Fuse blows when ignition is turned on?

[-Savage-]

Hi all,

CBR600FX 1999.

Trying to get it back on the road but I've run into a problem.

When I turn the ignition on (engine off) After about a minute, the main fuse blows.

Any ideas?

The bike was off the road as the 3 wires coming from the generator had shorted and went on fire, so I stripped them back and replaced connectors etc.

The only thing I'm not sure about is that all 3 wires were the same colour (yellow) so I presumed they could be connected in anyway to the 3 connections in the loom.

I don't think it would be the reg/rec as it wouldn't be involved in electrics with engine off?

I'm a bit stumped.





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My Bikes - - - Royal Enfield 500 with sidecar, VFR800, CBR600FX,
Peugeot Speedfight 100, Kymco Hipster 125, GSXR750K5, Jawa TS 350

[steven_191]

replace the fuse and feel the cables for any that are getting warm.

sounds like it could be a very high resistance short circuit, in which case the cable could be getting warm.

you need to find out the reason these cables melted in the first place and then work your way back. eleminate the orignal problem because you repairing that may have caused another.

make sure that the wiring you dont is correct before anything else. im fairly sure that those cables would need to be wired correctly.

[stinkwheel]

It doesn't matter which order they are connected BUT they are connected to the alternator windings, I'd worry about them having melted if the wires did.

As you say though, a short here shouldn't really be a problem with the engine off. It could however be your reg/rec breaking down and dumping current to earth having been damaged by whatever caused those wires to melt.

A tip I was given by someone older and wiser than I am:

Rig up a bulb holder with two male spade terminals on the end and attach this in place of the main fuse. Put a fairly high wattage bulb in it (maybe 21W, an old indicator is a good thing to use)

Pull all the other fuses out.

Turn on the ignition.

If you have a short circuit, the bulb will light up to full brightness (this takes the place of the fuse blowing, it's putting some load in the circuit so you wont risk melting the wiring). It'll glow slightly as current flows through it but the sign of a short is it being fully bright.

Now start replacing the fuses one at a time. Keep an eye on that bulb.

Once you work out which fuses circuit the problem is on, you can use the wiring diagram to narrow down exactly where the fault is.

For my number one guess though. When the bulb lights up, try disconnecting the reg/rec and see if it dulls.

Otherwise, where the wiring passes the headstock is a common place to get a short circuit due to wires wearing through and touching the frame.

If you want to test the alternator windings, with the yellow wires disconnected from the loom and your multimeter set to kiloohms, you should have identical resistance between each pair of yellows. You should have infinate resistance between any yellow and earth.

Good luck.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[The Shaggy D.A.]

I had that problem when I rebuilt my GPZ. I'd forgotten to reconnect the main earth wire to the frame, so everything was earthing through the brake light circuit
____________________
Chances are quite high you are not in my Monkeysphere, and I don't care about you. Don't take it personally.
Currently : Royal Enfield 350 Meteor
Previously : CB100N > CB250RS > XJ900F > GT550 > GPZ750R/1000RX > AJS M16 > R100RT > Bullet 500 > CB500 > LS650P > Bullet Electra X & YBR125 > Bullet 350 "Superstar" & YBR125 Custom > Royal Enfield Classic 500 Despatch Limited Edition (28 of 200) & CB Two-Fifty Nighthawk > ER5

[Pete.]

[Fortuna]

Rectifier wires are all the same and are interchangable. If it got that hot it caught fire you may have burnt the windings in the generator. Although with the engine off this shouldn't cause the fuse to blow.
If the reg/rec is broken it may still cause the fuse to blow if it's badly damaged internally. I've seen the insides of these and they can look horrendous.

If you do change it, I can recommend aquiring a newer FET type from a modern bike from ebay and not replacing it with the old type SCR unit.

Disconnect the reg/rec and turn ingnition on, if it still bows you will have at least eliminated it from the list

[-Savage-]