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#704420 - 08/09/17 1:21 pm Adding fuses ?  
Joined: Jul 2015
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Pdh1 Offline
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Pdh1  Offline
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Kent UK
The one and only fuse blew when switching on lights,(at an mot of course !) I had a spare and no problem since, but now considering fitting an extra fuse to the light circuit on the brown/white wire going to main light switch. My headlight is 55/65w, my main fuse is 15amp blade. What rating fuse do I need to blow if there's a short in lighting circuit, that won't blow main blade fuse so bike keeps running ? Also, are the amp ratings different if it's a mini blade or inline glass fuse ? I'll probably use mini blade if I can as replacement fuses are easier to come by. Whilst doing this, is it worth putting an extra fuse anywhere else, such as horn etc ?

Last edited by Pdh1; 08/09/17 1:23 pm.

A65L 1966
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#704439 - 08/09/17 5:08 pm Re: Adding fuses ? [Re: Pdh1]  
Joined: Jan 2004
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DMadigan Offline
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ca, us
Search back through the forum for fuse ratings. Some fuses are based upon blow rating, others on maximum current rating. Add the current draws for everything on and add a safety factor of 25% - 50% for the main fuse. Then add separate fuses to other circuits such as lighting and ignition. Probably group horn with lighting since I believe that is all the circuits that you have unless you are concerned with the horn taking out your lights.
You should use a resetable fuse for the main for convenience.

#704516 - 08/10/17 12:28 pm Re: Adding fuses ? [Re: DMadigan]  
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Stuart Online content
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Stuart  Online Content
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Hi,

Originally Posted by Pdh1
The one and only fuse blew when switching on lights,(at an mot of course !) I had a spare and no problem since, but now considering fitting an extra fuse to the light circuit on the brown/white wire going to main light switch. My headlight is 55/65w, my main fuse is 15amp blade. What rating fuse do I need to blow if there's a short in lighting circuit, that won't blow main blade fuse so bike keeps running ?

To calculate Amps from Watts, the formula is Watts divided by Volts equals Amps - I use a nominal 12V.

I suspect you mean the headlamp is 55/60 - dip/main Watts - as they're the standard quartz-halogen headlamp bulb ratings in GB? 60W is 5A using the calculation above.

As you intend to put a lighting fuse in the Brown/White wire to the lighting switch, don't forget that, by definition, the wire also carries the Amps for the tail-lamp filament (5W ~ 0.5A) and whatever pilot lamp filament the bike has. Tot up the Amps and use the next-highest standard blade fuse (10A). If you use blade fuses, no need to worry about Dave's "Some fuses are based upon blow rating, others on maximum current rating"; all blade fuses are rated 'continuous' ("maximum current").

Originally Posted by Pdh1
are the amp ratings different if it's a mini blade

No.

Originally Posted by Pdh1
or inline glass fuse ?

Depends:-

. "glass" fuses fitted to Britbikes when they were new were rated by 'blow' Amps, which is twice the 'continuous' ("maximum current") rating - hence 25A or 35A in original manuals;

. otoh, the Japanese use the US system - all fuse types rated/marked 'continuous' - so watch out for "25A"-"35A" "glass" fuses intended for old Jap bikes, the equivalent of 50A-70A 'blow'! eek

Originally Posted by Pdh1
I'll probably use mini blade if I can as replacement fuses are easier to come by.

Mmmm ... I've not had any trouble getting standard blade fuses? Only problem with mini blades is the "mini" - they're so close together in a fuse box, you must have the plastic tweezers to fit 'em and pull 'em. frown I'm not sure you can get individual fuse holders for mini-blades(?) and fuse boxes are a logistical pita.

Originally Posted by Pdh1
Whilst doing this, is it worth putting an extra fuse anywhere else, such as horn etc ?

When doing my first rewire, I added just a fuse between rectifier/Zener and battery and another between the handlebar kill switch and electronic ignition; however, headlamp main 'n' dip, starter (T160) and horns are supplied through relays, and I use the type that take a blade fuse in the 'high current' part of the relay.

Where is the main fuse on your bike - between battery -ve and ammeter/ignition switch or in a single Red wire attached to battery +ve (assuming 'positive earth')? Fuse in Red wire, bigt fuse between battery -ve and ammeter/ignition switch sick

Hth.

Regards,

#704564 - 08/10/17 9:23 pm Re: Adding fuses ? [Re: Pdh1]  
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Pdh1 Offline
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Pdh1  Offline
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Thanks Stuart. My fuse is between battery positive (earth) and frame, red wire. I meant blade fuses in general were easier to get, rather glass. I didn't realise you couldn't get single mini fuse holder, so standard blade it is. I must have read about blow rates on Brit bikes fuses being different rating to blade, not realising they were different to jap glass fuses. Thanks for pointing those out to me. So, I will put std blade 10amp on brown/whi wire. If you think I should use relays, have you a link to relays you use? Why didn't they use relays originally, or were they just not around at that time? If you have a diagram for relays that would be helpful. I guess it will just be headlight/rear light, and horn. Should I change my 15 main fuse for resetable as Dmadagan suggests ? It seems a helpful idea. Thanks!

