Topic: 1978 cb750f killswitch blows main fuse when turned to on.

[Rydog400f]

Hey all, I've been having trouble tracking down an issue with my killswitch. The main fuse is blown ONLY when the switch is turned to on position. When it's off everything works without blowing the fuse including the starter button. I've checked continuity at the switch and it seems to be working properly. Any help is greatly appreciated!

[BenelliSEI]

That would suggest there is a dead short somewhere on the down stream side of the switch. I’d start by pulling out the wiring diagram for your bike and follow it along. Test every connection to ground? And see what is bare or grounded. Start with the two power leads to th coils.

[scottly]

The black wire with the white stripe is the "output" side of the kill switch, and connects to both black/white coil wires. What is the background story? Is this bike new to you, or have you had it for a while and this is a new problem? If it is a new problem, have you been messing with the coils or pulling wires through the handlebars?

[Rydog400f]

The black wire with the white stripe is the "output" side of the kill switch, and connects to both black/white coil wires. What is the background story? Is this bike new to you, or have you had it for a while and this is a new problem? If it is a new problem, have you been messing with the coils or pulling wires through the handlebars?

The bike is at the end of a full rebuild and I just put the original coils back on. The bike ran before I tore it down for the rebuild. I'm not sure if it's the coils or something else. I can't find any bare or touching black/white wires. It has a new wire harness as well. Thanks!

[Rydog400f]

That would suggest there is a dead short somewhere on the down stream side of the switch. I’d start by pulling out the wiring diagram for your bike and follow it along. Test every connection to ground? And see what is bare or grounded. Start with the two power leads to th coils.

No bare wires that I can see. It's a new harness. What should I be looking for at the coils?

[scottly]

If the aluminum brackets the coils mount to are not orientated correctly, the bracket can touch the coil primary wire. For a quick test, disconnect the blk/wht wire from the coils and see if the fuse still blows; if it does, the problem is somewhere between that point and the switch.

[Rydog400f]

If the aluminum brackets the coils mount to are not orientated correctly, the bracket can touch the coil primary wire. For a quick test, disconnect the blk/wht wire from the coils and see if the fuse still blows; if it does, the problem is somewhere between that point and the switch.

I'm at work now but I'm almost positive it blows the fuse when the coils aren't connected.

[scottly]

If the fuse still blows when the black/white wire is disconnected from the coils, unplug the switch from the main harness and measure the resistance from the blk/wht wire to frame ground; it should be an open. If there is a low resistance, the soldered wire inside the switch may be shorting against the handlebar.

[Rydog400f]

If the fuse still blows when the black/white wire is disconnected from the coils, unplug the switch from the main harness and measure the resistance from the blk/wht wire to frame ground; it should be an open. If there is a low resistance, the soldered wire inside the switch may be shorting against the handlebar.

Just checked it and there is zero resistance. Also confirmed that the fuse blows with coils disconnected.

[BenelliSEI]

So..... with coils disconnected, ignition key switch “ON”, fuse blows as soon as you switch on the three way “kill switch”? If so, there is something going to ground when you rotate that switch. Anything else hooked up to that power lead to the coils?

[Rydog400f]

So..... with coils disconnected, ignition key switch “ON”, fuse blows as soon as you switch on the three way “kill switch”? If so, there is something going to ground when you rotate that switch. Anything else hooked up to that power lead to the coils?

Correct. I'm wondering if the switch has to be grounded to the handlebar to work? If I remove it from the bar should the switch still work? That might show if something is grounding to the bar.

[madmtnmotors]

I'm wondering if the switch has to be grounded to the handlebar to work?

No. Follow the diagram:

http://manuals.sohc4.net/cb750f/technical_reference/WD750F_78.jpg

I'll wager that when you installed the new harness you got something wrong. I'm guessing a black to green somewhere or there's a black wire pinched and shorted to ground.

[madmtnmotors]

Looking at the diagram, the only thing connected to the engine stop switch is the coils. Carefully check the wire from the switch to the point of connection to the coil wire connection in the headlight bucket. Disconnect the engine stop switch completely by disconnecting both the black and the black/white leads and check continuity through the switch in both the on and off positions between those two leads. Then with the switch still disconnected check for continuity between each lead and ground (first the black and then the black/white or vise versa) in both positions for both wires to the switch in order to determine if it's in the wires to the switch or the switch itself. Chasing electrical gremlins often requires determining what it's not.

