Starter Relay Fuse.

V

Vince

Hero member
I just blew a starter relay fuse playing around with a broken horn connection. It is a 30 and the spair fuse held in the same rubber holder is a 20. So whats correct so I dont risk any damage, 30 or 20.
 
If it's only feeding a starter relay, 20 should more than enough. The relay should only pull a couple of amps. If there's anything like 20A going through it you'll probably fry the contacts in the starter switch on the handlebar.
 
Thanks Cam, I was doing the anual filing of the not working rusty horn points after fixing the broken wiring connection didnt get the non working horn fixed. So nice to again have 2 working horns.
 
I bought 4 of 10 20 30 fuses, I supose I will try the 20 first and see what happens.The origanal fuse went when I bumped the live horn feed with the key on. It all works now with a 20 fuse installed, even both horns as well.
 
OK, so the horns are on the same circuit as the starter. Not that it makes any difference. It would be a very unusual situation where you need to use the horn and starter at the same time!

You'd probably get away with a 10A fuse.

Simon is quite right. Fuses are normally sized protect wires. You want the fuse to blow before the insulation melts off the wires. So a fuse needs to be sized a bit below the max current rating of the smallest wire it protects. I'd hazard a guess that the smallest wires might be the ones going to the handlebar switches. I don't recall the actual size of the wires that hang out of those switch blocks, but I'd guess no more than 1mm2 cross sectional area (about 17 AWG). That size of wire would struggle to take 20A without getting too hot. Normal rating would be in the 9-14A range depending on ventilation and proximity of other heat sources. So bearing that in mind, I'd be more inclined to go with a 10A or 15A fuse unless you have relays to take the load of the handlebar switches.
 
Stick with the 30A, he knows what's what in my application. It spent a week there sorting a couple of issues wiring wise and made a completely new ignition circuit that should stop it doing random loses of green smoke to the plugs, that would happen for 2 or 3 minutes and then come good. Hasn't happened since his magic fingers did there thing.Hope its good for good.
 
What does the wiring diagram say?
Mine says 25A.
My guess is that was there for a reason.
 
I suspect what Vince is talking about is the typical type of late model Japanese start relay with an integral 30A main fuse.  This allows for just one run of unfused +12V cable on the bike - from battery to start relay.  Any unfused permanent 12V cable on the bike is a big fire risk in the event of a short.  From the relay, two cables exit - one to the starter motor (unfused) and one smaller but still significant one (via the 30A fuse) which is the main supply to the fuse box.  Originally, Laverdas never had one, the supply to the fuse box was unfused.  You generally had 3 cables (start relay, fuse box supply, regulator charge wire) going to the battery +'ve, all of them unfused.
 
Yes to Steves point on the Starter Relay, from my limited knowledge.The wiring bloke knows his stuff so all is good.
 
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