electrics advice please - When do you need a relay?

[Steve Blackdog 687]

Hi folks

 

Can anyone explain to me when and why you need a sub-harness, a relay and /or a dedicated fuse when fitting an accessory?

 

I am about to fit a dual USB (touratech).  The instructions just say wire up to the negative/ground terminal of the battery and also to, eg, the headlight live wire.  It goes on to say 

On BMW CAN-BUS motorcycles such as BMW R1200GS, R1200RT & F800GS, we only recommend using a relay, or TPS 15 CAN-Bus Helper.

What's that all about?

 

I have seen on here that certain heated grips require a sub-harness.

 

Please can someone educate me as to what all these things do!

Cheers

Steve

 

 

[fred_jb 10,268]

Steve I'm no expert on vehicle electrics, but I do have an electronics background, so will tell you what I understand about this.

 

Firstly CAN-BUS wiring systems are basically computer controlled such that each circuit is monitored and can be electronically shut down if it takes more that a pre-determined amount of current. There are normally no traditional fuses for lights, accessories, etc, on such systems, and the fact that each circuit is closely monitored means that just taking a feed from one circuit, say the headlight one, to provide power for some add-on lights is very likely to result in the headlamp circuit being regarded as faulty and being shut down. Because the circuits are electronically monitored to fairly close tolerances there is not much leeway to add load beyond the normally expected level, so you need special CAN-BUS compatible devices to provide additional power feeds on these systems.

 

Traditional wiring systems such as that on the NC are more forgiving.   You can tap into one circuit to feed another, but you still need to be wary of putting to much load on the host circuit as it will be fused.  The fuse value will be chosen to be appropriate for whatever is normally attached to that circuit.  However with fused systems there is usually a bigger tolerance so the fuse rating may be significantly higher than the normal load on a particular circuit.   This means that you can usually get away with tapping in something which doesn't increase the normal current by too much, but if you add a heavy load to the circuit you may well blow the fuse, with all the inconvenience that could entail.

 

To take the example of adding auxiliary lights;  you may find that the load on these is too high to simply supply them from the same circuit which powers the headlight.  In this case you can fit the standard accessories circuit (sub-harness, relay, and fuse) from Honda and feed the lights from that as it uses the relay to provide a 12V supply whenever the ignition is switched on.  I can't remember exactly what the fuse rating on this is, but is probably something like 10A, so you still have to make sure that the total current taken from all the accessories you hook into this stays below that level.

 

Am alternative way of tackling this would be to use the feed from another circuit, not to actually power the accessory, but just to switch on a relay as this only requires a small amount of current and will not overload the host circuit.  The relay will then supply a feed direct from the battery to the new accessory, normally with a fuse in the circuit suitable for the current rating of the accessory.   I should add, though you probably know this, that the purpose of a fuse is to melt if too much current is taken by a faulty device, thereby breaking the circuit and stopping any further current flowing to the faulty device, hopefully preventing it from going up in flames, melting the wiring loom, or flattening the battery!

 

Relays can simply be regarded as switches which allow a low power circuit to control a much higher power one.  For example, when you start the engine, a very large current is required to turn the starter motor, and this requires very thick cables from the battery to carry the current without melting.  It would be completely impractical to have this amount of current going through the handlebar switch, so instead the handlebar switch just provides enough current to activate a special relay called the starter solenoid, possibly through another standard relay.  This relatively small current energises an electromagnet which causes the heavy duty switch contacts inside the solenoid to move to the on position and allow a large amount of current to flow into the starter motor.

 

Lots of other smaller relays do the same thing for other circuits which means that the switches involved can be small and reliable, but large currents can still be switched on and off.

 

Hopefully that is reasonably accurate, but I'm happy to be corrected by anyone who knows more about this.

 

Fred

Edited February 17, 2015 by fred_jb

[Tex 36,817]

Good job, Fred. And thanks for taking the time to write such a detailed reply to the question.

[Steve Blackdog 687]

Fred (or should I say Sir)

 

Thanks for that very thorough explanation - it all makes sense now.

 

It sounds sensible to go for the fuse and relay in most circumstances.

Thanks again, Fred - I really appreciate that you put so much effort into this answer.

Best

Steve

[fred_jb 10,268]

Fred (or should I say Sir)

 

Thanks for that very thorough explanation - it all makes sense now.

 

It sounds sensible to go for the fuse and relay in most circumstances.

Thanks again, Fred - I really appreciate that you put so much effort into this answer.

Best

Steve

You're very welcome Steve - and no need for the Sir!

 

The standard Honda sub-harness and associated relay and fuse are a reasonably easy way to provide accessory power because the fuse panel already has a spare slot for this relay and fuse meaning that you do not have to do any wiring to connect up the power to the relay. The only issue I have with this is that the socket the output of this relay goes to is hidden away under the LH side panels, and this is where you have to plug in the sub-harness.  

 

All the sub-harness does is take power and earth from this socket and split it to feed several accessories sockets which you can plug your accessories into - fine if they are Honda accessories because they will come with matching plugs to connect to one of these feeds.  However, you can buy the plugs separately and fit to non-Honda accessories if you want to power them from the Honda sub-harness.

 

The issue here is that all the sockets on the sub-harness are supposed to be tucked away under the side panels, meaning that whenever you want to add an accessory you have to take the panels off to connect it up.  I'm intending to extend the wiring the sub-harness plugs into so that all the sockets can be made accessible inside the battery compartment for ease of use in future.   I'm not sure if the standard sub-harness is long enough to do that anyway without extending as I've been putting off removing my panels!

 

Will report back on this soon!

