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Discussion Starter #1
Newbie I am, and like all newbies I have lots of questions.

I note on my '72 Bonneville a brass device fitted to the right rear of the airbox with an uninterrupted power connection attached. It looks for all the world like a sender of some sort, but for the love of me I cannot figure what.

Has anyone an electrical circuit diagram of this model Bonneville that might explain the device?

I hope someone out there can help me out on this.

Thank you. Rod Goodall
 

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A photo would be good, but I would hazard a guess that it is a Zener diode, a type of voltage regulator. Bolted to the air filter is the standard location on T140s.
 

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Discussion Starter #4
Thank you daveforty and GrandPaulZ for replys. I am no sparky, but I do know that electricity takes the path of least resistance and so with the 12v being on a common clip I cannot understand how this device might work.

As I've said, the 12v goes to and from this diode on the same clip and so I struggle with the idea of just how this device can effect voltage without a seperate entry and exit. I accept that it must do so somehow as the wiring looks original and the clip was firmly attached when I opened it up.

I am chasing a short circuit and have found worn bare wires that I've repaired, but checking the voltage across this diode with a multimeter caused the fuse to blow. This immediately makes me suspicious of the diode itself.

I do not know how to insert a picture of the device here, and so you must rely on my narrative for what it's worth.

Thanks again for your respective interest.

Rod Goodall
 

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The Zener diode is connected between ground and + or -12v. With 12v across the thing it doesnt conduct. When the engine is running the alternator & rectifier generate power. As the output voltage from the alternator / rectifier rises the Zener starts to conduct and limits the voltage to (approx) 14v. It is effectively shorting any volts above 14v to ground. It will get hot while doing this, hence it is fixed to a large piece of metal to dump the heat. On some bikes this is an alloy finned thing benath the bottom yoke, on your bike (and mine) it is fixed to the air filter box. If you consider the wire on the terminal to be the 'entry', then the 'exit' is the heat it generates. Trying to think of a simile - how about; think of your battery as a bathtub and the alternator/rectifier as the taps filling it up, the Zener would be the overflow outlet that drains excess away.

If you run the engine and measure the voltage across the battery, it will be approx 12v with the engine off and as you run it up the voltage will increase with revs up to approx 14v, it shouldnt go (much) above that - maybe 14.5v. More than that and the Zener isnt working.

It is not unknown for them to fail, it will be a case of getting a new one and trying it. Note that they are 'polarised' so you will need the correct type of Zener for positive earth wiring or negative earth wiring. Also check the airbox casting is properly grounded, layers of paint may insulate the airbox from the frame, especially if the frame is epoxy coated. Use the resistance range of the meter, it should be zero Ohms between the two pieces.

Last year the Zener on my bike failed into a short circuit and took the fuse out and stopped the engine. I was at speed in traffic on the motorway at the time so it was a little exciting. I was able to get going by pulling the wire off the thing and replacing the fuse.
 

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Discussion Starter #6
Thank you daveforty for your expansive reply regarding the operation of the Zener Diode. You have made it's operation very understandable and and I am grateful for your effort on my behalf.

As it turns out the Zener is not faulty, as I found a dead short in the tail lamp bulb holder. Thanks to you I am now enlightened about the Zener Diode function and can keep this in mind if/when I should need to test for charging rates etc. Again my thanks to you.

May you have a safe and prosperous New Year.

Rod Goodall
 
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