Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board Where does the fuse go?

Mine are on the + side, and as far as I know always have been. Something I read here tells me this is wrong.

On a + earth bike with no electric starter, ideally you should have a fuse in a sole return lead close to the battery + terminal.

Because the factory had the fuse on the - side or power/hot side. I have seen recommendations of a fuse on both sides.

So all my red wires go to a single point and I have a blade fuse holder from that point to the + side and it's a red wire. So I'm OK. Thanks.

https://photos.app.goo.gl/GskWNjEqsBRAZXmD8

So + , - or both?????

With lower voltage
it does not matter which side/ polarity has the one fuse . ( the fuse is just the weakest link in the conductive-loop )

with where your fuse is ... all connections that start at the negitive side ... are returning through that one fuse to
Complete all electrical connectionS ... its all good

the one fuse could be ... moved to the end of the batterry negitive terminal
and the fuse protection would be 99% as good . ( you lose one fused protection ... a dropped wrench between
battery negative terminal and fused positive frame )

if this was a high-voltage circuit ... which this is not ... the fuse would need to be on the negative or switched-side ,
so that ground-continuity would not be lost ... even with a blown fuse .
( many people follow high voltage convention for low voltage applications ... just because it's what they're used to seeing )

you can add more fuses ... but why ?
adding extra fuses can be more trouble than its worth .
the electrical system is simple enough for one fuse to do it all ...
with one fuse to blow there is only one place to look .
adding extra connections ( fuses ) adds maintenance .

Hi, I know for sure factory put fuse in the ‘69-73 years in the brown/blue wire a few inches or so from the battery negative terminal. These are all positive ground bikes.

The red wire was not fused from factory on above bikes.

When they switched to negative ground I don’t know without looking at wire diagram.
Don

In the grand scheme of things it doesn't seem to matter where the fuse is as long as you have one.

It could matter. If you get a short between the hot terminal and the bike frame, you will have no protection at all unless the fuse is on the ground side of the battery. wrench, seatpan, could happen a number of ways. My bike is positive ground, so my fuse is between the single grounding point (former rectalfryer lug) and positive terminal of battery, as close to the terminal as I could practically get.

According to the manuals, they first put a fuse on the 650 in 1966, in the return lead.

In 1968, they moved it to the live side.

They recommend that older bikes without a fuse should be fitted with one in the return lead.



Don’t use a 35 A fuse. That was a defunct British auto Industry rating.

TT That copy from the manual in the first sentence says the fuse is on the negative terminal. In the last it's on the positive???????

yes , so what ... its still one fuse , just moved to the other side of the battery
... the page says the fuse was moved .... at/after a certain engine production number .

i think it corresponds to other wiring changes ... like no more rotary lighting switches ... they weren't needed anymore .
moving the fuse to the live-side or switched-side ... moved the fuse into the main harness assembly ... and makes
Production a tiny bit simpler ... ground side connection at battery is just a wire and not a sub-harness assembly ...with a fuse in a wire

the move obeys the convention for having all switched components on one side or polarity ... it doesn't make it better or worse .

Hi tritonthrasher, Thanks for posting that.
It does raise more questions as to why.

Here’s something I’ve pondered. Suppose you are riding 65 mph, so alternator output is fairly high. The fuse goes open circuit like vibration made it fracture. Or an intermittent short.

Would bike keep running until rpm got too low to support spark?

If bike kept running with intermittent short would it ultimately damage wiring? Maybe until short took all power to ground & no spark kills motor.

Not often but occasionally I’ve read about this sort of thing happening.
Don

If the battery is isolated by a loss of continuity at the fuse, the alternator can keep the ignition working, but not if there is a short circuit. The alternator won’t burn out the sort of wiring that we see on old Triumphs.

Unless someone can convince me otherwise I'm going to leave well enough alone. Everything works and a fuse did blow a while back. Can't remember what I found but the fuse did what it was supposed to.

if the battery loses connection ...the alternator can still power the bike

but if there there is a short circuit... something totally different happens ... two independent Power supplies
are shorted ... but behave quite differently .
the fuse blows the battery connection ... as the battery tries to flow over 15 amps ( or whatever the fuse rating is )

and a short of the inductive output of the altenator ... pulls the voltage output to zero / quickly below ignition support
and the bike stops.
( the voltage from an inductor is proportional to the resistance )
this same principle is used to regulate the voltage with most modern Black Box voltage regulators
... where the output is shunted for part of each Hertz cycle )
but with a wiring short ... the shunt is permanent until the short is fixed/removed .

the alternator output does not need a fuse , it flows 10 maybe 15 amps into wiring rated for 20amps

the battery needs a fuse ... because it can flow more amps... than the wiring can handle .
because the battery is the dangerous source
even
the alternator and/or voltage regulator are fuse protected from the battery ... not the other way around .

Forgive me, but what does that mean? Plus or minus fuse connection?

Forgive me, but what does that mean? Plus or minus fuse connection?

Desco, I read a similar thread a while back. There were lots of opinions and lots of “I know better than you”. In the end I stuck a fuse on both sides of the battery. One or the other will blow!

Plus = positive
Minus = negative

Good thought and cheap insurance.

Useful info for someone who has not seen a battery.

I have been working on the 72 for over thirty years, the 68 for 16. I have owned Triumph's since I was 18. I am now 76. From total engine rebuilds to minor repairs and troubleshooting. I am not an idiot. Well maybe I am. Why else would I keep doing this? I am just looking for a simple answer to where the fuse goes. Seems to be about 50/50 opinion wise. Both sides will keep everyone happy.

It’s been answered. There is a slight advantage to putting the fuse in the return.



Two fuses in the same circuit is a bit silly.

One can put a fuse in every wire if that floats yer boat.

However, if one wants sensible protection with one fuse, it would be placed near the battery+ve terminal (in a +ve ground system, sorry Stuart) in the one wire to the batt+ve.

I don’t know why Triumph (Lucas really) changed their minds on this. Did it happen on BSA and Norton as well about the same time?