Causes of Alternator Diode Failure

by Kenneth V. Oster
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The diodes in an automobile's alternator serve the same function as a one-way check valve does in a hydraulic system. Diodes allow electrical current to flow in one direction only, and are the essential part that converts alternating current (A/C) to direct current (D/C).


As the alternator is running, it produces current that passes through the diodes to the battery connection on the alternator. The current then travels to the battery through the positive battery cable. As long as there is a good connection to the battery, the battery will be maintained in a fully charged condition and the electrical system will function properly. A small amount of current also flows through the circuit for the alternator light on the instrument panel to provide and indication of the alternator's health to the driver. Properly functioning diodes also protect all of the sensitive electronic equipment in the vehicle from reverse current flow.

Causes of Failure

When there is a defective connection between the battery and the alternator, charging current will be forced to find an alternate route to flow out toward the battery. A defective connection may be caused by corrosion or an open circuit from a break in several battery cable stands. The alternate route the current finds usually leads to excessive current flow, causing the diodes to overheat and fail. Diode overheating also occurs when the alternator is used to bring an undercharged battery up to a fully charged condition. As the vehicle is driven to bring an undercharged battery up to the correct voltage, the excessive current flow can overheat the diodes, leading to failure.

Human Error

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Diodes can be damaged when there is an attempt to swap out a battery while the engine is running. As soon as the battery cables are disconnected, the current will seek out another route to flow, causing the diodes to overheat and fail. Diodes can also be damaged when there is an attempt to jump start a battery, as an excessive surge of power can burn out the diodes on either vehicle. Additionally, diodes can be damaged if a battery charger that is set at a high amperage is connected to a battery in reverse polarity while the battery is still connected to the alternator.

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