AJS & Matchless Owners Club

Rob Harknett wrote:Just a thought. I guess the fuses are all the quick blow type. How would an anti surge fuse suit us??

A time delay fuse could take 10 or more seconds longer to blow than a fast acting fuse, you could end up with damage to the wiring before it blew.

Also automotive fuses are designed to withstand vibration, a slow blow fuse is likely to be damaged by vibration.

So you are likely to have the time delay fuse fracture in normal use rather than it opening under fault conditions.

Dr James Smith, of Matchless Clueless and Classic Motorcycle Electrics Manual fame, suggests a fuse rating that is 25% higher than maximum system load. See page 109 of the Manual.
Eg with a 35W headlamp, other small lights and brake light, max current draw would typically be about 10A (or 60W). Could be more with horn and brakes on.
Then there is the extra load to warm a cold filament in the headlight bulb, which can cause a surge.
He suggests a 15A fuse is the most appropriate commonly-available size for 6v systems, or 7.5A at 12.

In the text of the link put up by DM2 in the first post, the suggestion is made that the total load should be about 70% of the fuse rating - which is pretty much the same logic expressed the other way round.

A 10A fuse will often be fine on a 6v machine, but is IMHO a tad marginal. Any reasonable-sized fuse will blow immediately with a dead short, which is what it's all about really. I normally go 15A on 6v and 10A on 12v for a main fuse, being absolutely certain to be sure it covers the brake light and/or horn circuit if that comes direct from the battery. Can't go wrong with extra fuses, of course, in individual circuits, and it is worth noting that the makers of DVR2s specify a fuse on the output side of their gizmo (wire A from reg to battery).

Anyone know what happened to James Smith? His Website still existed last time I looked but no recent responses on it.

The key protection provided by the fuse(s) is to prevent the battery being shorted as you will see much high currents from a shorted battery than our aging dynamos can possibly provide and all you are doing is preventing the wiring from becoming the "one time, potentially very dangerous" fuse. Ultimately if a short occurs it doesn't care what the source of the current is of course, it will just sink it all until the weakest link fails which must by design be the fuse. Personally I have added multiple fuses to my AJ so each can be rated more closely to the load and the wiring supporting it, hence my secretly lurking air horn compressor has to be fed by a relay, upgraded cabling and a separate 25A fuse. I wouldn't want to be using that fuse to protect the rest of the bike!

I can also confirm the fragility of slow blow fuses - I spent 2 weeks last year (the joy of retirement!) searching for the intermittent fault that stopped my Mondeo dead in its tracks with no communication between the instrument cluster and the ECU which on most occasions cleared itself automatically after a cooling off period. Turned out to be a fractured main ECU fuse that under most circumstances would display continuity and operate just fine. Cost £2-52 to replace which was lucky as I was just about to send the instrument cluster off for a £250 "common fault" overhaul!

Alan

Its still there for me this morning, Samuel. Its well worth buying his book though, just in case - theyve not found a way of pulling the plug on printed paper yet

https://matchlessclueless.com/


Steve

Greybeard wrote:Its well worth buying his book though, just in case - theyve not found a way of pulling the plug on printed paper yet

Yes I have seen his Website recently, and I bought his book a few years ago. I meant that we have not heard from him personally since he ceased being a member and there have been no recent responses from him on his website.

Interesting about James - he was an enthusiastic member for a long while and was, as I recall, instrumental in developing an LED rear light "board".

Right, so 15 amps it is (thanks Groily and DM2) in no small part 'cos I have a pair left in the spares box. Gardening finished for the day (thank goodness) so I can isolate in the garage with a brace of fuses and, er, a beer.

Best wishes everyone!

Hello,
just to say I have experienced a blown fuse last saturday...I'm a very bad mechanic, and a worse electrician ! I was making a harness for the horn, as my wiring loom had two short wires Under the twin seat to fit the horn in this place, and for my bike the horn must be before the left hand barrel. I made accidentaly a short cut, and then no lights...
In a few seconds I have replaced the 10 A glass fuse I fitted on the live wire and all was O.K. with no more damages...
My battery is 12 V.
Conclusion : A 10 A fuse is ok for this purpose for a 12 V circuit.

Yep JN, that is exactly why I finally got round to fitting a fuse - avoiding a disaster! I think for 6v it has to be 15 A for all the reasons discussed above.

How the motorcycle manufacturers and the Prince of Darkness got away with peddling the "anti-fuse" logic for so many decades escapes me!
Still, we know different now and fuses are so much cheaper than new or re-built fire damaged motorcycles!

Alan