Topic: 350f stumped on electrical

[hoodellyhoo]

Try as I might, I can't seem to figure out why my fuse and rectifier keep getting hot. This only happens when the bike is running, no lights or anything else turned on. The fuse is correct (15A). All grounds and connections have been cleaned. The rectifier is good, but I also got similar results with a spare that I have. The ignition switch is new and checks fine. I also checked many of the wires in the harness such as those for the kill switch, rectifier, alternator, voltage regulator. I checked the alternator today and that checks out. I get 5ohms on the field coil and 1.2 on the stator. This is more that the stated values(.61-.69 I think), but my clymer manual says I should only be worried if the value is less than those stated. Besides, would anything in the alternator even cause the symptoms I have? Like I said, I'm stumped. Because it only happens when running, there's only so many things that it could be. Could it possibly be something like coils, plug wires and caps, points, or condensors? I wouldn't think so because I thought problems in those areas only gave you a weak spark. Any suggestions?

[TwoTired]

The rectifier will heat proportional to the power being used by electrical devices and the battery undergoing charge.  This is why is has a heat sink, to get rid of enough heat that the diodes will survive. Electric start draws a great deal from the battery (150 amps).  This has to be put back after the engine is running.  And, the alternator will give all it has until the recharge task is finished.  It provides more with engine RPM.  And, it all must go through the rectifier.

Check to make sure that the battery isn't getting above 14.5 Volts when the engine is sustaining RPM above 3-4000.  Higher voltages will incurr a higher draw amperage with fixed resistance devices like lighting, etc.  A charging battery increments slowly.

The fuse should get warm.  But not warmer than you can stand by holding your thumb on it. (engine reved)

If it is warmer than this, then you either have more than 2/3rds it's rating passing through it.  Or, the contact clips have some resistance heating going on.  Surface oxidation, even if it is transparent, can create heating and this can be transferred to the fuse.  Low clip contact pressure can also contribute.  You should need a tool to remove the fuse.  If you can remove the fuse with simple finger pressure, the clips have lost their retentivity and this increases resistance and contact heating.

Poor connections on the back of the fuse block can also heat, and the heat can conduct to the fuse.  Lastly, the fuse clips should "float" so they will align themselves for maximum contact surface area between fuse clip and fuse.  A small contact area, can also increase resistance and heating effects of passing large currents through them.

All this assumes you haven't changed the electrical load with higher wattage headlights, signal bulbs, or added some other current needy devices to the electrical system.

When checking your stator, you must subtract the measurement error introduced by the test leads.  Subtract the value you read when you simply touch the probe tips together.

Cheers,

[Bodi]

There is no fuse in the battery charging current path between the rectifier and the battery. The MAIN fuse carries only load power, basically just the ignition coils if you remove the HEAD and TAIL fuses. Measure the fuse current if you have a meter that can, it should be maybe 5 Amps maximum with the HEAD and TAIL fuses removed.

[hoodellyhoo]

Not quite sure what you mean by the head and tail fuses because the 350f only has one fuse.

[Bodi]

Right you are! The 350/400 manual has both diagrams and I looked at the wrong one near the end of the book, the 350 is closer to the middle.
Anyway, the fuse current should be easy to measure, just remove the fuse and attach ammeter probes across it. The meter has to be able to handle 10A minimum. You should have less than 10 amps, probably close to 5A with the stop light not on... assuming factory components (points, original headlight). Electronic ignition can draw more current and a higher power headlight definitely will, but if you're much at all over 10 amps then you're going to be discharging the battery most of the time, the alternator barely makes 13 amps on a good day.
Once you know the actual current you can tell if there's a fuse/fuseholder problem or a wiring/load problem. Under 10A blowing a 15A fuse says the fuse is defective or the fuseholder is loose or dirty and causing the fuse to overheat.
Over 10A suggests you have added too much electrical load or there's a problem in the wiring somewhere.

[333]

Something simple to check.  What shape is your battery in?  Does it have enough fluid in it?  A dry or bad battery will have resistance and the fuse will blow when the charging system wants to charge, while still having enough volts to allow kickstarting.

[Bodi]

The charging system can not blow the fuse. The alternator power goes - without any fuse - from the rectifier directly to the battery and to the battery end of the single 15A fuse. Only the bike's actual electrical loads go through the fuse - lights, horn, and ignition.

[hoodellyhoo]

Something simple to check.  What shape is your battery in?  Does it have enough fluid in it?  A dry or bad battery will have resistance and the fuse will blow when the charging system wants to charge, while still having enough volts to allow kickstarting.

The battery is brand spankin new with plenty of fluid so that wouldn't do it.

[TwoTired]

One way the alternator can blow the main fuse is if the field coil or the wiring to it is deffective and providing low resistance/high current draw.

Measure across the white and green wires while disconnected from the regulator.  The field coil s/b 4.6 to 5 ohms. and then add a little for the wire resistance.  If it is lower than this or intermittantly shorted, then you can blow main fuses from a faulty alternator or related wiring.  You could also check the black wire path going to the VReg, as this is what supplies battery power to it and the alternator field coil.

Cheers,

[Trevor from Warragul]

My 350F melted the fuse.  The contacts were corroded.  I cleaned them up as best I could but the fuse still got quite hot.  I wired in a new fuse with good quality connectors & the problem went away.