Generator Rectifier and Diode Failure Explained

The engine runs perfectly, the set is up to speed, and there is no voltage at the terminals. Before condemning the alternator, look at the generator rectifier — the rotating diode assembly that turns the exciter’s AC into the DC the main rotor needs.

Where the generator rectifier sits

In a brushless alternator, the exciter armature spins on the same shaft as the main rotor and produces AC. That AC is useless to the main field, which needs DC, so a set of diodes mounted on the rotating assembly rectifies it. Everything happens at 1500rpm on a spinning part, which is precisely why diodes fail: heat, vibration and voltage spikes, all at once, for years.

How they fail, and what you see

An open diode usually leaves you with reduced, wandering output. A shorted one can load the exciter enough to give you nothing at all. If diodes keep failing, suspect the surge suppressor — it is there to absorb the spikes that kill them, and a failed one takes the next set of diodes with it.

Rectifier or AVR?

Both present as “no output”, so separate them before ordering parts. The AVR controls excitation; the rectifier delivers it. A quick way in is to check whether the AVR is receiving sensing voltage and producing field output — if it is, and the machine still will not build up, the fault is downstream. Our explainer on what an AVR does is worth reading first if this is unfamiliar territory, and there are specials on selected AVRs if yours turns out to be the culprit.

Testing and replacement

• Test each diode with a multimeter on diode range — conducting one way only, both ways is a short, neither is open.
• Replace the full set, not the one that failed; the others have the same hours on them.
• Replace the surge suppressor at the same time.
• Check residual magnetism and flash the field if the machine has been standing.

We stock rectifiers and alternator parts alongside the alternators themselves — see our alternator guide for how the whole assembly fits together.

Not sure whether it is the rectifier or the AVR? Tell us the alternator make and model and the symptoms — we will help you narrow it down.