To make sure the voltage regulator is working properly, turn off all consumers except the high beam headlights and measure the voltage again. It should be within 13.2–14.7 V for the 9412.3701 generator and 13.6–14.6 V for the 371.3701 generator.
The removed voltage regulator of the generator 9412.3701 can be checked by connecting a lamp (1–5 W, 12 V) between the brushes, and a power source (only direct current, "minus" to "ground"!) to the terminals "D+" and "ground", first with a voltage of 12 V, and then 15–16 V. In the first case, the lamp should light, in the second – not. If the lamp lights in both cases, there is a breakdown in the regulator, if it does not light – there is a break or the contact between the brushes and the terminals of the regulator is broken. In both cases, the regulator should be replaced. To check the generator regulator 371.3701, the current source should be connected to terminals "B" and "C" ("plus") and "ground" ("minus").
To check the rectifier block valves, disconnect the wires from the battery, generator, and voltage regulator terminal(s). Connect the battery "plus" via a lamp (1–5 W, 12 V) to the "B+" terminal of the 9412.3701 generator (to the "30" terminal of the 371.3701 generator), and the "minus" to its body. If the lamp lights up, then there is a short circuit in both the "positive" and "negative" valve blocks.
To check for a short circuit in the "positive" valves, connect the "plus" of the battery via a lamp to the "B+" terminal of the 9412.3701 generator (to the "30" terminal of the 371.3701 generator), and the "minus" to the terminal of one of the phase windings of the stator. If the lamp lights, one or more positive valves are broken.
To check for a short circuit in the "negative" valves, connect the "plus" of the battery via a lamp to the terminal of one of the phase windings of the stator, and the "minus" to the generator housing. If the lamp lights up, one or more negative valves are broken or the stator windings are shorted to the generator housing. To eliminate the short circuit of the windings, remove the generator from the car and, having disconnected the windings from the voltage regulator and rectifier unit, check their short circuit to "ground" with a lamp or ohmmeter. The generator valves can also be checked with an ohmmeter, without connecting the battery and test lamp.
The short circuit of additional diodes can be checked by connecting the battery "plus" through a lamp to the "D" terminal of the 9412.3701 generator (to the "61" terminal of the 371.3701 generator), and the "minus" to the terminal of one of the phase windings of the stator (to one of the rectifier block mounting bolts). If the lamp is on, one or more additional diodes are broken.
A break in the main valves is determined by a sharp decrease in the output current (voltage drop under load). However, this can also be caused by a break or short circuit in the generator windings. A break in the additional valves can be determined by a low voltage on the "D" plug of the 9412.3701 generator or the "61" plug of the 371.3701 generator (below 14 V) at low and medium generator rotor speeds.
Determine the serviceability of each diode (primary or secondary) can only be done with the rectifier unit removed using an ohmmeter or test lamp. It is recommended to replace the rectifier unit as a whole if it fails. It is allowed to replace individual valves, but the main valves will require re-pressing in the holder - an operation that requires accuracy and skill. The stator and rotor windings are checked using a special flaw detector or electronic oscilloscope - based on the shape of the voltage curves.