What could be melting my main switch fuse

Hi all,

Welcome any advice on what could be burning out my 30A main switch fuse...

I'm working my way through (learning) the electrics of my 53 plate Monster 620 ie, to fix an ongoing intermittent electrical issue which

1) burns out my 30A main switch blade fuse after 30mins of riding (less if I'm using high revs), and 2) causes an infrequent 'miss' when riding... like the electrics miss a beat once in a while, causing the engine to stop firing. This happens for less than a second before it picks up again.

New stuff on the bike: reg/reg, battery, relays. New stator and fuel filters also in the post...
Wiring checked around battery; insulation looks ok, no visible breaks or shorts

I think the 30A main switch fuse protects the ignition circuit. Any clues on the culprit?

Cheers
Rob

[Mr Gazza]

I've read back on the history of your problem here. http://www.ukmonster.co.uk/monster/s...ad.php?t=57711
Seems like you've already had some good advice, and followed it.

As to the fuse blowing, my first thoughts would be a short, which could be hard to find and might even be hidden within the harness somewhere!! Prime suspect would be in that part of it that passes the steering head, where it flexes when the bars turn. You might be able to reproduce the fault by running the bike and moving the bars from side to side? Another vulnerable part of the harness is a "crook" more or less above the crankcase breather, just behind the vertical pot. Water can find it's way into the harness and pool at the bottom of the "crook" causing problems.. Sometimes.
Otherwise it's just a matter of carefully looking everywhere until you find it.. Not a great prospect!

As pointed out in the other thread, it could still be the fuel pump. This will draw more current if the filter is getting full as the load and therefor the resistance will build up. I can't think of any more obvious sources of resistance at the moment, but others might? You can do a resistance check on the pump with a meter.. StevieL would have the method and figures for that test.

You have fitted a new mosfet R/R. I can't emphasise enough, the need for good low resistance, weatherproof connections of the three yellow cables to the rectifier. I use Gold connectors on these, others hard wire, solder or crimp these cables. This won't be the cause of fuse blowing, but it is worthwhile checking the voltage at the battery terminals with the engine running to ensure that everything is working okay and there is not excessive voltage getting through. You should see a maximum of 14.5-15 volts. If you have a surge above that you might be lucky enough to blow the fuse before boiling the battery.

Finally. The missing phenomenon might be caused by dodgy connections to the injectors, which are fired electrically. The plugs can sometimes play up due to corrosion or wear making them loose. Mine looked perfectly normal but would not pass current from plug to socket! They only pass a signal current, so I doubt they would have anything to do with the fuse blowing, but worth checking thoroughly anyway.

This all sounds highly annoying, I hope you get to the bottom of it.

Thanks again, much appreciated. I may have been confusing myself earlier on which fuse is key - ie, the 30A fuse melting is not the 'main' fuse (the big bugger on the left of the tank) but the 'main switch' fuse which sits in the fuse box. I've read that serves the ignition circuit. in any case, I'll change the stator and fuel filter, plugs too would be worth it by the sound of things.

[Mr Gazza]

I had assumed that your spark plugs would be in good order if you were chasing an intermittent misfire, as those would be the first thing to check. I give mine a 6000 mile service life, having found that after that they can cause less than crisp performance. (that said the current pair have lasted much longer this time.)

The plugs I was alluding to in the last post are the electrical connector plugs on the harness that plug into the injectors. I had trouble with mine, or rather the previous owner did! I bought it as a non-runner and traced the problem to those plugs. As I mentioned they looked in perfect order and signals were coming out of the plug terminals but not transmitting to the socket on the injectors. It was a bit of a swine to diagnose, as everything checked out, except the injectors were not firing. I twigged it by spraying switch cleaner into the plug. The engine fired and ran when the switch cleaner was wet, but when engine heat soon dried it out, it died again one cylinder at a time.. and there was the smoking gun! I replaced the Marelli plugs and it's been no bother since.

I'm not saying that is necessarily your problem, but the moral is; take nothing for granted when testing.

Also not sure how the alternator stator could have any effect on either of the problems you are experiencing, replacing that sounds like a waste of money to me and a red herring in the equation anyway.

