
Over the winter, I replaced all of my rear lights with custom LEDs. I got everything together, and on my first ride out, discovered that my cruise control no longer worked. It would turn on, but ignored my pressing the "Set" button as if it wasn't even connected. The cruise control has always worked 100% before, so I had to assume something I did caused the problem.
I pulled the trunk off and took the cruise computer out. I hooked up a meter to the connector and tested all the signals from the various switches - brake, clutch, throttle disengage, and so on. Everything checked out 100% correct. I then had a look at the circuit:
The cruise computer monitors the brake lights - it has a connection right to the same wire that is turning the brake lights on. This is so it shuts off immediately when you hit the brakes. You hit the brakes, it sees +12V on that green/yellow wire, and shuts the cruise off. I started thinking - what if that wire needs to be pulled to ground in order to activate the cruise? Normally, it would be pulled to ground through the filaments of all the brake light bulbs. What if the various LEDs I had installed weren't presenting enough of a ground to allow the cruise control to activate? I decided to try an experiment. I added a SPDT relay to the circuit, so that when the brakes were not applied, the cruise control saw ground, and when the brakes were applied, the cruise control saw +12V:
I reassembled the bike and took it out for a test ride - and the verdict is, the cruise control now works again. So my hypothesis was correct: The cruise control not only disables itself when it sees +12V on the green/yellow brake line, it disables itself when it sees ANYTHING other than ground on that wire - and a floating input counts as "not ground".
So if anyone replaces their brake lights with LEDs and finds they can no longer set their cruise control, this is the solution.