Electricary issue and fuse rating

Stevo

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My ‘74 Mk ll 850 had been fitted with led lighting by the PO but the parking light in the head lamp didn’t work until I switched the wire positions at the switch that’s on the shell. Since I did that, although the lights work, after a few minutes riding with the main or dip beam switched on, the engine cuts out, then if I quickly move the ignition key to ignition only, she runs again. Eventually engine will cut again, then I discover the main fuse has blown.
The fuse fitted is 15amp but the book says it should be 35amp!
Can anyone confirm this please?
At the moment I’m unsure whether there’s a wiring problem or simply that the wrong fuse was fitted by the PO.
Thanks
 
Actually the correct "modern" rating for the fuse is 17A but since you won't find one of those in the parts store, most folks use a 15 though a 20 is probably OK. A 35A fuse will mean the fuse can carry more current than the wires! :eek: The 35A rating was a British standard that is not in use nowadays. The modern equivalents are essentially 1/2 of that listed in UK manuals of the period.

Something in your system is drawing too much power, down to the point where the ignition will not function with the lights on and eventually blowing the fuse. Most likely related to whatever was done during the conversion to LEDs OR the LED's do not work and play well with the rest of the system. I have no personal experience with LED installs but I recall reading that there can be issues involving the LACK of power draw by LEDs that can disrupt a conventional charging system.
 
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The fuse fitted is 15amp but the book says it should be 35amp!
Can anyone confirm this please?
The 35A book rating is the blow rating. If the 15A fuse is continuous rated then the two ratings are similar (35A blow = 17A continuous).
Thread 'Proper fuse' https://www.accessnorton.com/NortonCommando/proper-fuse.29738/

I switched the wire positions at the switch that’s on the shell.
Which "wires" did you switch? The pilot light would normally be powered by the brown/green from the ignition switch terminal 3, not from the headlamp selector switch.
 
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A 15A fuse is enough for a well working electrical system. Though no harm using a 20A fuse.
What I think is happening. With engine running and lights on, it draws much more current than the alternator delivers. So the battery delivers a lot of current, When the battery is depleted, engine cuts out. Switching to ignition only, disconnect the excessive current draw. Now the alternator has to charge the empty battery which will consume as much energy as the alternator can give. So current flow into the battery is so great that it blows the fuse.
As Mike says, likely something happened when changes was made in the headlamp switch.
 
Switching to ignition only, disconnect the excessive current draw. Now the alternator has to charge the empty battery which will consume as much energy as the alternator can give. So current flow into the battery is so great that it blows the fuse.
The standard alternator makes about 10A maximum so shouldn't blow a 15A continuous fuse.
 
My MK2a has ran for last 10 years with a modern 15A blade fuse, that's both with periods of normal and LED lights and its only blown a fuse when I shorted out a connection when replacing the tank. So there is something wrong with your wiring or a component.
 
After I had been running mine for about a year, I had the main fuse blow a couple of times. I was unable to track it down until I noticed the tachometer guage light was not working. There's a rubber-like material in the small bulb holder inside the rear of the guage, where the wires from the harness connect. This had perished and was allowing the wires to short.
 
The standard alternator makes about 10A maximum so shouldn't blow a 15A continuous fuse.
10A is how much the alternator can deliver at nominal voltage. (somewhere between 12V and 13V)
In this case the battery was at a voltage not enough for ignition to work. Depending on what ignition system somewhere between 6 and 10V.
I think output amperage in this case can be substantially greater.
 
Somewhat slow in getting back here as currently trailering the bike down to and around FL on a short vacation with her indoors. Appreciate all your responses guys, definitely stopped me from frying all the wiring or possibly the complete bike! Have been able to ride okay with 17amp fuse fitted by just not switching headlamp bulb on via the switch on the shell. I will go back to it and check properly that the correct wire colours go where they should and maybe change them back to how they were before I switched them around, if I can remember, at least bike ran ok even if sidelight didn't work!
Great timing also with Bonzo's recent thread and pics with great responses for reference, so cheers.

Edit: may not make any difference but forgot to mention bike has a Boyer ignition system fitted.
 
I will go back to it and check properly that the correct wire colours go where they should and maybe change them back to how they were before I switched them around, if I can remember, at least bike ran ok even if sidelight didn't work!

I suggest you check that moving the wires hasn't resulted in one of the bare switch tabs coming into contact with the headlamp reflector or any other item capable of grounding because the various tabs are, or become live/hot when the headlamp switch is operated.

Note that the headlamp switch should only select the main headlamp OFF-ON after the ignition switch has been turned to the "Ignition and Lights" position so the wires that would usually connect to the switch are blue/yellow 'UY' (from ignition switch terminal 4) and blue 'U' that goes to the handlebar switch cluster beam selector switch. All other lighting (pilot, tail and instruments) are powered by brown/green from the ignition switch (terminal 3).
Electricary issue and fuse rating
 
10A is how much the alternator can deliver at nominal voltage. (somewhere between 12V and 13V)
In this case the battery was at a voltage not enough for ignition to work. Depending on what ignition system somewhere between 6 and 10V.
I think output amperage in this case can be substantially greater.
No, the charge, regulated to around 14 V, will not be as high as 15 A into a motorcycle battery. The battery has internal resistance.
 
