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2015 BMW R nine T 2018 BMW 1200GSA Rallye, 2018 BMW R1200R Sport, 2005 Kawasaki ZRX1200R
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59 Posts
Discussion Starter · #1 ·
2015 R9T, LED tail and brake light.
Pull front brake lever, brake light works fine.
Press rear brake pedal, nothing.
My circuit diagram shows the switch to be 'normally closed' so logic dictates that if the switch is stuck, disconnecting it would simulate it opening and operate the brake light. Tried this, didn't activate brake light.
Plugged in my motoscan, got the following data.
Central Chassis Electronics, fault codes - none.
Live data, Front brake light switch voltage at rest 2.48V, lever pulled 2.48V, brake light activates.
Live data, Back brake light switch voltage at rest 6.6V, depressed pedal, voltage drops to 2.48V, no brake light.
One final thing I did was disconnect the back brake switch again and the 'at rest' voltage dropped to 2.48V.
So I'm of the opinion that the switch is functioning perfectly, as is the LED light.
Why am I getting a higher voltage across the rear brake light switch which I assume is the signal voltage for the ECU.
I'm seriously baffled so any help would be appreciated.
 

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21 R9T
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170 Posts
Not sure whether this right, but if there are any zip ties fastened tightly in that circuit they can cause a break in the cable. Found this on industrial fans, we had a graveyard of fans that the sparkie couldn't fix it was really hot so I got digging. Turned out the cable ties had severed the wiring, just an off chance if you have a circuit tester or multi.
 

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Registered
2015 BMW R nine T 2018 BMW 1200GSA Rallye, 2018 BMW R1200R Sport, 2005 Kawasaki ZRX1200R
Joined
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59 Posts
Discussion Starter · #3 ·
Not sure whether this right, but if there are any zip ties fastened tightly in that circuit they can cause a break in the cable.
Hi Fez,
I've had a quick look at the wiring that goes to the switch from the main wiring harness under the tank and there are a couple of very tightly cinched cable ties in that run. The other thing I've noted is that there is absolutely no slack in the cabling anywhere. It seems to be run as tight as a bow string, no doubt designed and measured to the mm to save a cent.
It's certainly an idea worth pursuing. There aren't any easily accessible test points other than at the switch connection itself and I'm reluctant to start cutting back the braiding around the main loom but I may not have a choice if I need to check continuity from the brake light switch back to the ECU.
 
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