Any simple diode will exhibit a ‘front-to-back’ ratio since they are designed to allow current to flow in only one direction.

　　The least expensive multimeter will show this characteristic on a diode and a cheap digital multimeter will have a setting with a diode symbol that will give one a rough estimate of the voltage drop in forward (conducting) direction.

　　Measuring on any resistance range will give little sign of ‘normal’ condition since the amount of current is too low for most diodes to reach their nominal forward drop.

　　If a diode is damaged, this may indicate a very poor ratio; a voltage drop little different when the probes are reversed.

　　There are several different technologies and materials used for diodes and each has a characteristic forward voltage drop which can range from the low 100s of millivolts to (as with LEDs or ‘stacked’ high voltage types) several volts.

　　Most digital meters cannot show a forward voltage drop of more than 1–2 volts.

　　Since alternators use more than one diode to convert the AC of the output to DC, you have a basis for comparison; all of the diodes (when serviceable) will show forward (conducting) voltage drops within ~10% of each other.

　　Once you have found the terminal orientation, a check can be made on each, sometimes while attached to the alternator, sometimes one lead needs to be removed to avoid possible parallel devices that can falsify the reading.

　　-On automotive alternators, diodes will often ‘fail-to-short’ and on the diode setting will indicate very low drop (under 100 millivolts or > 0.100 or just 100.)