Many older Delco (GM) alternators are equipped with what was infamously known as the diode trio. The diode trio was in millions of GM alternators known as the “SI” series. A diode is an electrical one way check valve that allows current to flow in only one direction. Trio, of course, means three.
A diode trio is 3 diodes connected together. Trio diodes also occurred
in many other manufacturers’ alternators with built in voltage regulators.
The diode trio is connected to all three stator windings. The current generated in the stator flows through the trios to the voltage regulator.
The diode trio is designed to provide current for the field circuit from the brushes and rotor and turn off the charge indicator light when the alternator voltage is equal to or greater than the battery voltage.
If one of the diodes in the trio is failed or open, the alternator may produce close to normal voltage levels but be reduced voltage enough to not completely put the charge indicator light out. In other words, the vehicle battery light will be on dimly. During this scenario the alternator can achieve approximately 66% of it’s total amperage rating, which can successfully operate most vehicles for long periods particularly if the vehicle is not using high amperage accessories such as A/C, lamps on high,and rear defog.
However, long term usage in this condition will over tax and overheat the other 2 phases of the alternator and cause them to fail. The lesson here is
if you’re out driving and you see your idiot light come on, here’s what to do:
Turn off as many electrical accessories as possible. Particularly do not use rear defrost. You can still continue to drive, but get the vehicle to a service center or home as soon as conveniently possible. After installing a new alternator or diode trio, make sure to fully charge the battery before driving the vehicle.