Brake light mystery

Hi all

After several decades of riding (dirt and street) I aquired a 2004 WR450 F last year...yes I know its old but it came with a grandfathered fully street legal licence plate setup (despite the fact it has no signals, horn, instrumentation or brake lights)...that was a necessity for my riding scenario.

However I have a strange electrical problem.

In a bid to at least have a rear brake light I have twice installed the appropriate hydraulic brake light switch just above the rear brake fluid reservoir. I have wired it so one wire from the switch connects directly to the positive terminal of the battery and the other wire is spliced into the yellow wire of the tail light wireing harness...(the blue and black wires are untouched). I have the appropriate dual filament bulb in the tail light assembly...see image below.


When I test this setup without starting the works perfectly. As soon as pressure on the brake pedal starts to engage the brake light comes on. Test after test is perfect.

HOWEVER....once I start the engine it almost immediately starts to take more and more brake pressure in order for the brake light to work..the tail light works just fine whether the brake light is engaged or not. After about 5 minutes the brake light stops working completely but the tail light continues to work. If I now turn the engine off...the brake light no longer works at all. The bulb is fine (neither filament is burned out) but the brake light switch switch no longer functions.

I have done this twice...with a new hydraulic brake switch each time. The result is always the same...everything works perfectly until I start the engine...then there is an immediate need to increase brake pedal pressure to get the brake light to come on and within 5 minutes it ceases to work at all..but the bulb is fine.

I appear to be "frying" the brake light switch..can anybody give me any insight as to why this is happening!

Is my wireing setup completly nuts?? (the place that sold me the switch said it should be OK)...

Could the bike have a modified stator setup that is putting out extra power that "fries" the switch?? (if come the bulbs don't fry)

Thanks very much for any assistance you can offer. I would love to get this working so I attract less attention during the unavoidable highway portions of my rides.


I would like to preface my post by saying that I am not familiar with the actual wiring in the WR.

Having said that, what you are describing and looking at the diagram, I am thinking the Yellow wire has a ground coming through it from the harness once the engine is running. If you cut the wire so that the switch is directly wired to the bulb and not "spliced" in to the harness as shown, I would think this would correct your problem. With a ground on the yellow and the switch trying to introduce a + it would seem you are trying to make a light bulb out of the switch and feeding + and - into the same line.

You think your bike is old, I got a '98 WR400 - how's that for old. Anyway, check the wires off the switch near the exhaust pipe to see if one got melted. This happened to me the other day.


You underestimate yourself my friend...I just did exactly what you suggested and everything works...including my existing brake light switch. Not sure how that came back to life but it has.

I am going for an all day ride tomorrow (playing hooky) and that will tell the tale, but I did run the engine for several minutes after doing the mods and all seems perfect.

You have my vote for the 2010 Nobel prize for tail light wiring on a 2004 WR450F!

Many thanks


Now make sure that its all waterproofed and go have a good ride.

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