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Discussion Starter · #1 ·
Hi all, new to the board. I own an ST4 but I'm here for a newly crowned Ducatisti buddy who's having issues with his 96 M900. The taillight works but the brake light doesn't. The 2nd filament in the bulb is in tact so that's not it. Neither front nor rear brake actuate the light. Any ideas/troubleshooting tips? Thanks!

Duc
 
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Discussion Starter · #2 ·
Check your fuse and verify that you are getting voltage at the lamp socket with a multimeter or 12v tester.
 

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Assuming the wiring system is the same as that for an '00 M900, which may not be true...

Use a voltmeter and check to see if you get voltage at the Green/Red wire at the taillight when you actuate either brake lever. If you do, then either the little brass contact that bears against the nub on the taillight bulb isn't touching, or that bulb that looks good isn't. I've had bulbs that looked fine, but a filament would not light up.

If you don't get voltage, then check the Green/Red wire at each brake light switch, see if you get voltage there when you actuate each brake lever. If you don't the switch may be bad, or...

With the key on, you should get voltage at the brake switches on the Violet/Black wire.

If the horn works, the associated fuse, #5 I think, is OK. That fuse also controls the voltage to the brake light.
 

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While it's not s'posed to be possible to reverse a three conductor bayonet-socket lamp.... it is!

The socket is cheezy and will take the bulb backwards. The result can be just as you describe.
 

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I agree with Don & Speeddog, the Bulb holder certainly is cheesy. I've had to bend the brass strip back into shape several times, & had the bulb in back to front also!!

All giving the same result.
 

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I had the same problem with my M750. Take a look at the metal tabs in the light which touch the bulb, mine had only one touching and the light would stay on but wouldn't light up brighter for the brake. Just bend the metal tabs to a harder abgle so they touch both nipples on the bulb this should solve it.
 
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Discussion Starter · #7 ·
The previous owner suspects the switch at the front brake caliper, but unless the wiring for the rear and front are connected there that doesn't make sense to me. Are the two wires independent (electrically)? If so I'd say it sure as hell isn't that switch.

What you've all said makes sense and I'll have him check that asap.

Thanks!
 

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The previous owner suspects the switch at the front brake caliper, but unless the wiring for the rear and front are connected there that doesn't make sense to me. Are the two wires independent (electrically)? If so I'd say it sure as hell isn't that switch.

What you've all said makes sense and I'll have him check that asap.

Thanks!
yes, they are independant. of course, both switches could have independantly died. check the bulb and socket first. then check the switches.
 
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Discussion Starter · #9 ·
Report from the monster owner: "replaced the front switch with a new one. still nothing. i tried to turn bulb but broke it so as soon as i get a new one i'll be able to tell if that was the problem."

This, combined with your responses, strongly suggest to me that the issue in fact the socket or the contacts (or bulb pts).

How does the brake switch work? By that i mean does the brake light go on when the circuit is broken (when the switch is depressed)?

Trying to understand the big picture.

Thanks.
 

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when the switch is depressed (brakes applied), the circuit is complete (closed). the switch will show coninuity with an ohmeter.
 

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why just replace the bulb in the first place ?? cheaper than a switch and most likely the culprit.. time will tell us
 
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Discussion Starter · #13 ·
Re: Brake light mystery - SOLVED

Well, surprise surprise...it was a bulb-related issue. I didn't get the details other than to say putting a new bulb in did the trick. Oy jesus...

Thanks for the replies!
 

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Glad to hear it ended up being an easy fix!
 
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