Topic: 15 amp main fuse blowing at operating temperature

[RSCB750]

Hello all, hoping you honda vets can help out here. I searched this topic and it seems to be a relatively common issue.

77 k7. Stock wiring, new coils, wires, plugs, ends, etc. Voltage regulator gapped and functioning properly (opening/closing, correct voltage @ various rpms as per clymer manual). My 15 amp main fuse blows when the bike is up to temperature. When warming up, no issues. If bike is repeatedly revved past 5k, the fuse blows.

Static timing set, dwell ok, points gapped, runs like a top (when it runs haha)

I have replaced the factory glass fuse today with a 15 amp blade to no avail. I am aware there is probably a short circuit somewhere.

Is there a common place to start looking for an issue on these machines? Could the rotor/stator be shorting out and over energizing the system?  When i unplug the rectifier problem persists.  Short inside the engine case?

Thank you for taking the time to read this post.

[Mr. Mike]

Sort of a loaded question.
But- say if you just rode around town at 3K rpm...would the same thing occur?
Could be vibration at higher rpm’s cause a wire to ground out or an intermittent connection arcing.

Battery connections +&-, behind the headlight bucket at the ignition switch, under the tank, seat and I suppose even at the handle bar wiring...if it’s vibration related could all be suspect.
Have you polished the old fuse holders and made sure the tangs were tight on both top and bottom? (Oops he replaced this 15A with the blade type).

Also check the solder connections on the back side of the fuse block. Inspect for loose/corroded connections from the battery on out to the harness bullet interconnections like inside the headlight bucket etc.

Think back...maybe when you switch on a blinker it popped?
You can see that you’ve got some investigation on your own to do. Don’t hurry.

Oh- Did your troubles seem to start after the new coils and other upgrades were completed?

[HondaMan]

The chromate plating on the clips that hold the fuses are 1/2 of the culprit: now it has become an effective resistor instead, heating the clips and the fuse to near-melting point. Second problem: the SFE type of vibration-resistant fuses that fit those holders have not been made since the 1990s, and instrument-grade fuses of the type you have now will not take ANY vibration when hot, making them break. Then they look like they "blew" for no reason.

My cure has been to make ATC-type fuseholders to replace them: PM me for details if you'd like me to make one for you. I need to know if the white plastic connector has BLADES inside, or RECEPTACLES, and Honda was not consistent with this on these bikes. And, the year/model...

[HondaMan]

Right you are, Cal: I am still suffering half attention problems from my miniature poodle service dog being killed by a pitbull last week...

On that particular bike: check the wiring harness where it bends around the steering head. Of all the 750, the K7/8 and F2/3 frames like to chafe the wires inside the jacket right there, making them intermittently pull a short when moving the steering head. This slowly eats the fuse.

But, another point: I (tried to) used a single blade-type 15 amp fuse on my bike (and one other) and the contacts inside were so poorly made that they would melt even a 20A ATC fuse in about 15 minutes of ride-time, on my way to work, normal riding. The problem was in the single-fuse ATC holders's contacts: poorly made, misaligned, and poorly crimped to their wires. It melted even the rubber holder, on both bikes.

[disco]

I have replaced the factory glass fuse today with a 15 amp blade to no avail.

Pay attention guys, he already replaced the stock fuse block 
If the fuse blows only after running the motor, then I would check the RED wire from the fuse to the key, and the key switch itself. Its possible the heat is accumulating along that circuit due to it being "in use" and thus overheating. Corrosion of the contacts, switch, etc would be the likely culprit if the cause is as I described.

Else, it could very well be a shorted wire that is making contact with bare metal due to vibration. If its only the MAIN fuse that is blowing, that narrows down the suspects somewhat. Trace that circuit from the battery POS, to the fuse, past the fuse and to the key. You'll discover the source pretty quickly.

I don't think he replaced the fuse block itself. He said he replaced the factory glass fuse, with a blade type. Not fuse block. It sounds more like he 'Jerry rigged' a blade type fuse into the OEM fuse block. RSCB want to confirm?

[scottly]

I have replaced the factory glass fuse today with a 15 amp blade to no avail.

Pay attention guys, he already replaced the stock fuse block 
If the fuse blows only after running the motor, then I would check the RED wire from the fuse to the key, and the key switch itself. Its possible the heat is accumulating along that circuit due to it being "in use" and thus overheating. Corrosion of the contacts, switch, etc would be the likely culprit if the cause is as I described.

