A while back, my Honda Shadow kept blowing headlight fuses so I thought I would share a technique that I use to hunt down shorts. This should work for most bikes or cars, so I thought I would share this problem-solving technique with you guys:

Thanks, sckewed. Fascinating! After closely examining the video, I see what you did.

You just hooked up another blinker by connecting that test light inline with the blinker circuit. The test light should flash with the blinker.

BUT the filament in the test light is now THE FUSE! The power no longer runs through the fuse in the fuse box because that one is burned out. The power flows through the wire to the test light ----and the filament burns out if too much power flows through (from a short) just like a fuse.

Only thing I don't understand is why the test light doesn't flash with the blinker after the short is eliminated. I understand why it wouldn't burn out but assuming you changed in a fresh bulb to the test light (which you must have done or else the blinkers could not get power), it should flash with the blinker instead of burning the filament---NO???

* Last updated by: Rook on 12/24/2017 @ 11:33 AM *

--or did you disconnect the test light and put a good fuse in after fixing the short in the switch?

Rook, Here's what is happening:
The light comes on when there is a short on the output side of the fuse. There is power on the input side of the fuse when the key is on. There is no polarity for a normal bulb. (Not as in a LED bulb).
When a short occurs on the output side of the fuse, it will act as a ground and cause the light to be bright

The light comes on when there is a short on the output side of the fuse.

Why doesn't the test light flash with the blinker after the short is fixed? There's power going through the test light and then continues to the switch. Seems to me the test light and the blinker should both flash when the switch is closed wether you have a short or not.

Rook, there would be a small amount of current flowing through the light circuit even if there was not a short, but not enough to notice the light, but when the short occurs, then there is a surge of current that causes the light to shine brightly.

Several years ago I had to learn assembly language programming for a project and the professor that taught the course was a retired electrical engineer and he used to love to go on tangents during his lectures and during one of these tangents he explained the physics of this technique. It fascinated me so I filed this in my memory banks and I have used this technique several times on both cars and bikes.

* Last updated by: skewedTotheLeft on 12/24/2017 @ 4:59 PM *

Rook, there would be a small amount of current flowing through the light circuit even if there was not a short, but not enough to notice the light, but when the short occurs, then there is a surge of current that causes the light to shine brightly.

Ah ok I gotcha. Like a flashlight with an almost dead battery, I see. The test light is flashing, you just can't tell because the filament is not getting enough power to glow. I thought the amps the test light used was similar to the blinker. If that were the case, it would seem both lights would be approximately as bright.

I'm still surprised by this. Being there is no fuse, it seems the test light would draw as much amps off the battery as it needs to light up whether the switch made the ground or the short in the switch made the ground. Then the residual gets sent to the blinker which would also draw as much amps as it needs to light up.

It seems like the blinker is dictating the amps that flow through the whole circuit. Interesting.

* Last updated by: Rook on 12/24/2017 @ 9:41 PM *

Rook, I've been thinking about some of your very logical comments and I decided to do some further investigation about what is really occurring in the test light example.

Once the headlight was removed in the test environment then the headlight circuit was not grounded so the circuit is not complete. The turn signal circuit has a separate fuse (I found the wiring diagram) but there is a common meeting point with the headlight circuit through a diode. So the test light would not have any grounded power because the headlight was removed. But when the short occurred in the turn signal, then the arch of power crossed into the common connection. This would cause the test light to flash because the short made its way to the output side of the headlight fuse and this acted as ground to momentarily flash the light.

* Last updated by: skewedTotheLeft on 12/26/2017 @ 11:46 AM *

So the short in the turn signal switch was grounding the test light even though the fuse you had the test light hooked to was for the headlight because the headlight and turn signal have a minor connection through a diode. The headlight probably flashed too when you did your test. ..but if you had a fuse in there instead of the test light, the fuse probably would have burned out before the headlight could flash.

I believe we have finally solved this mystery. Thanks man. If it were not for your intuitive questioning, I would not have fully understood what I was doing myself. You and I must be alike in this way--it is not good enough to simply know that something works---it is much better to know WHY it works--well in my case it took a little prodding.

* Last updated by: skewedTotheLeft on 12/26/2017 @ 6:29 PM *

You and I must be alike in this way--it is not good enough to simply know that something works---it is much better to know WHY it works--

Absolutely separates the men from the boys in the wrenching dept. Thanks for the great video and related convo.

Neat trick, I was thinking the same thing as Rook initially, but when skewed said:

there would be a small amount of current flowing through the light circuit even if there was not a short, but not enough to notice the light, but when the short occurs, then there is a surge of current that causes the light to shine brightly

It made more sense, I think the key here is the test light is wired in series and not in parallel (FYI, this terminology is always with respect to the battery/power source & other components such as the turn signals). When the circuit is functioning normally (right turn signal is on) as skewed said, there's not enough current going through the test light to illuminate the lamp as the test light is being limited by the amount of power that is flowing from the battery through the test light then through the switch & turn signal lamp.

When there's a short, power flows from the battery through the lamp and short-circuits back to the battery, bypassing the turn signals and as Rook put it "the test light would draw as much amps off the battery as it needs to light up".

In a different scenario, if there was no common connection between the turn signals and the headlights (what the hell is with that anyways??) and the short was somewhere in the headlight assembly after the light itself (ground side of the lamp). I would expect that the test lamp would illuminate the entire time the circuit is good/normal but the headlamp would be very dim.

Here's where I put my disclaimer that I'm NOT an electrical engineer and this is my understanding and not actual fact. So if someone else here is an engineer please correct me if I'm wrong.