I just finished a tail chop this weekend and in the course of wrapping everything up and testing signals, brake, etc. I found that my brake light is intermittent.  I don't know how long this has been the case - it could have been like this for weeks (scary thought).

OK - the facts:  '01 M900Sie, stock tail light assembly.  Initially after finishing the chop and custom bracket, I re-attached the wiring harness (with some very minor re-routing of the harness) and tested the signals/brake light out before tightening everything down.  Directionals - check.  Running light - check.  Brake light - nothing.  Swapped the running light wire and the brake light wire on the brake light to test bulb - still no response to brake input (front or rear).  Hmmm.  Re-seated bulb, cleaned contacts on bulb socket.  Nothing.  Scratch head....

Try again - re-seat connectors, mess around, scratch head.  All of a sudden, there is brake light response to rear brake pedal - but not to front brake lever!  So I think to myself:  could I have two problems - an intermittent connection to the rear brake light (bad crimp, etc.) AND a bad microswitch in the front lever?  How likely is that?

So I throw my hands up - it's late Sunday night and I can't deal with this.  I tell my son that the "chop" is done, looks sweet, but now I have an intermittent brake light.  He comes out to take a look - says the chop looks killer, and I go to show him the brake light issue.  You all know what's coming next.  Everything works fine!  WTF?

It would really help if any of you know the wiring for the brake light system.  Is it totally "passive" - in other words, do the front and rear brake switches simply complete a 12V circuit to the bulb, or are there other components involved (relays, ECU, bulb checking circuitry, etc.)?

The front and rear brake light switches run independent of each other.  You may indeed have two problems.  The front brake light switch or the cheesy bullet connectors could be one problem.  Follow the wires down the head tube from the microswitch.  You will come across the cheesy connectors.  Disconnect them and jump the two going to the rear.  Turn the ignition on.  If the brake light now works the problem is at the connectors or switch.  The other problem is probably either weak or dirty contacts at the bulb.  Try cleaning/bending as needed.  You can download a wiring diagram here: 
http://www.ducati.com/en/bikes/service/manuals/2001.jhtml?family=manuals

Great - thanks for the link to the diagram.  This makes it clear that there is nothing except a fused power to one side of each switch and a straight shot from the switches to the bulb (somewhere in the harness they must be spliced together since there is only one wire to the brake filament back at the bulb).  Maybe I'll take a soldering iron to all those cheesy in-line bullet connectors after I figure this out.

If it's the switch in the front break lever, is it a straightforward replacement, or do I have to do major surgery?  Is a whole new assembly the best option?

Fairly straight forward.  The switch is held in place by two 1 mm screws with nuts.  Do not lose them.  There is also a dowel in the lever.  Do not lose it.  The screws are also the adjustment for the switch. 

I had a problem with my rear brake light switch so replaced it wih a HEL s/s banjo switch at the master cyl. Since fitting it I found a 90° rubber boot  which will look better as in 2nd pic. Switch was only about £10 in UK so no doubt cheaper than Ducati switch.

Wow - that's pretty cool...does it work off of the brake fluid pressure?  If so that would probably be a lot more reliable than an exposed mechanical plunger type switch.  I'll have to see if I can track one of those down...

Quote from: sejman on October 07, 2008, 01:01:51 PM
Wow - that's pretty cool...does it work off of the brake fluid pressure? 

That's right - it replaces the banjo bolt that clamps the brake line to the master cylinder. Make sure you state the thread as M10 x 1
I tried to dismantle the plunger switch but it was crimped together and poor quality.

Did you check for power at the tail light assy with a 12 VDC test light?

I just replaced my tail light assy (96 M900) as the bottom of the bulb socket was partially broken out.  This allowed the bulb and contacts to move around and break contact with one another.

Got one from Chris at Motored Sports for $20.

I had the same problem. Turned out to be the bullet style connectors from the front switch where the wiring goes past the steering head. Pulled the headlight for better access and found the wires were pulled tight on full left lock. Cleaning connectors and a bit of rerouting was all it needed.  [moto]

Quote from: yeloduke on October 07, 2008, 06:35:16 PM
I had the same problem. Turned out to be the bullet style connectors from the front switch where the wiring goes past the steering head. Pulled the headlight for better access and found the wires were pulled tight on full left lock. Cleaning connectors and a bit of rerouting was all it needed.  [moto]

Ditto.  Got some slack in the lines and reconnected them.  It's been fine since.

Scott

That sounds about right - I'll check there first.  Right now it's working fine, but I'm certainly not leaving it this way.  Also the brake light assembly (socket, contacts, etc.) is pretty old and brittle, so I think I'll upgrade that to an LED version while I'm at it.