Ford Truck Enthusiasts Forums - Why do I keep burning out diodes in my alternator?

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What makes most diodes blow is when owners try to charge a dead battery with the alternator when driving. Alternators are designed to maintain a charge only and not recharge from scratch. As stated above look for excess current draw from whatever source.

Quote:

Originally Posted by rebocardo

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Bad ground or bad positive cable that for a moment disconnects the alternator (or a load) from the battery system when you hit a bump. When the load or battery reconnects to the alternator it blows the diode.

The above situation is called a "Load Dump Transient" that can easily hit 400V with enough power to blow just about any electronic component.

The amp rating on newer small frame alternators is only an intermittent rating. The older alternators were often rated continuous amps. Overloading newer alternators is quite common which results in burned out diodes. They just do not have the heatsinks necessary to do the job.