Interesting Starter Problem

[Kiwi_Roy]

I was asked to have a look at a loop with starter issues, the owner had told me previously the starter had engaged and remained cranking until he was able to disconnect the battery. I thought he must have screwed up the wiring somehow.

When I got to his place I found a very new looking Bosch. Sure enough when he pressed the start it cranked over and just kept on cranking until the battery was disconnected.

I pulled off the trigger wire and tried just hot wiring the solenoid, same thing again it would latch on even though there was no wire to the solenoid apart from the main battery connection.
I tried shorting the trigger terminal to chassis, just a weak spark and it remained cranking.
We pulled apart the solenoid, the contacts were in very good shape, lots of tension on the spring and everything looked normal. I didn't have time to do a drop test on the windings but my Fluke indicated that they are in the ballpark.
I sketched out the double coil arrangement and it's obvious that there is a path from the load side of the solenoid back through the heavy coil then through the light coil to chassis in other words the solenoid remains energized.
 
Searching for clues on the www I came across this from Vensel Enterprises
http://www.venselenterprises.com/techtipsfromdick_files/solenoidrebuildingarticle.pdf
It gives a very good description of how the solenoid works.
The key paragraph for me was the last one on page 1
“The coils must be balanced so that one does not overpower the other and cause the plunger to hang up and keep the drive engaged.”
I think on this unit a few of the coil turns may have become shorted putting it “Out of Balance”

I'm not discounting the starter hanging up in some other fashion but it drops out as soon as the power is removed.

Any other thoughts?

[rodekyll]

Does the starter do it on the bench?  It could be that the ring gear is notched.  Why is the starter being replaced?

[Kiwi_Roy]

Does the starter do it on the bench?  It could be that the ring gear is notched.  Why is the starter being replaced?

Never had a chance to try that, good thought.
No real reason for replacement, he just had a better looking starter on hand.
I will be checking out the original.

[Antietam Classic Cycle]

I had a Bosch starter rebuilt by a local shop and it did the same thing. Worked fine on the bench, stayed engaged when on the bike. Wasn't interference with the ring gear or anything else causing it to hang up. Just a defective (new) solenoid). They replaced it with another of the same and it's been fine since.

They'll do the same thing it you install the solenoid 180 degrees off. Numbers stamped into the solenoid body should face out, not towards the starter motor.

[Kiwi_Roy]

Thanks Charlie,
                       It didn't always do it but now every time, I suspect there may be a short in a few turns putting it out of balance.
I will post a sketch later. Orientation is correct.


The dot by the coil indicates the magnetic polarity assuming the current is in the same direction.
The Valeo solenoids are similar.

[crc]

ive had it happen before, I always thort it was the contacts in the starter arcing and fusing themselves together

[Kiwi_Roy]

ive had it happen before, I always thort it was the contacts in the starter arcing and fusing themselves together

That may happen also but not in this case, the contacts are pristine.

[Triple Jim]

Thanks for taking the time to make and post the sketches, Roy.  I was not aware the reverse current upon button release before.

[Wayne Orwig]

  I was not aware the reverse current upon button release before.

Yea, IF everything is balanced, you get half the current, and one half is reversed field, so it SHOULD kick off.

But if the coils were mis-wired on this new starter, or even if the spring is a bit weak, it may stay on.

I've also wondered if the solenoid slug itself should or should not be magnetized a certain way.

[Triple Jim]

I'd think the slug would become permanently magnetized to some extent from the high flux of the pull-in coil, and this would work to help release it at the right time.