My ST4s has had an odd problem for a while now. While carefully observing the speed limits on motorways, I have the found that using the flash (pass) button for the high beams, when the head light is already on dipped beam, will often drop the bike onto one cylinder. You then have to turn of the lights completely, close the throttle, open it again and she fires back on two and then you can turn the lights on again. That's OK in daylight but a bit hairy at night. It has done this for a while now and I've checked the charging system (OK), the battery (OK), the battery leads and earths (OK) and changed the coil for the cylinder that goes (always the horizontal cylinder), to no avail. However, I have noticed that fuse for the head lights, a 20a one, will melt its casing without blowing the fuse (see pic). What would do this and what's the best thing to avoid it? Out of necessity I only had a 25A fuse to use instead of the 20, is this a bad idea and will it result in a fireball? A
My tuppence worth....there's obviously an issue....I predict your 20amp fuse is on the limit, ie probably +/-20,5amp is flowing through it! This it's enough to cook it but not enough to blow it! As temporary measure ie to get me home a 25amp fuse will do the trick, but you run the risk of blowing something more expensive! Seriously I would have it checked out at a auto/mbike electrician to see what is going on!
Unfortunately it's par for the course for many an Italian beauty. It's down to gauge and quality of the 'conductive parts' within the fuse body and i've seen it on many Lancia, Fiat and even Ferrari. It's the additional light corrosion over time that has it finally giving in to heatsoak. I had to bypass the fuse body on my Cagiva Elefant for the same reason as the later ones are particularly prone to the problem. The fact that the fuse hasn't blown helps determine that it's heat rather than current. If you catch it at an early stage then meticulously cleaning the contacts at the fuse body might delay the overheating for a while but usually a weakness sets in there and more drastic action is needed. I don't know how easy it is to dismantle your fuse body - have done it on a 748 before.
deffo heat build up due to bad connection at fuse holder. rule of thumb fuse is rated to 1.5xpridicted currant flow.
may have gone off on a tangent here in my first post as it could be the circuit is overloaded despite not blowing a fuse, due to some other totally unrelated reason which would be your first port of call..
Not really answering the question, but what type of headlamp bulb does it run? H4? If its an H4 only one filament in the dual filament bulb should be lit at any one time they are usually rated 45/55 but can vary. However when you use the flash on most bikes I've come across it will light the Hi beam filament regardless of what the Lo is doing. So you are now pulling 100W through the bulb. It seems strange that the charge system and battery are so marginal that the system cannot cope easily with that and would cause the bike to start cutting out. May be enough though to bring the fuse current close up to its max rating. How old is the battery, what size is it and what's the voltage on it when the bike is shut off? Just a thought John
The battery is just over a year old, and charge at rest is about 12.7v, but when idling with nothing turned on, the system register about 13.4 across the polls rising to about 14v at 5k rpm. I also use a trickle charger fairly regularly to keep the battery well topped up. Yeah the dipped beam bulb is a H3, with the main beam a H1. A
Well I could have it wrong then H3 and H1 are single filament bulbs & I don't know the configuration when on main beam From a wiring diagram it looks like its a one or other situation Lo or Hi but the switch schematic is not that clear. The passing/flash does what I thought and throws the Hi beam on regardless. There is a relay on the Hi beam but not on the Lo and both get fed power from that 20A fuse. Now knowing Ducati's reputation with relays I'd take a look at the Hi beam relay base and relay, any corrosion on the base clean it up and either swap or try a new Hi beam relay as a start. Its not cast in stone but its a possibility. Battery & charging system sound fine.
Yeah, when the high beam is on, both the low beam and high beam are on together. And when the pass button is used, it is the same thing. Right, will check the high beam relay and see if there's anything up with it. Cheers, A
If you can't find any obvious signs of overloading in the circuit then it's down to the fuse holder contacts for certain.
It could a cheap pound shop job type of fuse. I put a cheap one in my 748 and it melted. Try using a decent fuse, you could try the BMW (car) E36 type of fuse, very good quality and that, if nothing else, will eliminate one possibility. Cheap fuses don't necessarily blow like a proper fuse should, they just get hot and melt, no don't because the rating of them is not that accurate.
I had to fix a friend's ST4s with this fault. It was caused by severe corrosion of the wires in the lower section of the 25 pin connector linking the fairing sub-loom to the main loom. I had to bypass the connector for 4 of the wires. Final solution was to replace the sub-loom and it's connector.