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Is the headlamp also LED? If so then that significantly reduces current rating needed for the fuse as the lamp would normally be the biggest consumer on the bike.
I've had a repeated blowing of the fuse that I eventually traced to a short happening inside of the 5 way multi block rubber connector under the tank. It was a new block, looked in great shape outside. The inside wall between two bullets was torn and led to the fuse blowing short.
 
No, the charge, regulated to around 14 V, will not be as high as 15 A into a motorcycle battery. The battery has internal resistance.
As far as I know, internal resistance in a lead acid battery is a fraction of an Ohm. Which makes it possible to blow the fuse. As OP Stevo described what happened.
Anyone have another explanation for the blown fuse?
 
A 15A continuous (assuming it is) fuse shouldn't blow until 30A...


...although I believe if subjected to a current significantly above the continuous rating it can blow before then.

Anyone have another explanation for the blown fuse?
A short from rearranging wires in the headlamp shell?

Also, it is not clear from the original description when the fuse blew. If the fuse blew due to a short then it could have caused the cutting out (at low RPM?) with the lights on because the electrical system would have only been running off the alternator output.
 
What an enigma this has turned out to be! For me anyway, as a total electrical things dummy. To recap, bike ran fine until I messed with wiring in the headlamp shell, namely the pilot lamp wasn't connected and I connected it to the flip switch so that it came on when the key switch was at ignition and lights position and it stayed on with dip and main beam on too. After say five or so minutes riding, engine would cut out and lights would also die, as my buddy riding ahead reported. When key switch was quickly turned to ignition only, engine would fire up.
On both occasions, I pulled off the road, stopped moving, engine idled then died almost straight away. Both times, investigation showed that main fuse had blown. Replaced fuse and bike could be ridden providing only pilot lamp was on.

So, after studying the helpful posts here, together with pics on another thread and shop manual diagrams, I can confirm all the connections in the headlamp shell are correct, plus I removed my connection between pilot lamp and the flip switch, made it safe and checked lights still work.

Also discovered possible smoking guns. The led head lamp appears to be very similar to that shown in another thread by Bonzo and appears to be a straight replacement fit, also the pilot lamp is not a led.
Does all this help to explain the problem I wonder?

Anyway, bike ran fine today for about a hour on the road with main beam shining bright all the way. Just because I'm curious, when we get back home I will connect the pilot lamp as per it should be and see what happens then!

Watch this space and thanks for your help everyone
Steve
 
So, after studying the helpful posts here, together with pics on another thread and shop manual diagrams, I can confirm all the connections in the headlamp shell are correct, plus I removed my connection between pilot lamp and the flip switch, made it safe and checked lights still work.

Make sure the two bare tabs on the bottom of the headlamp switch (terminals 5 & 6) cannot short to the headlamp reflector.
If necessary bend them as shown in the picture below because as previously mentioned those bare tabs can be live/hot.

Electricary issue and fuse rating
 
What an enigma this has turned out to be! For me anyway, as a total electrical things dummy. To recap, bike ran fine until I messed with wiring in the headlamp shell, namely the pilot lamp wasn't connected and I connected it to the flip switch so that it came on when the key switch was at ignition and lights position and it stayed on with dip and main beam on too. After say five or so minutes riding, engine would cut out and lights would also die, as my buddy riding ahead reported. When key switch was quickly turned to ignition only, engine would fire up.
On both occasions, I pulled off the road, stopped moving, engine idled then died almost straight away. Both times, investigation showed that main fuse had blown. Replaced fuse and bike could be ridden providing only pilot lamp was on.

So, after studying the helpful posts here, together with pics on another thread and shop manual diagrams, I can confirm all the connections in the headlamp shell are correct, plus I removed my connection between pilot lamp and the flip switch, made it safe and checked lights still work.

Also discovered possible smoking guns. The led head lamp appears to be very similar to that shown in another thread by Bonzo and appears to be a straight replacement fit, also the pilot lamp is not a led.
Does all this help to explain the problem I wonder?

Anyway, bike ran fine today for about a hour on the road with main beam shining bright all the way. Just because I'm curious, when we get back home I will connect the pilot lamp as per it should be and see what happens then!

Watch this space and thanks for your help everyone
Steve
I've read and understand your symptoms and how they seem to have started, but still: when running fine for a while, feel the fuse holder. If it feels warm it may be the problem. If it feels hot it probably is. If the terminals inside corrode, that adds resistance and therefore current to the system. In the last couple of years, I've fixed mysterious fuse blowing for two people due to 50-year-old fuse holders by replacing them (cleaning the contacts would have worked too). Another had an AGC 30 fuse - it was a re-wire for him as the wires melted, they took out other wires until the shorts reached 30 amps and blew the fuse - all caused by a corroded fuse holder and the wrong fuse..
 
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