Are my alternator brushes too short and what problems do short brushes cause?

Question

This is related to my question on trying to diagnose what's wrong with my alternator, but I feel it stands alone as it's own question.

I ordered a new rectifier based on the findings listed in the linked question, but when I opened the alternator up I noticed that the brushes looked really short:

I took a screen shot from a video of someone replacing the brushes on a similar model Mitsubishi alternator:

Just as a quick recap, it's an 80 amp alternator and the field control unit ( ECU does voltage regulation ) has a 14.3 volt set point. At 2100 rpms, it's maxing out at about 58 amps and between 12.6 - 13 volts while at the same time drawing about 3 - 5 amps from the battery as well.

So looking at this it makes me wonder if my problem may just be that the brushes are too short and it's not the rectifier?

What are typical symptoms of short brushes?

Answer 1

The brushes' job is to solve the problem of how you make a connection between the fixed and moving parts of the alternator. Over time the brushes wear, but as long as they are making a connection the length shouldn't matter.

So, check that the contact surfaces are clean and smooth and that the brushes move freely (so that they can make a solid connection). If one of the brushes was hanging up, that could explain the low output. You should be able to push the brushes back up into their housing and have them return smoothly and positively.

If the brushes are hanging up "in real life" because they've cocked in their guides as @Paulster2 suggests in the comment below you will probably be able to see this in the wear pattern on the face of the brush that rides on the slip ring. Instead of being one smoothly curved surface it will be "faceted" – you'll see one or more lines where the brush has worn in different positions. If the hanging up gets bad enough the alternator may drop off line or be intermittent in its output.

Another thing to check is that all the rectifier diodes and all three windings are in good shape. The max output you're seeing is pretty close to 2/3 of the alternator's capacity, that makes me think that you may have lost one leg (hopefully in the form of failed diodes rather than an open winding).