Author Topic: The Motorcycle Gods are Fickle - Blowing Through 15a fuses  (Read 1765 times)

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Offline cshanek

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I spent the weekend touching up the frame with paint, polishing and cleaning everything I could get my hands on, installing my new exhaust, and finally fixing a leak I had on the drain tube of carb #2. I even was able to mount the carb boots and carb body relatively painlessly tonight. I then ran fuel lines to my auxillary tank and turned the bike on. To my utter amazement, my tail light, which had been the last of the electrical demon Mohicans, was on. For a brief 10 seconds or so everything on my list was complete. I hit the ignition button for a few seconds, opened the throttle a little, then realized one of my spark plug cables was unplugged, got off the bike, and saw everything just go dark. No lights, no nothing. I checked the fuses and the 15a fuse had blown. I then thought that there was probably an obvious ground/short somewhere to I proceeded to go from one end of the bike to the other inspecting the wiring, blowing many more 15a fuses along the way. 

Aside from my tail light not coming on (accept when rear brake is engaged) my electrical situation had been very stable and predictable for months. Everything in the head lamp bucket is terminated and insulated. Because I worked on just about every square inch of the bike, I am assuming the issue lies elsewhere. The one thing that is throwing me off is that the bike was happy and working for that brief 10 seconds or so, which makes me think that shifting my weight when getting off the bike did something. Any thoughts or ideas? What is the best way to isolate this problem without blowing another dozen fuses?

Offline scottly

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #1 on: May 25, 2020, 10:38:45 pm »
What is the best way to isolate this problem without blowing another dozen fuses?
Take a brake light bulb, and solder wires on with alligator clips on the ends. Clip the wires to the main fuse terminals in place of the fuse. A normal load will cause the light to glow, while a short will cause the light to be as bright as a normal brake light.
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Offline madmtnmotors

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #2 on: May 26, 2020, 05:12:00 am »
When my main fuse started to blow periodically I attacked the high amperage connectors in the harness for inspection/cleaning/repair. These include:

>The clips for the fuses themselves. Make sure they are clean and shiny.
>The connectors to the stator, both the big multi connector block as well as the individual connectors under the sprocket cover behind the trans cover. I usually replace these bullet connectors to the stator anytime I delve into a new project.

http://forums.sohc4.net/index.php/topic,137351.msg1549191.html#msg1549191
TAMTF...


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Projects:
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"P.O. Debacle": http://forums.sohc4.net/index.php/topic,126692.msg1441661.html#msg1441661
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
                                           http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
                                           http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                           http://oldmanhonda.com/MC/wiring750K1.html
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
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                                    http://forums.sohc4.net/index.php/topic,48858.msg515204.html#msg515204
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
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Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
Late model K7/K8/F2/F3 front sprocket cover removal: http://forums.sohc4.net/index.php/topic,178428.msg2072279.html#msg2072279
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Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #3 on: May 26, 2020, 06:35:12 am »
I will give these suggestions a try when I can overcome the gumption trap. What do you make of the sudden nature of the problem, that is from all is well for about 10 seconds to immediately blowing fuses the second I turn on ignition? I figured this had to mean something changed right then and there, possibly shifted or a short or worse.

Offline Alan F.

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #4 on: May 26, 2020, 08:27:58 am »
Not all fuses are designed to blow instantly, there are fast and slow varieties. The slow type can take 10-15 seconds at peak current before blowing.

Offline madmtnmotors

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #5 on: May 26, 2020, 08:42:41 am »
I will give these suggestions a try when I can overcome the gumption trap. What do you make of the sudden nature of the problem, that is from all is well for about 10 seconds to immediately blowing fuses the second I turn on ignition? I figured this had to mean something changed right then and there, possibly shifted or a short or worse.

Start by looking for a short first and go from there.
TAMTF...


