The Darmah has a front and rear brake light switch with both screwed into their respective 'splitters' and work off the hydraulic pressure when the lever is operated. They're both connected to and run off the same live feed and their switched lives are also connected together. The front switch works fine & lights the bulb but the rear doesn't. When the rear pedal is operated 12.7 v is seen at the bulb holder but when the bulb is inserted it doesn't light. With the bulb inserted and the rear pedal operated 0 v is seen at the switched live terminal (of the switch). So do/can these stop switches fail only when under electrical load?
@Andy Bee I do not know how exactly they fail, or why, but I would try one that’s known to be ok (swap front to rear?) and see how that goes. G
Yep, unfortunately that is really the only way to go... but as you have to bleed the system after swapping them out I'll probably buy new so I only have to bleed one. The rear being a pain mind...
Seems a bit of an odd problem, can you just check both pressure switches are working (or not) and aren’t shorting to the switch case with a multimeter, and/or make some jumper leads to connect the rear switch to the front harness and vice versa, before opening the brake system? I wonder if it is a problem within the wiring harness.
Both switches work fine with when tested with no wiring attached i.e. terminals shorted when lever/pedal operated. I also did think it could be summat up with the loom but the live & switched live on one are connected to the other (live & switched live). So why does one work & not the other... Edit: I'll recheck each switch again tomorrow but will take proper notice of the actual resistance value when it shorts.
You wouldn't normally be able to swap them over as they would not be the same , the rear having a banjo fitting
If you buy new go for a VW polo type switch/intermotor, it will be half the price or less than ducati https://www.gsfcarparts.com/products/brake-light-switch-int51650 Yours is M10x1 If its banjo type drop me a PM
Hmmm... the rear on mine isn't a banjo fitting and uses the same switch as used on the front and shown in the first post. It is screwed into a splitter that has input from the master cylinder & output to the caliper.
OK thats fine. People do away with splitter and mount it on the caliper or master cylinder usually as its easier to bleed. the splitters are M10 x 1 so the link above is what you need
When the rear brake lever is operated there is still a resistance of around 15k between the switch terminals so it looks like the switch is faulty. Although thinking about it, it could be some debris/air in the fluid that is hampering the internal mechanism. They surely can't be that complex, just a spring and a ball bearing that moves with the pressure. I might just bleed out the system before ordering a new switch, to see if it makes a difference. That's a fine idea. The set up on mine stems from the late 80s when Tony Brancato, as part of a rebuild, fitted braided hoses all round which I was loathed to replace when I recommissioned it a few years back.
It's not an odd problem at all. It simply means that there is a high resistance across the internal contacts of the brake switch. A digital voltmeter draws no current so it registers 12V but when a bulb is connected the resistance prevents sufficient current from getting through and the voltage is dropping too low to light the bulb. It looks likef Andy Bee has connected his meter across the brake switch connections set to read resistance and read 15kΩ. It should read zero or very close to it.
This was what originally stumped me also... I could see the voltage but not when the bulb was connected... it didn't help I forgot all my electric circuit basics from 40 years ago. Every day is a (re)learning day The good new is, it's not the switch per se but contaminants in the fluid because after pumping a little fresh fluid through the bulb will light up but only intermittently at present. And what would you know? the resistance changes in harmony. Doh! cheers guys.
