Alternator leakage test

The leakage test determines whether a shorted diode conducts power from the battery to the alternator windings while the engine is off. As a parasitic draw, it leads to battery consumption in rest mode.

When the battery is weak at start-up, and after charging you find it has good capacity, the suspicion falls on parasitic draws. The procedure for detecting a parasitic draw requires connecting an ammeter to the battery. By removing the fuse in order, determine the line where the fault is located. However, after removing all fuses the problem is not detected and there is still a leak, it is necessary to check the alternator leak.

The alternator leakage test is performed with a multimeter while the engine is not running and with the ignition off. The multimeter is set to measure current in mA. Disconnect the supply plus wire from the plus terminal (B+) on the alternator. Connect the plus measuring pipette of the multimeter to the B+ of an alternator, and the black pipette to the removed terminal of the supply line. Depending on the type of vehicle, there is always a tiny amount of current flowing between the battery and the alternator when the car is at rest. However, if a current of more than 60 mA is flowing, there is an alternator leak that acts as a hidden consumer draining the battery. When there is a leak, it is necessary to check the condition of the diodes on the alternator diode plate.