[Nickj]

The main fuse is probably a short or maybe the regulator is a bit dodgy and fails when it starts to warm up. Possibly one of the diodes in it has gone bad, that could easily give you that kind of problem.
If this doesn't happen when you take the bars from lock to lock then the loom is probably OK and it is something that like above.
Alternator isn't likely to be the culprit, volts out do rise with revs (hence the regulator) but it shouldn't be a real issue. It ought to happen as soon as the revs rise.

Intermittent bad running.. plugs and injectors. Change the plugs maybe clean out the fuel system.
Both might be linked if the reg is dumping more random voltages, sometimes low but sometimes high enough to trip the fuse

[manwithredbike]

Only going by the wiring on my carbie but, the main 30A fuse (the one in the fuse box) supplies the next 4 or 5 fuses below it in the box via the ignition relay which is operated by the key switch. (The charging circuit is protected by the big separate fuse and this plus the alternator and reulator cct are all upstream from the 30A fuse in the box so wouldn't seem to be involved). All the sub ccts eg, all various lights, horn, ignition system etc are sub fused after the ignition relay so you would expect that any fault or overload on these ccts would blow their individual fuse. The fact that the fuse is blowing after 30mins would suggest that it is either running at or just over full load causing it to blow gradually or an intermittent short circuit and very possibly the supply to/from the ignition switch in the loom where it meets the front forks/yolks?? Possibly the cutting out could be related to loom damage round that area??
Phil

[Darren69]

If you dont mean the big 30A fuse but instead the one in the fusebox then I'd check or replace the relays esp the main ignition one.

[Darren69]

Check and change fuel filter, if blockec can overlisd fuel pump.

[manwithredbike]

You could be right Darren but an over loaded fuel pump should blow the sub fuse on that circuit first.... unless there's already a close to full load on the main fuse due to other components i suppose?

[Darren69]

So do you mean the big 30A overall fuse is blowing or tbe main one in the fusebox? I'm confused.

The main fuse housed in the fuse box. On a normal ride A brand new one of these 30a blade fuses will melt within 30mins (but not totally blow) and the bike will stop as dead as a duck until I’ve replaced it. So I take 3/4 when I’m out

[Mr Gazza]

This sounds very strange, I don't think I've ever heard of a fuse slowly melting. The idea is that they blow before damage is done.
So it probably isn't a dead short. It could be something creating a large load or high resistance.. The fuel pump we have covered and this is still to be checked. The only other thing I can think of that would create a high load would be a super headlamp bulb of illegal proportions. You can get very high wattage bulbs for off road use only, might be worth checking that, unless of course the headlights are not even on when this happens?

I'd be looking for softened cable insulation. It usually shrinks back from ends and tried to wrinkle up, sometimes it will melt and reveal conductor and distort connector blocks too. If you can find evidence of that it could narrow things down. Have you had a feel round for warm cables when the fuse goes?

I think you need to get a multimeter on the battery to check the voltage too. As mentioned before, it shouldn't exceed 15 Volts really. You should see up to 13.5 volts when switched off. This will go up to no more than 15 volts as the revs rise. Also expect a large drop when the starter is engaged, it should recover from that immediately.
If you are getting 17 or more volts into the harness then it will certainly damage things, starting with the battery.

I think the "missing" and fuse melting are separate issues. Concentrate on what is blowing the fuse and then address the misfire if it still present.

[Darren69]

Could be poor/dirty connections in the fusebox itself causing heat build up. I probably wouldn't ride it unless you really have to, when they catch fire they really go up!

[Mr Gazza]

As Darren rightly says, it's probably not a good idea to persist with riding it with this potentially dangerous fault.
In line with Darren's thoughts on the fuse box, are your fuses a good tight fit in the holder? Mark9 mentioned in your other thread that he uses good quality thick bladed fuses to ensure a good fit, as cheaper ones have thinner blades and don't fit properly.
A loose fuse would explain your misfire too.

[Darren69]

As Gazza said high resistance not a short, that's what I suspect too. If 30A is a fair bit of current and if it cannot flow freely it will start to cause a lot of heat, enough to melt wires. Check connections are all good. The fusebox may already be damaged, so check that too.