Topic: 1975 CB 400/4 - electrical issue

[MDW]

Has anyone had this issue? All of a sudden the main fuse has started to overheat then blow when the ignition is on. With the ignition off there is power flowing to the fuse (which doesn't sound right even if the wiring diagram shows a wire running directly from the battery to the fuse). Someone suggested clipping a test light to the hot side of the fuse holder (without the fuse in place) and testing for a short. When I do this the light lights up in various places (I assume it should light when touching a ground wire, but not in random other places). I disconnected the battery and tested for continuity. Even after watching many YouTube videos I can safely say I am not proficient with a multimeter so the following probably sounds very rookie.  When the multimeter is set for continuity I get a peep and then an readout of 0-40 ohms depending on the wire. What perplexes me is that 10 days ago everything was fine. All I have done since then is replace the coils and zip tie the loom in place. I have checked a lot of the connections but so far haven't found any obvious places where a wire could ground. I kind of feel I must have done something wrong or I am missing something obvious.

Does anyone have any suggestions of what the issue could be, or what else I can do to locate the problem? It is driving me crazy! Thanks

[scottly]

All I have done since then is replace the coils

Try this: with the kill switch set to off, does the fuse still blow, or does it only blow with the kill switch set to run? If it only blows with the kill switch on run, I can tell you exactly where to look, assuming the 400 has the same coil mounts as a 750.

[MDW]

Thanks. I will check tomorrow. If it only blows with the kill switch on, where should I look? See coils photos.

[scottly]

Here's where to look: the aluminum coil mounts have a relieved area on one side, to give clearance for the wires. Both of the mounts have to have the relief facing inwards towards the coils. The correct orientation looks wrong, so it's an easy mistake. I bet a beer you have one turned around, shorting a coil black/white wire to ground.

[MDW]

Thanks I will check and let you know

[scottly]

The 400 coil mounts are different, but it wouldn't hurt to check for clearance, as well as any other place the blk/wht wires could short to ground, if the fuse only blows with the kill on.

[newday777]

Thanks. I will check tomorrow. If it only blows with the kill switch on, where should I look? See coils photos.

That harness and the blue and yellow wires look out of place. Run the harness lower than the coils and the blue and yellow under the frame (shouldn't be above the frame as the tank tunnel could put pressure on the wires)

[MDW]

Yes, the harness is not in its final position. I will see if the blue/yellow wires are long enough to run under the frame - thanks for the tip.

[MDW]

@Scottly, the fuse blows with the kill switch on and off. One other reference point is that the loom is new, so I am guessing the issue must be with the old parts being plugged into it. I am looking for corrosion around the fuse box etc. I will check the coils as a precaution, but with them mounted on the bike, I don't see any trapped wires.

[scottly]

If the fuse blows whether the kill is on or off, you are looking for a solid black wire, or something connected to one, shorting to ground. BTW, any solid green wire is connected to ground.

[MDW]

Thanks Scottly.

So, latest developments. I lightly sanded the fuse box terminals to remove any corrosion (I couldn't really see anything obvious), cleaned the 7 amp fused which looked really old and removed/replaced the coils. Probably not related but I also removed some paint where the coil mounting bracket touches the frame. I left the ignition on for 6 mins and the fuse didn't blow, and didn't feel to have heated up much (I could maybe feel a little heat from the two red wires going into the fuse box. I don't know if this a coincidence or not but it is an improvement at the least.

[robvangulik]

Those yellow and blue wires are supposed to go OVER the frame, see both middle drawings...

[MDW]

Yes indeed. They are too short to go under.

[jonda500]

I like to avoid leaving the ignition on for long without the engine running or the kill switch turned to off as the coils aren't designed to have continuous power running through them?
John

[MDW]

Agree, would never normally do that.