Topic: Hacks to Save Fuses When Diagnosing a Short

[cshanek]

Hello everyone,

Background: The spawn-of-satan right handlebar switch on my 1976 CB750 K6 is at it again (I think) and making work/troubleshooting difficult. 05c50 helped me work through an issue with this switch right after I bought it. So far I have blown about 10 15A fuses attempting to determine the cause of the short.

Question: What's a good way to diagnose shorts without blowing through a bunch of fuses? I hooked my multimeter to the contact points for the fuse (without the fuse in), hoping I would see a spike in current or something, and it provided consistent readings, and isn't super easy to check while working. I've seen a couple of videos/forums where people suggest hooking up a little light bulb so you can see it light up/go dim as you wiggle or unplug components? Any hacks, additional tricks, or sage advice would be appreciated.

-Shane

[Tracksnblades1]

Ohms law…🤔

Low Ohms = high current = Poof…😳

VOM meters still work finding direct shorts…😇

[leovich]

Hello everyone,

Background: My spawn-of-satan right handlebar switch on my 1976 CB750 K6 is as it again (I think) and making work/troubleshooting difficult. 05c50 helped me work through an issue with this switch right after I bought it. So far I have blown about 10 15A fuses attempting to determine the cause of the short.

Question: What's a good way to diagnose shorts without blowing through a bunch of fuses? I hooked my multimeter to the contact points for the fuse (without the fuse in), hoping I would see a spike in current or something, and it provided consistent readings, and isn't super easy to check while working. I've seen a couple of videos/forums where people suggest hooking up a little light bulb so you can see it light up/go dim as you wiggle or unplug components? Any hacks, additional tricks, or sage advice would be appreciated.

-Shane

Try self reset fuse from Harleys, you can find also aftermarket, see like this in picture

[bryanj]

Use a 21 watt bulb with leads accross the fuse

[Nicklopic]

Go to an autoparts store, you can purchase circuit breakers to use in place of a fuse. Most places will have either blade style ones like ATC/ATO styles or ones with studs. You can always add spades/eyelets on them and add a wire to make them suit whatever you’re playing with. You’ll hear them pop and then a few seconds later they will automatically reset

[cshanek]

Use a 21 watt bulb with leads accross the fuse

I have one of these rigged up already https://www.carid.com/bosch/long-life-halogen-bulbs-1034-mpn-1034ll.html but it is only 13w apparently.

[Rosinante]

Use a 21 watt bulb with leads accross the fuse

Yup, in principle.  I'd just put a resistor there.  Modern bulbs v. incandescent makes things more complex. 

Or better yet, use an ohmmeter. To find the short, which is basically what you have.  You'd be looking for very low resistance.

[scottly]

I have one of these rigged up already https://www.carid.com/bosch/long-life-halogen-bulbs-1034-mpn-1034ll.html but it is only 13w apparently.

That will work fine. BTW, that bulb has two elements, each with a different wattage. Use whichever one is brightest when you connect it directly to the battery terminals, and make a mental note of the brightness. When connected to the fuse holder, it will be dimmer with normal loads, such as the resistances of the headlight, taillight, at least one ignition coil, field coil etc, and much brighter when the short is happening, and will limit the current to a safe level. You may want to remove the headlight and taillight fuses while you are trouble-shooting.
Now, what is the history of this problem? Does the fuse blow only under certain conditions, such as by wiggling wires or turning the handlebars? Does it only blow when the kill switch is set to Run?

[cshanek]

Copy all.

The most recent problem just began and is annoyingly intermittent. I was able to run bike and sync carbs twice (wanted to get them better seated on boots and give it a go again).

Some notes:

• It seems to happen regardless of switch being on or off (buttons), however when I dismounted the switch from the handlebar, all was well. I played around a bit, mounted it, blew a fuse. I taped the bar with electrical tape, thinking the bar was the contact point for the short, mounted the switch again, and blew fuse.
• I think I have eliminated rear light and blinker
• I have the headlamp removed, and the rats nest in there (re-did a few years back) looks good. I have moved things around in there, and did not see anything change on multimeter, and more recently no change in the test bulbs brightness
• I think I tested all of the ground connections, and they look good
• I know this might be sacrilegious on this forum, but I decided to buy one of these https://www.amazon.com/gp/product/B00G4YUWMC/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1 to make troubleshooting a bit easier (also to use on an old truck). I just received it.
• I am using a small lithium battery (https://www.amazon.com/gp/product/B08SR3FZNS/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&th=1), which might be adding to the confusion. I noticed that when it has a decent charge my multimeter says it is putting out 16A. I hooked my test bulb up for about 45 minutes to take a bit of power out of the battery, multimeter got down to 15A, tried again and blew fuse. The interesting thing about this theory was that I convinced myself that running the bike for 30 mins for carb syncs charged the battery just enough to blow my fuse. It seems on a couple of occasions that the short occurred without me even touching the bike or moving anything around. This might be debunked now.

