BMW K100 starter motor won’t stop turning

[Gregor]

Went to start the K yesterday and it would only turn the starter slowly and wouldn’t fire up, not been used for a couple months so battery was not fully charged.

Like a muppet I had to give the starter another thumb instead of charging it fully, resulting in the starter motor slowly turning even with ignition off, killswitch off and all the fuses pulled ……only stopped when I disconnected battery.

Took the relay apart….



Left hand upper contact looks smaller than the right.

I’ve cleaned up the earth point on the engine case and +ve to starter motor, used another battery and it still runs on without firing up inly stopping when -ve to battery disconnected.

I’ve taken the starter motor out and cleaned out all the carbon dust inside, brushes look good, gave the commutator a clean and it runs freely on the bench.

I’ll put it back tomorrow and try again but before ordering a new relay could it be anything else and what causes the starter to continue to run with ignition off, fuses out and kill-switch off ?

[roadster]

There may be what BMW call a load shedding relay which also connects to the starter positive ( black wire?). presumably it is meant to turn off non vital drains while the starter is operating but I wonder if it could be a voltage source due to some fault elsewhere.

[Rockburner]

My guess would be the relay.

[Gregor]

Well the relay needs replacing, the left upper contact is eroded badly.



I’ve cleaned up the contact points where starter bolts to the engine, threads on the bolts even the spring washers.



I’ll fit back in later and see what happens with a freshly charged battery.

[Gregor]

I was struggling to understand what is going on here when the starter continues to run with ign and killswitch off, fuses pulled etc.

This was on the excellent K100 forum.

[Rockburner]

I was struggling to understand what is going on here when the starter continues to run with ign and killswitch off, fuses pulled etc.

This was on the excellent K100 forum.

All sounds logical enough.

Keep the battery charged and get a good relay.

[Gregor]

New relay arrived yesterday and it fired up this morning but the old battery is definitely knackered and it also needs a new earth cable to replace the skinny BMW item.

So I’ll lay the bike up for now, spent enough on parts this month already, it wasn’t getting ridden enough anyway.

[Gregor]

The old battery negative cable to ground on the engine did not look very healthy.

[David]

The old battery negative cable to ground on the engine did not look very healthy.

And, the winner of this year's "Understatement of the Year" goes to......

[Le_Fromage_Grande]

If the relay get's stuck in the on position the starter motor will spin regardless of ignition or kill switches, there's usually a direct +12V feed to the relay, which then goes to the starter motor, the return earth is usually through the engine and isn't switched, so you've got a complete circuit if the spring in the relay fails.