Wilbur



Projects:
"Evolution": http://forums.sohc4.net/index.php?topic=100352.0
"P.O. Debacle": http://forums.sohc4.net/index.php/topic,126692.msg1441661.html#msg1441661
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
                                           http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
                                           http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                           http://oldmanhonda.com/MC/wiring750K1.html
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
                                    http://forums.sohc4.net/index.php/topic,110931.msg1248354.html#msg1248354
                                    http://forums.sohc4.net/index.php/topic,48858.msg515204.html#msg515204
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
Taiwanese Cam Chain Tensioners:  http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841
Gumtwo Seat Cover: http://forums.sohc4.net/index.php/topic,164440.msg1897366.html#msg1897366
Primary Drive: http://forums.sohc4.net/index.php/topic,166063.msg1919278.html#msg1919278
Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
Late model K7/K8/F2/F3 front sprocket cover removal: http://forums.sohc4.net/index.php/topic,178428.msg2072279.html#msg2072279
630 to 530 conversion: http://forums.sohc4.net/index.php/topic,180710.msg2094423.html#msg2094423

Sent from my Tandy TRS-80!

Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #6 on: May 26, 2020, 08:45:42 am »
Alright thanks for the input. I have a couple of hours to take a crack at this crotch-punch before hiking across the Black Hills of SD for a while. I will post updates.

Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #7 on: May 26, 2020, 08:52:58 am »
I should also mention the harness, plugs, cables, dongles, rectifier, fuse box, and pretty much every other electrical component on the bike is about 2-3 years old.

Offline Alan F.

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #8 on: May 26, 2020, 11:33:29 am »
Can I ask where you bought the harness?

Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #9 on: May 26, 2020, 11:40:57 am »
Everything was bought from Vintage CB750. I have had a whole slue of problems concerning the handlebar switches, because first, they were shipped to me broken, and second the wiring isn't an exact match. Other than that everything has been working very consistently and predictably for a while now, except that I could not get my tail light to come on unless the rear brake was applied. It was not the fuse or second bulb either. 3 things, right or wrong, came to mind last night ... 

  • That my tail light was actually on when I turned the ignition on. I know I thought about jumping it to work, and was confident I knew how but hadn't. I was working around the back fender to paint the frame and polish so this was my "Occam's" razor. I am hoping it is an issue there.
  • The headlamp nest was the only thing that wasn't touched really, and I am somewhat confident everything in there was well connected, insulated and terminated.
  • Something had physically changed while I was cleaning/painting/polishing.

Offline scottly

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #10 on: May 26, 2020, 11:53:11 am »
Does the tail-light wiring pass through a hole in the fender with a rubber grommet to protect the insulation? 
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Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #11 on: May 26, 2020, 11:55:43 am »
It does.

Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #12 on: May 26, 2020, 11:57:56 am »
I could easily disconnect the tail light and possibly even rear signals and turn the bike on to test/isolate that area.

Offline scottly

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #13 on: May 26, 2020, 12:03:03 pm »
Whenever something suddenly stops working, always ask "what was the last thing you did before the problem started?"  ;)
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Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #14 on: May 26, 2020, 12:10:32 pm »
Yeah I am generally pretty good about that process, and even keep a daily work journal which describes what my assumptions are, what I plan to do test, results, notes and any other info. Unfortunately I threw all of that "process" out the window yesterday due to being in a rush to get her up in purring before I turned in. I polished the back 2/3 of the bike, especially the engine block and rear wheel assembly, removed the center stand, finished fitting new exhaust, installed all of the freshly polished foot pegs, pedals, and kick start, fixed carb #2, mounted boots and carbs, ran throttle cables, and then ran into this. Lesson learned though. I think it had been maybe 2+ days since I had turned the bike on, and I spent the entirety of the previous day cleaning and polishing.

Online HondaMan

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #15 on: May 27, 2020, 11:59:21 am »
I spent the weekend touching up the frame with paint, polishing and cleaning everything I could get my hands on, installing my new exhaust, and finally fixing a leak I had on the drain tube of carb #2. I even was able to mount the carb boots and carb body relatively painlessly tonight. I then ran fuel lines to my auxillary tank and turned the bike on. To my utter amazement, my tail light, which had been the last of the electrical demon Mohicans, was on. For a brief 10 seconds or so everything on my list was complete. I hit the ignition button for a few seconds, opened the throttle a little, then realized one of my spark plug cables was unplugged, got off the bike, and saw everything just go dark. No lights, no nothing. I checked the fuses and the 15a fuse had blown. I then thought that there was probably an obvious ground/short somewhere to I proceeded to go from one end of the bike to the other inspecting the wiring, blowing many more 15a fuses along the way. 

