Topic: 73 750K blown main fuse

[mnvkngs65]

Just finished rebuild due to loss of oil to top end. Prior to top end failure, the electrical was fine with new main wire harness.

Now after the bike has idled for 5 min or more the main fuse will blow. I started systematically unplugging the blinker relay, safety unit, rectifier and finally the regulator. The fuse will only blow when the black wire on the regulator is plugged in and the bike is warm.

I quickly swapped out another old spare regulator and had the same problem (so I assume it is not the regulator unless both happen to be bad).

I'll test the field coil and stator - on the bike- next. I didn't see/feel anything wrong with initial inspection at the ignition switch or in headlight bucket.

Any suggestions would be appreciated.


Thanks!

[PeWe]

Problem is permanent now?
- No melted wire shorted to exhaust?
Brake switch wires for rear brake will blow main fuse when shorted.
I got it due to aftermarket switch had bad bullet plug insulation the glided off down the wire.
Fuse blew when using the brake. If on other side of switch it can blow without using the brake.

It might be a short in ign switch, inside headlight bucket or the wires to-from it or handle bar switches.
Measure with buzzer after optical inspection. Disconnect things and see....

[mnvkngs65]

Thanks for the info. PeWe.

Only blows after bike has warmed up. If the black wire to the regulator is unplugged it will NOT blow. Doesn't that preclude the other possibilities you suggested?

I'll keep looking.

Thanks.

[PeWe]

Missed that
"If the black wire to the regulator is unplugged it will NOT blow."

Yes, that's a good lead.
The rectifier fails when warm?
Any wire to from rectifier that feels warm?
The green can melt when battery minus has bad grounding to engine/frame.

[TwoTired]

Please relate how the glass fuse is "blown".  Near the end caps, or in the center?
Two different avenues to explore.  Internal heat vs, external heating.

Cheers,

[mnvkngs65]

Hi guys,

The fuse blows right in the center and I haven't felt any warm/hot wires. Because it only happens when warm I feel like it is a stator/coil issue (although I checked those 50 year old wires prior to reassembly).

Electrical is definitely not my forte but am slowly learning thru sheer necessity = )

Thanks

[bryanj]

If you have a stator/field coil fault it will be the first ever

[mnvkngs65]

Hi Bryan,

Sorry, meant the wiring from the stator/coil.

John

[bryanj]

That is possible, the bullets get corroded and loos which causes high resistance and heat.

[RAFster122s]

Did you have engine out when doing top end? Or, do you have a frame kit installed? Wondering if stator wires got pinched or similar black wire to regulator has a pinch short somewhere if the motor was out.

[ekpent]

Did you have engine out when doing top end? Or, do you have a frame kit installed? Wondering if stator wires got pinched or similar black wire to regulator has a pinch short somewhere if the motor was out.

I was kind of thinking along those lines also if you had to pull the engine that maybe a wire or plug got damaged.

[mnvkngs65]

Thanks for the input guys.

I did remove the engine from the frame but had carefully disconnected the appropriate wiring and connectors (i.e. stator/coil, starter, rear brake, etc.) prior to removal.

I tested the alternator with the ignition in the off position last night and the readings were w/n specs and was without obvious shorts.

I also used a new reg/rect combo from another 750 last night but didn't get a chance to run it long enough to see if the problem resolved (battery died). Which leads to another problem, bike is not charging  despite a new regulator/rectifier. I don't know if the two problems are connected or not?

I'm going to start by chasing down the electrics fed by the black wire attached to the regulator and go from there (confusing since it doesn't happen unless the bike is warm and I haven't ridden it yet -  so not using the headlight, blinkers, kill switch, rear brake light, etc.).

Any other suggestions appreciated.

I'll let you know what I find, if anything.

Thanks again.