If one of the rectifier diodes is damaged, the entire rectifier unit must be replaced.
A short circuit of the rectifier block diodes can be checked without removing the generator from the car, after disconnecting the wires from the battery and generator and removing the casing from the rear cover of the generator. The wire from the "D+" terminal of the voltage regulator is also disconnected. You can check with an ohmmeter or a lamp (1-5W, 12V) and the battery as shown in Figure 7-9.
Figure 7-9. Circuits for testing rectifier diodes:
1 - battery;
2 - control lamp;
3 - generator;
I - checking both "positive" and "negative" diodes simultaneously;
II - checking the "positive" diodes;
III - checking "negative" diodes.
Note: To simplify the fastening of the rectifier parts, three diodes (with a red mark) create a "plus" of rectified voltage on the case. These diodes are "positive" and they are pressed into one plate of the rectifier block, connected to the "B+" terminal of the generator. The other three diodes ("negative" with a black mark) have a "minus" of rectified voltage on the case. They are pressed into another plate of the rectifier block, connected to the ground.
First check if there is a short circuit in both the "positive" and "negative" diodes. To do this, connect the "plus" of the battery through the lamp to the "B+" terminal of the generator, and the "minus" to the generator body (figure 7-9, I). If the lamp is on, then the "negative" and "positive" diodes are short-circuited.
To check for a short circuit in the "positive" diodes, connect the "plus" of the battery through a lamp to the "B+" terminal of the generator, and the "minus" to one of the phase terminals of the stator winding (figure 7-9, II). The lamp lighting up will indicate a short circuit in one or more "positive" diodes.
A short circuit of the "negative" diodes can be checked by connecting the "plus" of the battery through a lamp to one of the phase terminals of the stator winding, and the "minus" to the generator body (figure 7-9, III). The lamp burning means a short circuit in one or more "negative" diodes. It should be remembered that in this case the lamp burning may also be a consequence of the stator winding short circuit on the generator housing. However, such a malfunction occurs much less frequently than a short circuit of the diodes.
A break in the diodes without disassembling the generator can be detected either with an oscilloscope or when testing the generator on a bench by a significant decrease (by 20-30%) in the value of the output current compared to the nominal value. If the windings, additional diodes and the voltage regulator of the generator are in good condition, and there is no short circuit in the diodes, then the reason for the decrease in the output current is a break in the diodes.
Information taken from the official website «ChevyMan»