Alternator Diode


How do you check the diode of the alternator?


There will be more than one diode. They are set up in a bridge rectifier arrangement. Most of the ones I’ve seen have 6 diodes arranged to rectify a 3 phase current. If you have the alternator running you can see the excess ripple caused by a bad diode with an oscilloscope. If it’s not in the car, then you have to take the alternator apart to measure forward and reverse biased ohms on each individual diode.

If none of the above makes sense to you, you probably should take it to someone who works on alternators.


Exactly, but unless you are planning on rebuilding it yourself, it’s less hassle to just have the alternator tested in the car. It will either be “good” or “bad,” replace it if required.


A diode is a one way switch. It will allow current to flow in one direction but not the other. So if you use an ohm meter, you should get a reading with the probes in one position and when you reverse the probes not get a reading. In GM’s anyway there is a diode trio or three of them. In this day and age though, it is just too hard to get them apart so you just get a good quality rebuilt alternator instead like an AC rebuild.