Motorcycle starter safety circuit

[Hondaboi1729]

I don't know if it's the same for a car but bikes uses a starter relay to set off a starter motor which cranks the engine.

Bikes have a side stand for you to lean the bike against when stationary. In case the rider tries to start the bike with side stand deployed (could cause crash if forget to lift up out of the way when cornering the bike) there is a side stand switch to cut off the ground to the starter relay if stand down.
In conjunction there is a clutch switch in series to prevent being started in gear. there is also a neutral switch in parallel to these two connected to ground so the bike can be started in neutral even if the side-stand is deployed
i.e side stand switch off (open)

But in every bike there's a diode below in the pic:

Basically as far as I can see the way each switch works is when it's open it cuts off the ground to the coil in the starter relay.
So either neutral switch is closed or BOTH clutch and sidestand switches are closed for current to be flowing into the relay...
Now I really can't see what difference would be made if you deleted that diode (presumably to protect the switches from backwards current). But I mean why on Earth would current go back the wrong way to potentially damage the switches? Is it something to do with the collapse of the magnetic field causing current to try and flow the other way when the starter button is released (since no more current to +ve so relay so field collapses)? Just a suggestion here- I'm asking not telling- I've no idea

IF this was the case though, then why is there no diode to the fuel pump relay, the cooling fan relay and any other kind of relay on the bike?

Also I've just thought- they're just bloody switches so unpolarised so why would the direction of the current matter- it makes less sense to me what this component does...

Edit I think I'm missing another diode to the clutch and sidestand switches in the diagram sorry.

 

[jim hardy]

My guess is there's also a neutral light and it shares the neutral switch.
The diode keeps it from lighting via the kickstand and clutch switches.

old jim

 

[Hondaboi1729]

Oh my lord Jim how clever are you? goddamn you figured it out without me even posting the complete circuit.
yeah on the small motorcycle I have there is indeed a neutral light connected to where you put it. The only thing though is that is in the instrument panel
a block connector which has pins which go into the dashboard. there is a designated neutral light pin. So that neutral pin must act as a +ve for conventional current to start flowing from it throught the neutral switch to ground. That explains that.

However because I don't understand the internals of the circuitry inside the dashboard or how it's programmed which leads a certain pin to be +ve or -ve the following also confuses me:

This time this is for a different bike of larger engine size and it has two diodes now:

https://ibb.co/mHws6G

The labelled pin "NL" in the instrument cluster is the Neutral Light pin:

We have established what the vertical diode is for: it's to stop current flowing from the +ve Neutral light pin through the closed clutch and sidestand switches to ground making the neutral light come on.

However the horizontal diode is for what? I mean looking at the current flow of the circuit the only thing it can be for is stop current flowing from the +ve at the top down throught the vertical diode and into the neutral light pin. But why would that ever happen? We said that it was connected to +ve already?

 

[CWatters]

I think it might be to prevent a fault in the instrument panel (short to 0V) grounding the starter relay?

 

[Hondaboi1729]

But had we not established the NL pin was acting as +ve? why would current flow from +ve at top of coil to +ve NL pin? Unless the polarity of a pin is not fixed and can actually vary as controlled by some chip or whatever in the circuit board which the pin is connected to in the instrumental?

 

[CWatters]

But had we not established the NL pin was acting as +ve? why would current flow from +ve at top of coil to +ve NL pin?

It wouldn't...unless there was a fault in the instrument cluster. For exampe a fault that connected NL to 0V.

 

[jim hardy]

@CWatters is close, I think .

My guess is it's so that ---
when you're not in neutral, no current can flow backward into the instrument cluster's NL pin.
Current should only come OUT OF that pin to light the neutral light, not sneak back IN and get to who knows where through the neutral light bulb.
Furthermore, if it's a LED bulb you can't afford to reverse bias it very hard.

That's called a "Sneak Circuit" and "Sneak Circuit Analysis" is a field of its own. "Sneak Circuits" cause strange things to happen, like aircraft cargo doors to open in flight. I used to chase them in the power plant...

My two cents, and that's overpriced .

 

[Averagesupernova]

@CWatters

That's called a "Sneak Circuit" and "Sneak Circuit Analysis" is a field of its own. "Sneak Circuits" cause strange things to happen, like aircraft cargo doors to open in flight. I used to chase them in the power plant...

My two cents, and that's overpriced .

Jim, how many aircraft cargo doors came open in your nuke plant? :)
I recall missing the boat one time with push button switches all being used to switch some relays on the low side. This particular test fixture was also controlled from a PC and I used a relay to open the ground connection to all the switches. This way the operator couldn't screw up the test when the PC was doing the auto test by accidentally pushing a button on the fixture. Sounds foolproof right? That is until the user would push two buttons at the same time. Doh!

 

[Tom.G]

My guess is it's so that ---
when you're not in neutral, no current can flow backward into the instrument cluster's NL pin.
Current should only come OUT OF that pin to light the neutral light, not sneak back IN and get to who knows where through the neutral light bulb.

If the fuse supplying the Instruement Panel blows, then all connections in the Panel are at ground... another case of a "Sneak Circuit."

 

[jim hardy]

... another case of a "Sneak Circuit."

yep, right around the neutral switch.

I've had two '83 Ford Rangers that will start in gear and clutch not depressed. THAT was a surprise first time... Must be a sneak circuit there somewhere. Both were diesels, i think the gasoline ones are okay..

 

[Hondaboi1729]

No Jim that was a very valuable two cents thank you :)
Thanks to Tom and C Watters too I now understand :)

 

[jim hardy]

Jim, how many aircraft cargo doors came open in your nuke plant? :)

Hmmm. I'm glad we didn't have any of those. ( See my signature...)