 

Fred

Edited February 17, 2015 by fred_jb

[Tigertail 64]

_{The standard Honda sub harness is long enough (just) to feed into the compartment above the battery, so you don't need to remove side panels to add extra goodies.  That is where I put my sub harness when I fitted it, in retrospect due to the poor quality of the Honda cables and fittings I am planning on making up my own harness at sometime in the future, preferably when the weather is a bit warmer to work in my non-heated garage!  I would seriously recommend anyone wanting a sub harness to do a search through the many threads on here about this subject and make up their own harness - just get the relay & fuse from your Honda dealer.}

Edited February 17, 2015 by Tigertail

[fred_jb 10,268]

_{The standard Honda sub harness is long enough (just) to feed into the compartment above the battery, so you don't need to remove side panels to add extra goodies.  That is where I put my sub harness when I fitted it, in retrospect due to the poor quality of the Honda cables and fittings I am planning on making up my own harness at sometime in the future, preferably when the weather is a bit warmer to work in my non-heated garage!  I would seriously recommend anyone wanting a sub harness to do a search through the many threads on here about this subject and make up their own harness - just get the relay & fuse from your Honda dealer.}

Thanks for that confirmation Kev.

 

I had pretty much decided to make up my own sub-harness anyway because of the poor quality and very thin cables in the Honda one.  It will be shorter than the Honda one because I will make up an extension cable that plugs into the 6 way socket to allow me to put a 6 way socket for the sub-harness in the battery compartment.  I've already got the bits - just need to make up my mind to get on with removing the side panels!   I need to do this anyway because I need to get at the air filter housing to fit a K&N filter.

 

Fred

[embee 7,288]

Good write up by Fred.

 

Remember, the fuse is there to protect the wiring.

 

The expensive delicate piece of electronics you attach to the wiring will usually protect the fuse by blowing first.

[nabrU 2,294]

fred_jb and Tigertail,

 

Last Sunday I fitted my heated grips and Honda sub-harness. However I cannot confirm that sub-harness is long enough to feed into above-battery compartment. Well it is long enough physically but then there is a problem with fitting LH plastic fairing. You have to push the side cover tab of the LH panel into the grommet (seen at the picture left of the sub-harness connector). I was unable to do this with sub-harness fitted like on the picture (connector is to thick to allow this) so side panel simply sticked out somehow.

 

That's why according to manual sub-harness connector has to be secured the other way. But then it's not possible to reach battery compartment.

 

  

[TheEnglishman 401]

It's just easier with the sub-harness and associated relay 'pack'.  You can do it without, but if you've spent thousands on a bike why not avoid any pain with a few quids worth of wire and connectors specifically designed to help you?

 

Chopping into existing wiring will lead to a whole world of pain you really don't want to get into.  And if you look carefully, the headlight wire really isn't the one you want to get into.

 

Oh and CANBUS is a communication protocol - it has nothing to do with fuses and was designed to let automotive devices talk to each other without the need for computer to be involved (cheaper, lighter wiring(copper is expensive) - sounds technical so they can justify charging more).  One of my wife's cars was controlled from a joystick and a gameboy type console on the steering wheel, all hooked into the CANBUS network.  Driving it was ace.  For her because it meant she could get out and for me 'cos it felt like a driving computer game!

[fred_jb 10,268]

It's just easier with the sub-harness and associated relay 'pack'.  You can do it without, but if you've spent thousands on a bike why not avoid any pain with a few quids worth of wire and connectors specifically designed to help you?

 

Chopping into existing wiring will lead to a whole world of pain you really don't want to get into.  And if you look carefully, the headlight wire really isn't the one you want to get into.

 

Oh and CANBUS is a communication protocol - it has nothing to do with fuses and was designed to let automotive devices talk to each other without the need for computer to be involved (cheaper, lighter wiring(copper is expensive) - sounds technical so they can justify charging more).  One of my wife's cars was controlled from a joystick and a gameboy type console on the steering wheel, all hooked into the CANBUS network.  Driving it was ace.  For her because it meant she could get out and for me 'cos it felt like a driving computer game!

You are of course correct that CAN-BUS is actually a communications protocol running over a serial bus which lets multiple control units (effectively computers) within a vehicle talk to each other.  However, in the context of the original question the practical impact of such systems is in the way they are normally associated with computer based management and diagnostics of electrical devices and accessories, which is what allows the elimination of conventional fuses on a CAN-BUS based wiring system.

 

 This consequently limits the ability to add additional loads to existing circuits, so I was trying to keep it simple!

 

Fred

[Tigertail 64]

Neil, in your photo if you route the sub harness below the grommet hole and into the gap between the plastic panels it should reach easily and I had no problems fitting the side panel back on.  I believe that when I did mine I taped the 6 pin connector flat along the existing loom and then up into the gap between the two black plastic inner panels, this leads straight up into the battery compartment.  It was tight getting the plastic weather cover through the gap into the compartment but not too difficult, and you don't really need it once your cable ends are inside the compartment anyway.

[Tigertail 64]

View of Sub-harness from inside Battery compartment.  Easiest way is to route it from L/H side of fusebox, if you look from above you will see a (fairly big) gap down to the frame tube.  Use a zip tie and thread it down the gap to see it comes out quite close to the sub-harness 6 pin connection.

 

http://i296.photobucket.com/albums/mm174/kevin_mansell1/battery%20comp1_zpsqb8fihzp.jpg

 

http://i296.photobucket.com/albums/mm174/kevin_mansell1/battery%20comp3_zpsybr41xyq.jpg

 

 

Edited February 19, 2015 by Tigertail