Last edited by Pdh1; 08/10/17 9:26 pm.

A65L 1966
#704720 - 08/12/17 12:46 pm Re: Adding fuses ? [Re: Pdh1]  
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Stuart Online content
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Stuart  Online Content
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Scotland
Hi,

Originally Posted by Pdh1
I didn't realise you couldn't get single mini fuse holder,

I'm not sure you can't, I've just never looked. I've pretty-much always used Vehicle Wiring Products and Autosparks, they don't sell 'em the cars have mini-blades, it's a pita keeping track of loose ones so I use ordinary blades on the bikes.

Originally Posted by Pdh1
use relays,

Up to you. I used relays on my T160's because I always intended fitting either 100W-main-beam or twin 60W-main-beams.However, I've fitted relays as a matter of course on any subsequent rewire simply 'cos I think even with only a single 60W main (55W dip), with ageing standard switches and other wiring, it's a good idea; and, if I want to upgrade to 100W main-beam subsequently, it's just a simple bulb change. T160's have a relay for the starter as standard; I upgrade horns to twin Fiamms, but afaik all twin-horn installations have a relay.

Originally Posted by Pdh1
have you a link to relays you use?

http://www.vehicle-wiring-products.eu/product.php/643/12v-4-blade-fused-relay.

The front part of the T160 rear mudguard is plastic and incorporates an open-topped compartment originally for the tool roll. The sides of the compartment just perfectly-accommodate two interlocking relay sockets back-to-back with the wires in the compartment. So I have a block of relays/bases under the seat, detachable from the harness with a multi-pin connector.

Otoh, the T100 doesn't have that compartment (unless I fit it with a T160's 2-piece rear mudguard), but the two headlamp relays fit inside the headlamp shell and the horns relay is mounted on the frame bracket under the tank that originally took the horn itself. smile On this bike, I've connected the wiring to the relays with "Trailing Relay Sockets".

Originally Posted by Pdh1
Why didn't they use relays originally,

They did. Indicators use a relay (Lucas 8FL); twin horns on '69 and '70 Lightnings and Bonnevilles, and '69-'74 triples, had a relay (Lucas 6RA); T160 original points ignition had 6V coils fed through a ballast resistor; a starter-button operated relay (Lucas 22RA) operates the starter solenoid, and bypasses the ballast resistor to feed full battery Volts to the coils just for starting.

However, apart from some Commandos, all the British bike makers used the shonky Lucas BPF headlamp bulbs, which were variously 50/40 or 45/35 (or 45/40?), none of which draw more than ~4A so, at best, relays would've been an unjustifiable extra cost when the bike makers paid Lucas as little as possible.

Btw, if you do a Forum Search for "relay", you'll find some contributors also use a relay to supply electronic ignition, in theory taking the load off the handlebar kill switch. In practice, any ignition coil(s) only draw(s) a similar ~3.5A to a 40W headlamp dip beam; if the kill switch is dodgy, it's still in the supply to a relay, so it'll still affect the ignition. I prefer to fix the kill switch and forego the relay. smile

Originally Posted by Pdh1
If you have a diagram for relays that would be helpful. I guess it will just be headlight/rear light, and horn.

Mmmm ... at its most basic, a relay is simply a remote on/off switch, that you operate with another on/off switch; all you're doing is taking away the Amps of, say, an electric-starter, 100W-main-beam, etc. so the hand-operated on/off switch and its wiring can be small and light. As the rear light or single standard horn are very small loads (~0.5A and ~2A respectively), imho relays for them become an added complication without any benefit.

Otoh, particularly twin accessory horns major on being LOUD grin so more Amps each, doubled, without a relay, through the standard handlebar button's tiny contact area risks welding the contacts together. eek So a relay for them.

With a more-powerful headlamp, depends how you want to do it. I prefer to keep the standard principle, of the dipswitch switching off one headlamp bulb filament when switching on the other. So I use two basic on/off relays, like the linked one with the fuse, one relay for main and one for dip. Then the dipswitch switches the relays, not the headlamp; each headlamp filament is connected to a relay, along with the battery.

Decide where you want to site the relays and if you want to use one or other of the sockets I linked.

If you look at the relay link again, you'll see a circuit diagram as one of the images; the terminal numbers will be moulded into the relay too:-

. 85 and 86 are the 'low-power' terminals; the 'oblong-with-slash' in between is the electro-magnet operated when the terminals are energised.

. As you've upgraded your bike's headlamp, you know the bulb main and dip wires are connected to the corresponding wires from the dipswitch (with bullet terminals and snap connectors?).

