Dead spot on Aprilia Leonardo starter motor

Another electrical problem but I found out how to prove this and discovered a very useful get-you-home fix; The problem is that I tried to start the bike but just heard the click of the starter relay.

The quick answer to what I eventually found was that the brushes in the starter motor had worn down to the last little bit (on 43,000 kms). Minor variations in the commutator meant that the brush did not touch at one point. It had a “dead spot”.

I discovered a get-you-home fix; Turn on the ignition and push the starter switch then bounce hard up and down on the seat (like you are testing the shocks). This rotates the starter motor around a little to step over the dead spot if it is a dead spot and it may start – it did for me.

If that fixes your problem then get to a garage or home in one go because what it means is probably that your starter bushes are down to the last bit of metal and this will fail to start again and the bouncing on the seat trick may not work.

As I mentioned I accidentally discovered the temporary fix but in proving what was actually wrong I also worked out a test routine as it could be one of 4 different things,

  1. the relay contacts not working
  2. no 12VDC +ve in
  3. no Earth
  4. problem with starter motor

When you push the ignition switch then you should hear a solid click of the relay every time you push in the switch. The relay is located under the right hand side of the seat and the contacts are visible through an air vent just above the right hand inspection opening. They are two large lugs.

If you don’t hear this click then you have a problem with the relay circuit from the starter switch on the handlebar to the relay. You’ll have to trace this out with a multimeter though if you are on the road and don’t have a multimeter then I would,

  • Check fuses are OK
  • check wiring connections to relay and to starter switch look ok (no corrosion)
  • check starter switch making contact (if you can you could try to prove by shorting the switch contacts with a wire)

If the relay is making a click and if you don’t have a multimeter you can test (1) and (4) by shorting the contacts of the relay with a bit of wire. Don’t short to frame – just between the contacts. One side is always 12V +ve and the other connects through the starter motor to the engine earth. With the ignition OFF if you try and short the relay contacts and they do NOT spark then the starter motor (or Earth or +ve supply) probably has a problem. If they DO spark then the relay probably has a problem. Take care with shorting this obviously.

If you do have a multimeter then testing is very easy. You should see 12 volts on one side of the relay all the time and when you push the starter switch then it should appear on the other side.  If it does not then the relay is probably faulty.

If you do see the relay working then the 12 volt supply goes to the starter motor via the heavy cable (then through the motor to the earth). With the ignition off then if you measure the resistance from the battery -ve/Earth post to the starter relay lug that goes to the starter motor (not the other side of the relay as that is always live !) then you should see less than an ohm e.g. only half an ohm (0.5 Ω) – basically a near short as the starter motor is about 500 watts so at 12 Volts that means about 0.3 ohms. If you see a high reading (more than an ohm) or an open circuit then check that the case of the starter motor is low resistance (it should be at earth) and then check that the lug into starter motor is low resistance. The starter case will be corroded a bit so make sure you are touching actual metal.

If your finding is that the starter motor has a high resistance through it then it is probably the brushes. The starter motor is easy to remove though it will clash with the carburettor so not much wiggle room. The starter motor is also easy to pull apart and if it is a worn brush then this is obvious. You may be able to get a few more starts by bending the brush housing a bit but the brush wire is what will stop you in the end. If you do bend the housing to gain some starts then you can test the motor on the bench by checking the resistance stays low between the case and terminal lug whilst you slowly rotate the motor by hand one whole revolution. It should stay low ohms at every point.

If it is an original Mitsui starter motor then I don’t believe that you can get replacement brushes assembly though an auto electrician may know of an equivalent brush part. AFAIKS the starter motor is the same for all  Leonardo 125 and 150 models for all years (96-04) and for some other models that use the Rotax Type 120/154 (120S-154S) engine (e.g. Scarabeo 125/150/200 from 99-03 and also the BMW C1 125/200 from 00-03 ). The OEM Aprilia starter motors are very expensive – but there are much cheaper “Economical” or equivalent motors that are made that are a 1/4 of the price new. Unless you know the kilometres of a second hand bike or you can actually see the bushes to see how much is left on them I wouldn’t buy a second hand starter motor.