Topic: Main fuse blows when regulator is hooked up?

[Slapguts]

1976 CB750K 25,00 miles. Newish battery, full. Bone stock and unmolested.

Here's a strange problem. As long as my voltage regulator is connected, my main fuse pops. Went to pull away from a red light yesterday, and the bike shut down. No lights, nothing. Push it to the side of the road, and went through all my spare fuses trying to figure it out. Got a ride to work, went back the next morning with a handfull of new fuses. Started trying to find bad wires. Nothing, everything looks great, no worn or frayed wiring. I unhooked the voltage regulator on a hunch, and the bike fired right up, and ran great until the battery died. Pulled the battery, took it home and charged it up, went back and the bike ran great again.

Checked the voltage regulator adjustment section in my Clymer's manual, and it seems to be spot on.

I don't have a working multimeter right now. Should I get one, or save the cash for a new regulator? I'd rather not drop the cash on a meter if I know I need a new regulator. They seem pricey.

What wire should be disconnected so it will still charge the battery, even if it does overcharge it? Bike runs with any two wires hooked up, but as soon as all three are connected, the fuse pops.

[Bodi]

Well, if you can stand to lose one more fuse you can check if it's the regulator.
Charge the battery.
Disconnect all 3 regulator wires then connect the black wire to the white wire at the regulator. Turn on the key.
Did the fuse blow?
yes: problem with the field coil, hopefully the white wire has worn or melted through its insulation and is touching ground. the coil itself is pretty robust but anything can happen, if it has gone bad you need a replacement.
no: regulator trouble, but that's pretty unusual.
Let us know what happens.

[Slapguts]

Thanks Bodi, I'll give it a shot. Just connect the black and white straight together?

[Bodi]

yes. all the regulator does is connect them when the voltage is too low and disconnect them - actually put a resistor between them - when voltage is too high. so if you connect them together you're just pretending to be the regulator telling the alternator to make as much power as it can. don't connect the green wire to anything.

[Slapguts]

Well, gave it a shot. Connected the black and white, turned the key, and the fuse popped.

Tracing the white wire for shorts is the next step, right?

[TwoTired]

Use an ohmmeter to measure the white and green wires from the Field coil.  The resistance is supposed to be 7.2 ohms. Your's is likely lower than that.  Check the field coil itself and the disconnected wires to the Vreg for connectivity to the green wires in the main harness.

Cheers,

[hoodellyhoo]

Out of curiosity, could this same trick be used on a 350 four? The problem is one that it similar to my own. Although I haven't tried fixing any electrical problems yet since acquiring the bike from my dad, I do remember that when he was working on the electrics, the right side coil would get warm while the left side remained cool. This is why I think I have a similar issue.

[Slapguts]

Use an ohmmeter to measure the white and green wires from the Field coil.  The resistance is supposed to be 7.2 ohms. Your's is likely lower than that.  Check the field coil itself and the disconnected wires to the Vreg for connectivity to the green wires in the main harness.

Cheers,

Thanks again, TwoTired. I'll pick up a multimeter. Going to need one eventually, might as well bite the bullet.