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Advice please - starter motor

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Chairman, Jul 15, 2005.

  1. Whilst trying to get a recalcitrant bike to start today, I ran into starter motor trouble. I'd appreciate any suggestions as to the cause:

    The motor was successfully turning over, but not firing. I pulled a plug and lay it on the cylinder head, and cranked the motor with the starter - weak spark, so I cleaned it a bit and tried again.

    This time, the starter motor was sluggish - classic flat battery symptoms but the headlight wasn't dimming, even when the starter button was depressed. I tried a few more times and the starter would barely turn over, or not turn at all.

    The battery was showing 12.8V - so it isn't a flat battery.

    After thinking for a bit more, I removed the starter motor and the shaft felt very "gritty" when I turned it - like a worn bearing does. I opened the starter motor to find that the carbon brushes had broken off, as had one of the copper tongues on the commutator. Another of the tongues was loose. The starter was full of carbon grit from the brushes.

    I'm not a big believer in coincidences, so it seems odd that the starter would suffer a catastrophic collapse just when I was working on the bike. However, since the brushes normally slide over the commutator without interfering with it, I can't think of anything I might have done that could have caused the sort of damage I saw.

    I've seen worn commutators - you can usually refinish them with some wet&dry or by skimming them in a lathe - but I've never heard of one breaking. There was no sign of burning or arcing on any components, and the broken commutator tongue was clearly mechanical damage - the copper on the fracture line was bright and clean, while the rest had the darker color of oxidised copper. The tongue itself was twisted and lying in the motor housing, clearly it has been broken by something catching on it.

    For the curious, the motor is a Denso and there's an exploded view of a very similar model at http://www.dansmc.com/denso_starter1.JPG

    Can anyone offer an explanation (preferably based on expertise rather than tea-leaf reading) of why a starter might collapse like this?

  2. Sorry I drink coffee otherwise I would help you. 8) 8)
  3. How old is the motor, and are the brushes held in with springs on that model
    Last time I had an electric motor go on me like that , a small screw had fallen out of the cover, no rattles or anything until , the brushes got ripped apart..

    Only discovered the screw head by chance after pulling it apart..

    If theres no other foreign objects in there could it be metal fatigue. gets kinda warm in there if you keep cranking
  4. Only thing I've seen similar to what you describe is when the bearing in the end plate collapses, the armature can move enough to allow the commutator "tongues" as you call them to hit the metal brackets that the brushes slide in.

    But that ripped a number of "tongues" off, not just one.

    It could be a foriegn object (like HHB's screw) falling into the starter motor, but in the 19 years I was a mechanic, I can't remember seeing it happen outside poor assembly by the trainees we taught.

    In anycase, looks like a new armature eh

  5. (points to reply above)

    Wot 'e sed...

    ...especially as the bike in question is, what, 25-ish years old? Good odds the starter might've gone through a sloppy overhaul at some stage in its life.
  6. Only thing I can think of is maybe the starter got a bit warm with continuous cranking & the potting compound decided to let go on the commutator? This in turn could have chewed up the brushes etc. Just guessing.
  7. Thanks everyone. My guess is that Roarin is on the money. The contact next to the broken one seemed loose - I could wiggle it like a loose tooth. I suspect that it's got hot, the compound has softened allowing a couple of the contacts to lift and these have, in turn, caught on the brush.

    Three cheers for the KTM place in Hoppers Crossing - they're doing a reco with a guarantee.