. Disconnect the headlamp from the dipswitch at those connections; connect each dipswitch wire with a new wire 'n' terminal to either 85 or 86 on one relay each; these then become respectively the 'main relay' and 'dip relay'. Connect the other of the 85/86 terminals on each relay to the bike's (Red?) 'earth' wires network.

. Also with new wires 'n' terminals, connect the headlamp main and dip wires to terminal 87 on the corresponding 'main' and 'dip relays.

. On my T160's, the relays' terminal 30 are connected directly to battery -ve with "28/0.30" wire (28 strands, each 0.3 mm. dia., 'thinwall' insulation, rated for 25A) through a 15A fuse (100W main is 8-and-a-bit Amps, 55W dip is ~4.5A, I might flash main when dip is on).

. Otoh, the T100 has the later version of the Lucas on/off ignition switch, with two double-spade terminals riveted to the back:-

.. As I don't go in for the 'parking lights' malarkey, the lighting toggle switch supply wire is on the same double-spade as the White wires. So the Brown/White from the ammeter is on its own on the other double-spade.

.. As the Brown/White from the ammeter, and the Brown/Blue from the battery to the ammeter, are also 25A-rated 28/0.30 thinwall, I connected the headlamp relays' supply wire just to the unused terminal on the ignition switch. This means any headlamp supply from the battery indicates on the ammeter, rather than bypassing it, so the ammeter still gives a true whistle indication of charging or discharge of the battery.

Originally Posted by Pdh1
Should I change my 15 main fuse for resetable

Up to you. I haven't used 'em because in the back of my mind is they have a finite life (1000 switches?), might not be a problem now but, when I did my first rewire thirty-five years ago, I didn't want the problem that, if it wouldn't reset years later, was there still a short or had the resettable reached the end of its life (when I'd be stuck without a fuse frown ). Maybe I'll carry a resettable as the spare? smile

Hth.

Regards,

#704839 - 08/13/17 2:03 am Re: Adding fuses ? [Re: Pdh1]  
Joined: Aug 2001
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Mark Z Online content
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Mark Z  Online Content
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Owego, NY, USA
I usually try to avoid expressing opinions on the board, but I'm deviating from that policy tonight.

First, to state what I hope is obvious: If you have a short, you must find it and fix it ASAP.

There are only two reasons for having multiple fuses. One applies more to automobiles, where there are so many circuits, switches, and appliances and so much wiring that's not accessible for inspection, that you need many fuses to help isolate the problem. The other reason, more applicable to motorcycles, especially those with relatively simple wiring, is to be able to disable non-essential circuitry in order to limp it home (and fix the problem!). Unless you have turn signals, the only non-essential circuitry is the lighting. That is, if you have a short in the ignition circuit, you aren't getting home without fixing it.

In this case (short in the lighting circuitry), "getting home" can be accomplished by simply carrying an extra fuse and turning off the lights - that is, unless it's nighttime, in which case the lighting is NOT non-essential, and you're not getting home without fixing it. Either way, there's really no need for multiple fuses.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
#704844 - 08/13/17 2:42 am Re: Adding fuses ? [Re: Pdh1]  
Joined: Jan 2004
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DMadigan Offline
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DMadigan  Offline
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ca, us
Stuart, sorry, but I would have fixed the problem before exhausting the life of a resettable fuse. The point in using one is you do not have to carry a spare and you can reset it whilst looking for the problem without wasting a lot of fuses or always carrying a meter on the road to find wiring shorts.
Mark, I would agree except for the odd fault where something accidentally shorts the wiring and blows the fuse. You would have to carry at least one spare. Having one for the lighting/horn and another for the ignition at least keeps you going if the fault is not the ignition (and not night time).
I had a pull ring that holds the coil bracket short against the coil's power terminal intermittently. That took a while to find but I did not need a load of fuses on hand and could keep going by resetting the fuse.
But, to keep it original, just install one fuse holder, the spring loaded type that holds a glass fuse dangling between the battery and harness.

#704865 - 08/13/17 2:00 pm Re: Adding fuses ? [Re: Pdh1]  
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Pdh1 Offline
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Pdh1  Offline
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Kent UK
For my part, the fuse blew when switching on lights, so easy to solve behind Mot station. Pull wire on lights. Rectify the short (water in dip switch) reconnect wire, fit spare fuse which I had, and mot sorted. What I don't want is something in the light/ horn circuit shorting when riding, blowing the one fuse and therefore cutting the engine cutting out. Sod's law says this will happen on a bend at night not a straight during day. Several fuses also makes fault finding a bit easier. Reading all the advice, I think I'm going to fit re- setable main 15amp, and further add blade fuses for lighting and horn.

Last edited by Pdh1; 08/13/17 2:01 pm.

A65L 1966

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