Topic: Ignition Switch Short (ignition position ON, Entire bike dead) 1976 Honda CB750F

[Resto89miles]

I have power to the ignition (red wire).

When I turn the key to park, the tail park light works.

Now when I turn the key to on, the entire red (hot) circuit goes dead.
I changed the ignition switch out, same results.

If I pull the black wire out of the ignition switch connection, I have power to the red (hot) circuit again.

I tried grounding the harness black connection with the black wire disconnected from the harness, no change.

I noted that the black wire circuit grounds in through the regulator then out to the green wire to the chassis ground.

[bryanj]

You have all your colours wrong.
Red is positive live from battery

Black is SWITCHED live ,12volt positive with key on

Ground is green.

Black at regulator is power supply to field coil when ignition on and battery needs charging.

I would sugest a proper wiring diagram from the Honda manual

[scottly]

Now when I turn the key to on, the entire red (hot) circuit goes dead.

The red wire is always hot. If the voltage goes away when you apply a load (turning the key on) it means you have some resistance in the circuit between the ignition switch and the battery + terminal. The 15 amp main fuse feeds the red wire that goes to the switch. Start by checking the fuse box and the clips that the fuse snaps into. The clips should be clean and shiny. Also look for any signs of overheating or melting.

[Jerry Rxman Griffin aka MuthaF'er]

Now when I turn the key to on, the entire red (hot) circuit goes dead.

The red wire is always hot. If the voltage goes away when you apply a load (turning the key on) it means you have some resistance in the circuit between the ignition switch and the battery + terminal. The 15 amp main fuse feeds the red wire that goes to the switch. Start by checking the fuse box and the clips that the fuse snaps into. The clips should be clean and shiny. Also look for any signs of overheating or melting.

Take the back off the fuse box and take a look

[newday777]

I have power to the ignition (red wire).

When I turn the key to park, the tail park light works.

Now when I turn the key to on, the entire red (hot) circuit goes dead.
I changed the ignition switch out, same results.

If I pull the black wire out of the ignition switch connection, I have power to the red (hot) circuit again.

I tried grounding the harness black connection with the black wire disconnected from the harness, no change.

I noted that the black wire circuit grounds in through the regulator then out to the green wire to the chassis ground.

Welcome to the forum Resto89miles

Have you tested your harness individual wires for proper continuity? You could have melted wires under the tape on the harness, there was a post earlier this week of a melted ground (green) in the harness that he had intermittent lights happening and found the cause was burnt insulation and shorting the black power wire in the harness.

Post back replies here in this thread what you find.

[Resto89miles]

Thank you for the responses.
 Yes, into it I realized red is battery, black is 12v to the system. (I'm an old car mechanic.)

I tried to open the old ignition switch and the plastic back broke, exposing the well-worn contacts, so I got a new repop switch.
The symptom remained unchanged.

I made a short testing harness out of the same colors to plug into the switch and the harness, so I could isolate each wire (3 brown, 1 black, 1 red) at a time.

Only when I unplug the harness black wire, then the test light comes back on.

I have unplugged every black source from the harness. I unplugged the brown/white strip wire in the triangular box and got low level lights on the dash indicator lights.

I just rode this bike, and it sat. Nothing else was done except changing out the hi/lo beam switch parts. I turned on the switch, tested hi/lo, checked out OK. That's it.

It's just weird. 

The next step is looking for a melted wire hidden in the harness.

I haven't heard of a system component causing this.

[Resto89miles]

PER FUSE BOX:

That fuse box was buggered up by previous owner (added some overloaded circuit to it) things were done to it where the plastic overheated. I cleaned it up as best as possible. It is still poor.

I will get a new box. I may even just get a new harness, that way it will rule out the main harness as a problem. Thanx for the tip!

[scottly]

First just replace the fuse box. BTW, you might consider upgrading to blade type fuses instead of the old glass type.

[bryanj]

Fuse box upgrade go to member Hondaman, plug and play and well worth it.
From your symptoms my first thought is your battery has killed a cell, which happens quite quickly, i had a car do it whilst driving. Just testing voltage is no real test it needs load testing

[newday777]

As I said, test your harness continuity to each end of the harness color by color to KNOW IT IS NOT a problem.