Aside from my tail light not coming on (accept when rear brake is engaged) my electrical situation had been very stable and predictable for months. Everything in the head lamp bucket is terminated and insulated. Because I worked on just about every square inch of the bike, I am assuming the issue lies elsewhere. The one thing that is throwing me off is that the bike was happy and working for that brief 10 seconds or so, which makes me think that shifting my weight when getting off the bike did something. Any thoughts or ideas? What is the best way to isolate this problem without blowing another dozen fuses?

Do you have my ATC Blade-type fuseholder? If not, get a small wire brush (.22 caliber gun bore brush is perfect for this) and spin it in the clips for the fuses (all of them) until they shine. They have now lost their OEM chromate plating, now being a good resistor on each clip that heats up and melts the fuses. To make matters worse, you do not have the OEM type fuse, which is model SFE. All we can get today are instrument-grade fuses, not made for vibration nor surge heating. They just fail because they literally melt apart from hot clips and soft internal elements when carrying current.
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Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #16 on: May 27, 2020, 12:31:24 pm »
the fuse box looks great, connectors are clean and golden. I will do this none-the-less, no reason not to. Tonight I plan on running some continuity tests, and unplugging parts of the bike and seeing if I can isolate the issue.

Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #17 on: May 29, 2020, 07:18:35 am »
UPDATE: Only had about 20 minutes to spend on the bike since this happened. I disconnected all of the rear lights and tried again, another 15a fuse instantly blew. I then disconnected the fuse box, rectifier, and pretty much everything else in the area and cleaned the fuse clips (pic attached). Tried again and blew another fuse. I am fairly certain I have been using the same fuses for everything since this began. Below are my current assumptions:

  • It's not the tail light, break light, or rear turn signals or their grounds.
  • It's probably not in the headlamp bucket (since that was about the only thing that wasn't touched in the 2 days of work, other than tightening the right bucket ground nut.
  • Something changed when I got off the bike and everything was on ... this is what I keep coming back to. Did something shift? Everything seems to be very tight (not loose). Did something else fail?
  • Next step is to go through and perform continuity tests.
  • Eventually work my way to the stator connections.

I will not be able to touch the bike for about 10 days, so I will try not to let it get me down, regroup and attack this early June. 
« Last Edit: May 29, 2020, 07:21:18 am by cshanek »

Offline scottly

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #18 on: May 29, 2020, 09:00:44 am »
The stator doesn't connect to the main fuse.
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Offline scottly

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #19 on: June 03, 2020, 06:20:30 pm »
When you get back to this, check if the fuse blows with the handlebar "Kill" switch set to Off.
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Offline evinrude7

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #20 on: June 03, 2020, 08:14:49 pm »
This happened to me. One of the wires on the break light sensor had slipped if its male connector and was touching the frame. Kept blowing fuses til I looked down at it.

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Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #21 on: June 08, 2020, 08:59:27 am »
Spent a few minutes playing last night ... turned bike on and same initial behavior, tail light was glowing bright, everything was good. I applied the brake and noticed the tail light did not get any brighter. I tried starting the bike and then fuse blew. I replaced it with an identical fuse and it blew immediately. (Same bag of fuses I bought for testing). I also tried the kill switch set to off and it made no difference. I also eliminated the horn as a potential problem. My current assumptions now:
  • Not the tail light, or brake light, or rear signals since I can unplug all of them and this still occurs
  • Not the horn, since I can unplug them and this still occurs

I will try the brake light sensor thing next time out, and I suppose take a stab at actually tracing the short.

Offline cshanek

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Re: The Motorcycle Gods are Fickle - Blowing Through 15a fuses
« Reply #22 on: June 13, 2020, 08:07:25 am »
Well get a little time in today and tomorrow testing ... have the multi-meter, continuity lamp and about 3 dozen fuses. The plan is just to eliminate possibilities.

Offline cshanek

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