Replacing the K75 Starter Relay with a Solenoid

The classic BMW K bikes have an issue where the starter relay will stick if the rider starts the bike with a low battery. The battery’s lower than optimal voltage causes higher temps and arcing in the relay and the relay’s contacts weld themselves together. The bike will start but then the starter motor continues to run and the load-shedding relay continues to disable voltage to other parts of the bike like the headlight. This can potentially damage the starter motor. Once the starter relay’s contact stick, they can often be unstuck by knocking the side of the relay box near the relay with a heavy tool; I use a socket breaker bar wrench. A once stuck and released relay is very likely to become stuck again, even with a fully charged battery.

This has happened to me many times. I’ve tried many fixes like heavier cables, a larger alternator and  new relays and yet the problem persists.

The quality Bosch relay that BMW put in the classic K bikes is barely rated high enough to crank the motor, so I wondered, what if it could be substituted for an even higher rated automobile/marine solenoid that would probably never stick? After discussing possible solutions with fellow Brick owners, I decided to go ahead and add a Cole Hersee 24047 solenoid into the relay box.

The voltage supplied by the K75’s ECU computer is strong enough to flip the relay’s switch, but not strong enough to flip the solenoid’s switch, so it was necessary to keep the Bosch relay and assign it to switch the solenoid which in turn would switch on power to the starter motor. The Bosch relay also contains a diode that prevents current from finding it’s way back to the ECU and potentially causing damage.

Solenoid to Switch the Starter Motor
Solenoid added to starter motor circuit. (click to enlarge)

I do not have a wiring diagram available, but basically… I inserted the solenoid into the circuit between the starter relay and the starter motor. So the starter switch activates the relay (as usual) and then the relay activates the new solenoid which in turn activates the starter motor. This eliminates any high amp current from the relay.

It is possible that simply replacing the relay with the solenoid might work, but I kept the relay to assure that no current inadvertently back-flowed into the ECU.

Fitting the solenoid into the relay box was a challenge, but since I’m running all LED lights on the bike now I could permanently remove the Bulb Monitoring Unit and make some room.

It works!