[Rydog400f]

Looking at the diagram, the only thing connected to the engine stop switch is the coils. Carefully check the wire from the switch to the point of connection to the coil wire connection in the headlight bucket. Disconnect the engine stop switch completely by disconnecting both the black and the black/white leads and check continuity through the switch in both the on and off positions between those two leads. Then with the switch still disconnected check for continuity between each lead and ground (first the black and then the black/white or vise versa) in both positions for both wires to the switch in order to determine if it's in the wires to the switch or the switch itself. Chasing electrical gremlins often requires determining what it's not.

Thanks, I'll check when I get home. The killswitch for the 78 has a plug so I don't think anything is connected wrong. So far I know the continuity for the switch works properly on the plug end. None when it's switched to off and good continuity when its switched on.

[madmtnmotors]

Looking at the diagram, the only thing connected to the engine stop switch is the coils. Carefully check the wire from the switch to the point of connection to the coil wire connection in the headlight bucket. Disconnect the engine stop switch completely by disconnecting both the black and the black/white leads and check continuity through the switch in both the on and off positions between those two leads. Then with the switch still disconnected check for continuity between each lead and ground (first the black and then the black/white or vise versa) in both positions for both wires to the switch in order to determine if it's in the wires to the switch or the switch itself. Chasing electrical gremlins often requires determining what it's not.

Thanks, I'll check when I get home. The killswitch for the 78 has a plug so I don't think anything is connected wrong. So far I know the continuity for the switch works properly on the plug end. None when it's switched to off and good continuity when its switched on.

With the plug disconnected check the switch leads for continuity between each lead and ground with the switch in both the on and off position. Since the fuse only blows when switched to "On" I'm guessing the black/white wire between the plug and the switch is pinched and shorted.

[madmtnmotors]

When you disconnected the coils, where did you disconnect at? At the connectors right at the coils? If so, the black/white wire between the engine stop switch connector and the connector at the coil could also be the culprit. It sounds like it's somewhere in the black/white between the switch and the connector you used to disconnect the coil. There's a slim possibility that it's the actual switch, but testing for continuity to ground at the switch connector will help determine whether that is the case.

[Rydog400f]

When you disconnected the coils, where did you disconnect at? At the connectors right at the coils? If so, the black/white wire between the engine stop switch connector and the connector at the coil could also be the culprit. It sounds like it's somewhere in the black/white between the switch and the connector you used to disconnect the coil. There's a slim possibility that it's the actual switch, but testing for continuity to ground at the switch connector will help determine whether that is the case.

Yes I disconnected the black/white wires at the coils.

[scottly]

If the fuse still blows when the black/white wire is disconnected from the coils, unplug the switch from the main harness and measure the resistance from the blk/wht wire to frame ground; it should be an open. If there is a low resistance, the soldered wire inside the switch may be shorting against the handlebar.

Just checked it and there is zero resistance. Also confirmed that the fuse blows with coils disconnected.

The blk/wht wire is shorting to the handle bars. Remove the switch from the bars and look for where it's touching.

[scottly]

If the fuse still blows when the black/white wire is disconnected from the coils, unplug the switch from the main harness and measure the resistance from the blk/wht wire to frame ground; it should be an open. If there is a low resistance, the soldered wire inside the switch may be shorting against the handlebar.

[Rydog400f]

If the fuse still blows when the black/white wire is disconnected from the coils, unplug the switch from the main harness and measure the resistance from the blk/wht wire to frame ground; it should be an open. If there is a low resistance, the soldered wire inside the switch may be shorting against the handlebar.

Just checked it and there is zero resistance. Also confirmed that the fuse blows with coils disconnected.

The blk/wht wire is shorting to the handle bars. Remove the switch from the bars and look for where it's touching.

Thanks I'll check again tonight. I've checked once already and didn't see it touching but I may have missed something.

[scottly]

With the switch removed from the bars, check for continuity from the blk/wht wire to the switch housing; if none, look for any signs of arcing on the bars below where the switch mounts.
I see you're in SLO town. I lived there when I bought my SOHC in 1979.

[Rydog400f]

With the switch removed from the bars, check for continuity from the blk/wht wire to the switch housing; if none, look for any signs of arcing on the bars below where the switch mounts.
I see you're in SLO town. I lived there when I bought my SOHC in 1979.

That was it! My horrible soldering job was touching the housing. Bent them off the housing and no more blown fuse! I checked by removing the switch from the bars and checking if it would blow the fuse. It didn't. Thanks SLOCAL!

[scottly]