At malfunction of the generator in most cases I'll just point out the warning light placed on the dashboard. With ignition on and engine off it should be lit – this indication is necessary to check the serviceability of the lamp. After starting the engine, simultaneously with the start of rotation of the rotor of the generator, the lamp goes out, indicating that the generator produces a current denomination.
In the case where the warning light or the power of its glow increased, the charging current to the battery is not supplied at all. The Troubleshooting should start directly from the generator, then checking the whole chain charge.
If at the working engine the control lamp glows dimly, or flickers, hence the generator produces a low-voltage current. The most common cause of such a failure is the insufficient tension of a belt of a drive of the generator, although not eliminated, and malfunction of the voltage regulator.
Check voltage regulator
The main purpose of the voltage regulator is to submit to the vehicle battery and electrical system DC voltage, in the range from 13.5 to 14.5 In (different car models range of operating voltage values may vary slightly).
You need to measure voltage at the battery terminals before starting the engine. Depending on the "freshness" of the battery and its state value of the potential difference will be from 12 to 12.8 V.
After that the engine start and by multimeter measure the voltage at the battery terminals. As has been said, it must be in the range of 13.5 – 14.5 V and not exceed the threshold value even in the case of engine operation at high rpm. If the voltage regulator is faulty, the charging current coming from the generator will increase in proportion to engine speed, sometimes reaching critical values. The result of overcharging the battery would boil-off the electrolyte, and the output of the battery system.
In addition to the battery, too high voltage charging current is able to disable the devices on-Board electronics of the vehicle.
If with the engine running voltage at the battery terminals is not increased, but rather decreased, hence the charging current to the battery is not supplied.
checking the diode bridge
Usually the diode bridge consists of 6 diodes, 3 of which are "negative", and 3 "positive." Placing the multimeter in the ohmmeter mode, you must first connect the probe to the positive plate, and the second touch alternately to the terminals of the three diodes by fixing the value of the resistance shown by the ohmmeter. Then to the positive plate of the diode bridge, connect the second probe and the first alternately measure the resistance at the ends of the diodes.
The ohmmeter should show conductivity (to fix any resistance) on all the diodes in one case only, after the change of the probes places the meter should show no conductivity. If at least one of the diodes shows the conductivity in both directions, hence it is broken, and therefore the diode bridge needs to be repaired or must be replaced with a new one.
Similarly checked for the breakdown of the "negative" diodes.
Check the rotor and stator
Multimeter, translated in a mode of an ohmmeter, measure the resistance of the excitation winding of the rotor by attaching the multimeter probes to slip rings.
Resistance 2 to 5 Ohms will witness the normal state of the winding. Low resistance, close to zero, will indicate a turn-to-turn circuit, and tending to infinity – the break in the winding. A too high value of resistance – evidence of poor contact between slip rings and the terminals of the winding.
To assess the state of insulation of the excitation winding of the rotor is possible by measuring the resistance between the rotor and slip ring. In case of leakage current to ground the device will show a certain resistance value.
Before checking the stator do not forget to disable the findings from the diode bridge of the generator. The resistance between the terminals of the windings of the stator should be about 0.2 Ohms. Between the common terminal (zero) and the output of one of the windings the resistance value should be about 0.3 Ohms.
Check condition of stator winding insulation is performed in a manner similar to the insulation of the rotor.
Good luck to you! No nail, no wand!