Topic: Blowing main 15A fuse

[Crcb]

Hi all. I have a '78 CB750K and have discovered that I have an issue with blowing the 15A main fuse due to the right side coil grounding to the frame through the steel mount bracket. When I unbolt the end of the coil from the mount bracket, the fuse doesnt blow and all is well. The wiring diagram shows the coils ground through the spark plugs.
I'm not sure where to go next. Do I insulate the entire bracket so it doesn't contact the frame in any way or is this indication that I have a short somewhere or possibly a bad coil?

[calj737]

There should be a BLACK/WHITE wire coming to the coil from the RH switch pod. That brings 12v to the coil. It is far more likely that wire is chafed and is shorting out. You may need to remove the tank and have a very close inspection for any wires other than solid GREEN making frame contact.

[scottly]

Check that the coil is mounted to the bracket correctly, with clearance for the primary wires. The bracket has a recessed area on one side for clearance. This had all of us stumped a couple of years ago..
 

[Bodi]

The coils don't ground through the spark plugs: OK, the high voltage secondary does "ground" through two plugs but that secondary HV part of the coil should have no direct connection to the bike's 12V power.
So no way the plugs can blow a fuse.
Your coil may have failed and be drawing way too much current. That happens. Rarely.
The coil gets +12V power through the MAIN fuse and key switch, through the kill switch. The ground connection to each coil comes from the points., when a points set opens and removes the ground connection, power is cut to the coil, and it generates the spark voltage.
So a main fuse blowing could be a bad coil, more likely it's a wiring harness problem - a wire worn through or melted and touching where it shouldn't - or a problem in the kill switch or key switch.
You should have a meter that can measure the coil resistance. It should be close to 5 ohms, but meters do not usually measure accurately at low resistance. As longs a the two coils measure pretty close together they are probably OK. If one measures much lower than the other then it's probably NFG.
Since the problem seems related to the one coil check the wiring to it carefully.

[b1jackson]

Check that the coil is mounted to the bracket correctly, with clearance for the primary wires. The bracket has a recessed area on one side for clearance. This had all of us stumped a couple of years ago..

YES!  Check this first! Through a lot of trial and error and learning wiring, how to read my multi meter etc., and through the stick to it ness of this forum, we finally figured this issue out on my bike a few years back.  It took a lot of popped fuses and they'd go as soon as the key was turned on.