If one of the rectifier valves is damaged, the entire rectifier unit must be replaced.
A short circuit of the rectifier block valves can be checked without removing the generator from the car, after disconnecting the wires from the battery and the generator and removing the protective casing. The wire from the terminal "B" of the voltage regulator is also disconnected. For a generator with an old voltage regulator, it is also necessary to disconnect the terminal "B" of the voltage regulator from the terminal "30" of the generator.
You can check it with an ohmmeter or with a lamp (1-5W, 12V) and the battery, as shown in Fig. 7-8.
Fig. 7-8. Schemes for testing rectifier valves:
1 - battery; 2 - control lamp; 3 - generator; I - checking both "positive" and "negative" valves simultaneously; II - checking the "negative" valves; III - checking the "positive" valves.
Note: In order to simplify the fastening of the rectifier parts, three valves (with a red mark) create a "plus" of rectified voltage on the case. These valves are "positive", and they are pressed into one plate of the rectifier block, connected to the "30" terminal of the generator. The other three valves ("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" valves. To do this, connect the "plus" of the battery through a lamp to the "30" terminal of the generator, and the "minus" to the generator body (fig. 7-8, I). If the lamp is on, then the "negative" and "positive" valves are short-circuited.
A short circuit of the "negative" valves can be checked by connecting the "plus" of the battery through a lamp to one of the mounting bolts of the rectifier unit, and the "minus" to the generator body (fig. 7-8, II). The lamp burning means a short circuit in one or more "negative" valves. It should be remembered that in this case the lamp burning may also be a consequence of the stator winding turns shorting to the generator housing. However, such a malfunction is less common than a short circuit of the valves.
To check for a short circuit in the "positive" valves, connect the "plus" of the battery through a lamp to the "30" clamp of the generator, and the "minus" to one of the mounting bolts of the rectifier unit (fig. 7-8, III). The lamp lighting up will indicate a short circuit in one or more "positive" valves.
A break in the valves 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. If the windings, additional diodes and voltage regulator of the generator are in good condition, and there is no short circuit in the valves, then the reason for the decrease in the output current is a break in the valves.
[The original is available on the online resource VazBook]