Keep blowing main fuse

I finally bought my dream bike after almost 30 years of non bike owning/riding. I bought it from a guy on ebay who claimed the bike was 98% restored. I've had to rust treat the tank, fix a significant oil leak, trouble-shoot a charging system issue with a hodge-podge cobb job wiring system nightmare consisting of some very questionable spliced bypasses of a melted junction block from the regulator, and a non-working horn and 4-way. The bike was running entirely off the battery for a while while I worked up the nerve to attack the rat's nest of harry homeowner wiring. I've replaced the stator and regulator and wired them up with proper connectors, but I occasionally blow the main fuse, and always when I'm getting on it hard. It invariably happens as I'm accelerating in 2nd or 3rd gear. The 1st time it happened it was a real puzzler cuz it didn't look like the glass fuse was blown. I threw in a new long 30 amp fuse(can't find the shorty style), and problem solved for a few weeks. It's happened 3 times since then and twice in 1 ride just yesterday. It doesn't seem like it would be the charging system putting out too much juice because I noticed after I got home and let things cool down, I turned on the ignition switch w/o starting the motor and noticed the copper contacts on either side of the fuse were almost immediately too hot to touch from dead cold seconds b4. What could be causing so much resistance in the line so fast w/o the motor running? I'd appreciate any input to help me trace this problem.

patwelborn wrote:

I got home and let things cool down, I turned on the ignition switch w/o starting the motor and noticed the copper contacts on either side of the fuse were almost immediately too hot to touch from dead cold seconds b4. What could be causing so much resistance in the line so fast w/o the motor running? I'd appreciate any input to help me trace this problem.

It's not too much resistance blowing your fuse, it's too much current flowing somewhere. With a hacked up wiring harness, the likely suspect is a frayed wire insulation somewhere letting it touch the frame. Interesting it pops when you accelerate hard. I wonder if it's because that shifts the harness and causes something to touch it shouldn't?

Intermittent electrical problems can drive you crazy.

I thought heat equals resistance...that a properly sized wire running a normal amount of juice produces a normal expected amount of resistance, and increased juice through a wire rated for less will create heat through the resistance to flow. I don't understand how a short could produce increased juice. Wouldn't it just short to ground w/o the heat? Or could it be that the short is drawing the juice faster through the fuse than it normally would and that is the cause of the heat/blown fuses? You're right about the intermittant electrical problems driving you crazy, that's for sure. I'll inspect the harness as much as I can w/o a tear down,...I'd like to get through the riding season b4 I get into it heavy. Thanx for your response.

The fuse blowing problem might be caused by a pinched wire or two inside either of the handlebar swich pods. Vibration may be enough to have a contact or wire touch to ground.

The short fuse is an AGX type found at most good auto parts stores. It is 1" long. The other is a AGC type which is 1 1/4" long may not fit well in the holder.

I'd look at the ignition switch too, as they have been known to act up.

patwelborn wrote:

I thought heat equals resistance...that a properly sized wire running a normal amount of juice produces a normal expected amount of resistance, and increased juice through a wire rated for less will create heat through the resistance to flow.

Heat equals power dissipation. Current flowing through a given resistance causes power disspiation given by I(squared) x R.

Lower resistance allows higher current flow. If you have a short to frame, basically the small amount of resistance offered by the wire will let many tens of Amps flow and blow the fuse.

Thank you for the tips guys. I'll check things out.

Let me know what you find out, I just started having the same problem.

Here's an electrical troubleshooting procedure for intermittent problems:The Dreaded Intermittent Electrical Problem!

Here is the most basic method I know:

1) Charge your battery if you can.
2) Disconnect the Black lead from the (-) Battery terminal... or Red form the (+) Battery terminal, it does not matter which one.
3) Connect one of the following test setups in series with the Battery terminal and lead:
3.1) A 12 V light bulb,
3.2) A 12 V test light,
3.3) A 12 V test buzzer or,
3.4) A 12 V horn... you get the idea.
4) With the Ignition Switch OFF, go through your harness and wiggle the wires while looking/listening for the test setup to go on/start buzzing.
5) With the Ignition Switch ON, repeat the test except this time the looking/listening for the test setup to go on/stop buzzing.
6) Be prepared to open the Ignition switch and check/test for solder joint failure and or circuit board micro breaks (don't ask how I know this ).
7) Be prepared to pull the wires out of the Head Light to test for failures at or near the grommet.
Be prepared to open the harness at or near the Steering Neck for failures. This is where wires tend to exhibit fatigue due to repetitive movement.
9) Be prepared to open the left and right switch gear to search for rust and or broken parts. CAUTION: watch out for fling springs, ball bearings and stuff. Do indoors on White sheet (again don't ask ).
10) Be prepared to follow the heavy gauge wire from the Starter Solenoid (Relay) to the starter for bare ware wire exposure. Especially near bends and grommets.
11) If you can reproduce the fault symptom your are pretty much home free. Be prepare to find and repair/replace any internal wire breaks, insulation break downs, exposed wires, rubber grommet failures, etc. Often, shrink tubing will solve the problem temporarily.

Wow! Thanx for that excellent troubleshooting guide. I'm looking forward to tracing the problem as you've outlined, but I'm afraid it'll have to wait a bit as it's Father's Day and I should do stuff with the kids today, (and not dissappear into the garage).

That's a good technique, but on my bike the lights all go on with the ignition switch and can't be turned off. The headlight and running lights are a low enough resistance to light up a 12V buzzer or small lamp so they would look like a "short" to the test. You may need to disconnect the headlight and running lights (unscrew them) and unplug the coils so the big current draw components are not connected when you are trying to wiggle the harness to find a short to frame.

Pat, Heat at fuse connetions "bad".. if pre-existing condition, two things to check, 1st good clean contacts to fuse, 2nd that if plastic holder is deformed at said holders and spring tension to fuse is poor. high resistance can cause enough heat to melt fuse element where joined to cups at ends (as opposed to high amps which usually blow middle of strand and are visually obvious) I'd start there, if in doubt, temmporarily bypass stock fuse holder for Main Fuse /w an in line unit.,,,period reg/rect,s often deliver moderate charge rates at idle & climb to 15 vdc at revs, as opposed to more current systems which peak just off idle & drop at cruise. years of wear & tear take their toll, w/new projects, I strip & examine harnesses in total especially in areas where movement is prevelent. this is simply a stop-gap cure to possibly get you on the road until you have more time to fiddle. Luck g

You guys are great! Thank you all for the great help. The plastic at the base of the contacts is deformed/melted and it's not that I've really blown fuses. The first time it happened I assumed that the old fuse that was in there for who knows how long had deteriorated and broke inside itself, cuz a new fuse in it's place corrected the problem immediately. That replacement was a 20 amp(all I had at the time), and that I think is the only one that actually blew a few weeks later. The last couple times the bike quit, I removed the assumed blown fuse(I'm old and can't see shit w/o reading glasses which I don't have when riding), and threw in a new one and was off again. When it quit again is when I first noticed the heat, and removal and replacement got me going again. When I got home and let things cool is when I noticed the heat on the contacts when the key was turned on, and you know the rest. Perhaps it is just at the fuse block. I'll start there, and again thank you all for your time and info!