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:

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

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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?

  • 1
    I doubt the length of the brushes would make a difference, that is until they are worn down to the springs. The main thing with brushes is how they contact the armature slip rings. Weak springs could cause an issue. I think the main thing you need to do is pull out the carb cleaner on the internals. Then check for cracking/wear on the connections. Make sure nothing has worn into something else, etc. Ensure everything has a clean connection. If the brushes are really short, replacing them will not hurt anything. Ensure you regrease the bushings/bearings for smooth operation. Commented Aug 7, 2016 at 12:45
  • I would replace the brushes with new ones, those are at end of life.
    – Moab
    Commented Aug 7, 2016 at 15:12

1 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).

  • 1
    I was thinking along the same lines, but didn't know the finer points. I can see if the brush gets too short and start cocking-over in the brush bore. That would cause an issue, but I think you'd be able to see the brush face not looking "square" (using the term generically) to the brush body at that point. Brushes are typically made out of graphite materials, so should slide in their bore pretty easily. Commented Aug 7, 2016 at 13:51
  • 1
    Good point. Look at the faces of the brushes if they are faceted then I'd take that as a sign that they are too short.
    – dlu
    Commented Aug 7, 2016 at 13:54
  • "Faceted" is a great way of putting that ... very succinct and accurate! I always use too many words, lol. Commented Aug 7, 2016 at 13:56
  • Once I got the stator and "regulator" / rectifier assembly out from it's casing it became pretty obvious that the stator was bad. Everything in there had a kind of burnt look to it and the insulation on the stator windings was cracking and chipping off in multiple places. Looks like this alternator is toast ( literally ) so I'm just going to junk it and either pick one up at the junk yard or maybe get a rebuilt one. Commented Aug 8, 2016 at 11:50
  • I'd lean towards a rebuilt one. The shop may want the old one as a core.
    – dlu
    Commented Aug 8, 2016 at 11:56

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