Alternator leakage current is a relatively easy problem to diagnose and fix. It is extremely unpleasant - the battery is continuously discharged with a current of 3-5 amperes during parking. This is 100 times the current leakage rate for a serviceable car. In addition, the generator works incorrectly and defectively, with all the ensuing consequences. From this material on the ABS website, you will learn how the alternator current leakage manifests itself, how to remove it, how exactly the current leaves the battery, how to fix the breakdown without a specialist.

Symptoms of current leakage through the generator

A symptom of current leakage through the car's alternator is the rapid discharge of the battery. Even a freshly charged battery runs out in just one night. And if we also take into account the defective operation of the generator, then the battery during the next trip will not be charged to an adequate level. Sometimes they are not in a hurry to fix this breakdown, because they think that problems with the battery are associated with cold snaps or other factors. As a result, everything ends with the fact that the morning of a car enthusiast always begins with recharging the battery using a charger.

Pay attention. Cars with gasoline injection engines "do not like" when they are trying to start from a dead battery. Often attempts end in errors in the electronic control unit, which have to be reset using a special key from specialists.

The second symptom confirming the high probability of leakage of current through the generator is the unsuccessful search for leakage by the traditional method. It consists in measuring the leakage and pulling out the fuses step by step. If the alternator is leaking, pulling the fuses to 100% does not help to find the problem, since the charging system on the car is connected directly to the battery.

Can the generator drain the battery? With a current leakage of 5 amperes, a charged new battery with a capacity of 60 ampere-hours will be completely discharged in 10...12 hours. If the battery is not new, or not 100% charged, it will be discharged to zero in just 3...5 hours of parking. Accordingly, the answer to this question is yes, the alternator can discharge the battery.

Current leakage through the generator - how to check?

If the car's battery is suddenly discharged, you should not rush to think about the generator. Any problem must be solved methodically. Car won't start due to dead battery? Make sure it's really loaded. Set the multimeter to DC voltage measurement mode and measure the voltage directly at the battery terminals. If it is below 11 volts, it is a deep discharge. If there is at least 12 volts, but the starter does not turn, it may need to be repaired.

Next, make sure that the battery is dead "through no fault of yours". Perhaps you forgot to turn off something before parking - side lights, interior lights, music. Not? Start pulling fuses.

Brief instructions. Take any multimeter. Turn on the DC current measurement mode in the range up to 10 amperes. Do not forget to rearrange the red probe into the appropriate connector on the device. Remove one terminal from the battery terminal. Connect a multimeter to the open circuit - one probe to the battery terminal, the second to the removed terminal. The device will show the leakage current. If it is higher than 0.07 amperes, pull out all the fuses one by one until the readings decrease. About current leakage there is a separate material on the ABS website.

Pulling the fuses did not help to find the cause of the current leakage? There are two options. Or you may not know where there are other fuses in the car. Often there are additional blocks with them in the cabin, near the driver's feet or under the glove box opposite the front passenger. Sometimes it makes sense to find out where they are and include them in the check too. Or current leakage - through the generator.

How to confirm this? Disconnect the multimeter. Don't put on the clamp just yet. Find the generator under the hood, and the power wires that fit it. They are thick, and go the shortest way from the "positive" terminal of the battery. They are attached to the generator with a nut. Disconnect the wires from the generator. So you exclude it from battery consumers. The removed wires must be connected to each other and temporarily isolated so that they do not accidentally touch the "mass" of the car. Connect the multimeter back.

If the indicators have decreased significantly, this is the current leakage in the car through the generator.

It remains to confirm the diagnosis. Recharge some batteries from the charger. Connect the battery and alternator to the car's electrical system. Ignition off. Bring a steel object (wrench) to the generator pulley. If there is current leakage through the generator, the metal will stick to the pulley. This will happen because the generator now works on the principle of an electromagnet.

Additional sign. If the current leakage is 3 amps or more, this also indicates a faulty generator.

How does the generator discharge the battery?

In 99% of cases, current leakage through the generator occurs in the following way:

  1. "Positive" battery terminal.
  2. Power "positive" diode.
  3. Additional bridge diode.
  4. Through one of the brushes, current flows into the rotor winding.
  5. From the winding, the current goes to the relay-voltage regulator through the second brush.
  6. From the relay-regulator through the transistor, which is currently constantly open, the current flows to the "mass" of the car.
  7. It turns out a closed circuit. Due to the fault of a broken power diode, the excitation winding of the generator becomes a permanent consumer. It consumes a current of 3...5 amperes. It's safe for her. But the battery is quickly discharged.

There are other reasons for current leakage through the generator. However, the one described above is much more common.


Since the cause of current leakage through the generator is most often a broken power diode, it must be looked for. The search is simplified, since it is known that only the “positive” diode can be broken. There are three in total. The rest are negative.

How to check the generator for current leakage with a multimeter? Disconnect the battery from the on-board network. This is required for security reasons. Remove the generator from the car. Disconnect all stator windings from the diode bridge (there are three of them).

A diode in good condition passes electric current in only one direction. If he misses it in both directions, he is broken. If it does not pass current at all - in the open.

Pay attention. If a diode break occurs, then the battery charge control lamp on the dashboard lights up in the glow floor (temporarily goes out when gas is added).

Your task is to find a broken diode. To do this, you need to methodically ring each of the power diodes, applying one probe to the "horseshoe" into which it is pressed, and the second probe to the diode output. In order not to figure out where which diode is - “positive”, “negative” - just ring everything that is on the bridge. First in one direction, and then in the opposite direction (by swapping the probes). If a diode rings in both directions, then it is broken. The leakage of current through the generator occurs precisely because of it. In rare cases, two or three diodes are broken. More often than not, only one fails.

How to eliminate current leakage through the generator?

Repair most often comes down to replacing the diode bridge with a new one. To save money, sometimes they use an old, but obviously serviceable one. After installing the diode bridge, assemble the generator in the reverse order and connect it to the car's on-board network. Make sure the leakage current is fixed. To do this, use the methods described above.

VIDEO: current leakage