I plan to test the functionality of the switch with a multimeter tonight.

[robvangulik]

Are you sure you are measuring 16 Ampere, and not 16 Volt? That would mean you are using some 200 Watt as in 4 headlamps on high beam.....

[scottly]

• It seems to happen regardless of switch being on or off (buttons), however when I dismounted the switch from the handlebar, all was well. I played around a bit, mounted it, blew a fuse. I taped the bar with electrical tape, thinking the bar was the contact point for the short, mounted the switch again, and blew fuse.

Go back to the switch!! Does the short still go away with the switch dismounted? Maybe the tape got torn when the switch was mounted?

[cshanek]

Tested a bit more tonight with RH switch off the bars. I did not blow a fuse all night (new ceramic 15a fuses, and battery not fully charged). I did notice on a few occasions, that when I would turn the ignition on (regardless of switch settings), the battery would drain very quickly, and in a matter of 5-10 seconds, it would get below 9v and shut itself off/protect itself, and I would lose indicator lights. I am guessing this was representative a short, with the battery not producing enough current to blow my 15a ceramic fuse? I hooked the tender up and repeated the test. Eventually I moved the battery around a bit, even though nothing looked even close to making contact with the frame, and this behavior stopped. I tested the right hand switch in this "good" state. Results are attached.

[TwoTired]

Most of the ceramic fuses I’ve encountered are fast blow types.  This is not what you want. You want a slow blow.  The latter will take transient over current, while the fast blow types will not.  I’m assuming you are using the tubular type of fuse, the same shape as the glass fuses.

Do check the run temperature of the fuse.  If you can’t hold your thumb on it while it’s been operating with all lighting on, you have a fuse holder issue that will melt any fuse you put in there, eventually.

[scottly]

Tested a bit more tonight with RH switch off the bars. I did not blow a fuse all night

There's your sign, as Jeff Foxworthy would say. The switch is shorting out on the handle bar.
The battery voltage dropping rapidly under load is typical of a LiFe battery that is run down. Hopefully with a proper charge it will come back to life, but with deep discharges like you have done it may have suffered permanent damage.

[cshanek]

Tested a bit more tonight with RH switch off the bars. I did not blow a fuse all night

There's your sign, as Jeff Foxworthy would say. The switch is shorting out on the handle bar.
The battery voltage dropping rapidly under load is typical of a LiFe battery that is run down. Hopefully with a proper charge it will come back to life, but with deep discharges like you have done it may have suffered permanent damage.

Scottly, the odd thing there is that the super quick discharge happened on 4-5 occasions out of maybe 30. So I would attempt a fix, observe results, and then hook up a little junior tender.  I had my headlamp unplugged the entire time. During the 25ish "normal" observations, I would turn the key, and observe the voltage drop from 12.3 (or similar) to 12.1 or so in 10-15 seconds. During the super quick drainage events, 12.3v would drop to below 10v in 10 seconds or so, until the battery shut itself off. For each of these tests, all I was doing was turning the key in the ignition to see if the lamp would get brighter or if my ceramic fuse would blow.

Thoughts?

[scottly]

Just use the test light in place of the fuse. You are confusing the charge of the battery with trying to fix an excessive current draw. If the light is bright, and the battery voltage is high, you have a short. If the light is dim, and the battery voltage is low, you have a dead battery.

[cshanek]

I rode her in to work today, and aside from the left signals being solid (should be easy enough to fix), everything on the electrical side seemed to be in order. I have to assume it was the right hand switch potentially grounding out against the handlebars.

While I was playing with electrical issues, I had the wheels rebuilt with new spokes, I completely redid the front brake, and I rebuilt the carbs.  I had an annoying front brake squeak every rev, and power issues this morning, so onward and upward.

Thanks to everyone who provided guidance.