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Electrical System

Checking Out Diode Trios

When you say diode trio, General Motors charging systems come to mind. But "trios" are commonplace in Asian and European alternators, too. These units are also vulnerable to the same problems that cause repeated trio failure on GM systems. Fortunately, simple GM diode trio tests also apply to Asian and European designs. Two quick voltage checks are all it takes to prevent repeated trio failures.

Electrically, Asian and European trios work just like GM's. That is, the trio is a parallel output path to the standard diode set or rectifier. When the alternator is charging, most of the current flows out of the rectifier. However, a little bit of current also flows out through the trio to the charge indicator light terminal. On GM alternators, you know the indicator light terminal as terminal 1. On European alternators, the light terminal is usually called 61 or D+. On Asian alternators, it's usually called L. This terminal leads to the ground side of the alternator warning light on the dashboard. When the alternator is charging, the trio applies voltage to the ground side of the alternator light. Current must flow through the alternator light for the light to remain on. But after the alternator begins charging, there is no voltage differential across the light so the current stops flowing through it and the light goes out.

The word trio refers to three diodes in one assembly. When one of the trio's three diodes burn open, the alternator light comes on very dimly. When all three diodes open up, the light turns on brightly. Remember, the trio also powers the field circuit after the alternator begins charging. So, losing one or two "legs" of the trio causes undercharging. When the entire trio assembly burns open, the alternator won't charge at all.

The same two problems that cause repeated diode trio failure on GM charging systems also affects Asian and European alternators. The first is a bad connection or open circuit between the alternator output terminal and the positive battery terminal. These problems force charging current to take that parallel route - through the diode trio - out of the alternator. This relatively heavy current flow overwhelms and burns up the trio.

Performing a routine voltage drop test between the alternator and the battery is the best insurance against repeated trio failure. To do this, connect the positive test lead of a digital voltmeter to the alternator output terminal (usually marked BAT or B+). Connect the negative lead to the positive battery terminal. Select the meter's autoranging mode or else set it to its lowest voltage scale. Next, start the engine, turn on plenty of electrical accessories and note the voltmeter reading. When the meter reading is greater than about 0.5 volts, check and repair the circuit between the alternator and the battery. If you don't correct the excessive voltage loss in this circuit, the trio in the replacement alternator eventually will fail too!

Whenever the alternator output circuit tests good and the vehicle has a history of repeated trio failure, always check for a shorted indicator light terminal. On some vehicles, the alternator indicator light terminal powers an electrically heated choke. If the choke heater or its power wire shorts to ground, it destroys the trio. This problem is not uncommon on popular carbureted Toyotas such as Tercels and Corollas (Bosch p/n AL216X, AL217X, AL365X)

Last but not least, don't forget the basic voltage test for all diode trios. Start the engine and turn on plenty of electrical accessories. Use a digital voltmeter to compare voltage at the indicator light terminal with the voltage at the output terminal. The diode trio is bad if voltage at the indicator light terminal is NOT within about 0.2-0.3 volts of the output terminal's reading.

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