Else, it could very well be a shorted wire that is making contact with bare metal due to vibration. If its only the MAIN fuse that is blowing, that narrows down the suspects somewhat. Trace that circuit from the battery POS, to the fuse, past the fuse and to the key. You'll discover the source pretty quickly.

I don't think he replaced the fuse block itself. He said he replaced the factory glass fuse, with a blade type. Not fuse block. It sounds more like he 'Jerry rigged' a blade type fuse into the OEM fuse block. RSCB want to confirm?

I'm also wondering if he "jerry rigged" a blade type fuse???

[PeWe]

I had an issue with the rear brake switch. The insulation fell off the connection, the bullet plug for 12V was in contact with engine/engine mount. Main fuse blew when using the rear brake.
It took 2 fuses including stop when dark for me to find the problem.  I was happy that both stops happened close to my home and my friends home.
I ordered a bunch of AUTO fuses from Germany after that. I could not find that size here at all except for really expensive instrument fuses.

Brake lamp should have its own fuse or use the 5A fuse my K6 has and most likely later K7 has too.

[scottly]

Brake lamp should have its own fuse or use the 5A fuse my K6 has and most likely later K7 has too.

No, the brake lamp is powered by the main fuse. The Tail lamp has it's own 5 amp fuse.

[PeWe]

Brake lamp should have its own fuse or use the 5A fuse my K6 has and most likely later K7 has too.

No, the brake lamp is powered by the main fuse. The Tail lamp has it's own 5 amp fuse.

Exactly, that's why I wrote it should have its own fuse or connect to the 5A fuse to avoid  blowing the main fuse as this case is about. Bike will stop completely dead when brake lamp circuit is shorted.
This can be the case here. I do not think when using the rear brake, I just use it.
I had to think about the events I had and what really happened

[TwoTired]

I'm wondering if there are now 3 ohm coils installed, and if the vreg has been tweaked to make an over voltage condition during cruise after the battery is fully charged.

Cheers,

[RSCB750]

**no jerry-rigging has occurred during the making of this bike**

I cut the wires from the 15amp glass fuses and spliced into a blade style fuse holder from an auto parts store (and by spliced, i mean soldered and double shrink tubed, good continuity). Not pretty but hey, thats what side covers are for

I will investigate the red wire from the fuse to the ignition switch.  Thanks all for the speedy responses.

One thing ive noticed is there is constant power going to the  15amp inlet fuse, even when the key is off (pos term, starter solenoid, fuse). Is this normal?

[RSCB750]

Sorry for the double post.

Twotired, what do you mean by 3ohm coils? Between the boots (secondary resistance?) im getting 25.5k ohms and the inlet wires measure about 4.8-5.2 ohms. Is the primary resistance what you were referring to?

[TwoTired]

Yes. You said you had new coils but didn't specify which ones.  Your reported ohms are like stock coils, and unlikely a power draw issue. (Unless you also have Dyna ignition).

You still need to measure the voltage with the engine revved and the battery at full charge.  Should never ever go above 14-15V, or you have a regulator issue.

Stock bike normally draws around 10 Amps with lighting on and cruise RPM. Average voltage 13.8V.  Using ohms law, that means the bike systems total about 1.38 ohms.

Raise the voltage to 20V and the amps draw with those ohms will be 14.5 Amps, and vibration could make the fuse part as it will be warm/hot near melting.

Does the bike have all stock components (headlight)?  More draw would make a fuse "blow" sooner with an over voltage condition (which isn't good for the battery, either).

Cheers,

[RSCB750]

Gentlemen,

Had a very limited time to work on the bike this weekend.. still have yet to investigate the red key to main fuse wire.

I did notice that the large(r) copper winding on the back of the voltage regulator is getting hot.. like pretty darn warm. Is this normal?

[HondaMan]

Gentlemen,

Had a very limited time to work on the bike this weekend.. still have yet to investigate the red key to main fuse wire.

I did notice that the large(r) copper winding on the back of the voltage regulator is getting hot.. like pretty darn warm. Is this normal?

If you are referring to the Honda voltage regulator: yes, it is a resistor that limits power to the field coil when the regulator is in 'mid-charge' mode, usually above 13.2 volts or so.