Starter Relay Failure

By Tom Coradeschi

The battery in my K100RS went dead the other day. I tried to start the bike, but it wouldn't catch. The problem is that when I jumpstarted it (installed a new battery in it) the starter kept running and running until I disconnected the battery cable. When I reconnect it, the starter begins running again.


What you describe is a common, or fairly so, symptom of trying to start a K-Bike when the battery is not fully charged.

Pretty much what happens is this:

  1. The battery is low on charge, but is not totally dead.
  2. The low voltage condition of the battery means that it does not fully pull the starter relay contacts closed.
  3. Once the relay contacts close (however little they might), the remaining energy in the battery gets dumped to the starter.
  4. Your choice of:
    1. The low contact pressure and the high current through them combine to cause the contacts to become very hot and to weld themselves together.
    2. The low contact pressure, combined with the high starter current (which then lowers the system voltage even more), which results in the contacts bouncing open and closed. The resulting arcing causes the contacts to become very hot and to weld themselves together.
Either way, pretty much the only fix (you can try to pull the relay and repair it) is to replace the starter relay. It's found in the electrical box, under the fuel tank. Refer to Paul Glaves' tech note to identify the specific relay for your bike.

Hope this helps...

tom c


All contents Copyright © 1996 - 2018 Skylands BMW Riders

Maintainer: Tom Coradeschi WWW - email
Last Update: 10 June 2007