KTM Electrical Puzzle - I'm Confused - Please Educate Me

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6/12/2018 1:19 PM

My 2017 KTM 450SX-F lost all electrical functions including starter, fuel pump, solenoid, etc. I'd hit the starter button and not even a "click" or cycling of the fuel pump or anything. Battery is fine. Traced it down to the ground strap off the battery to the subframe had NO continuity. The grounding strap subframe mounting plate welds had broken so there was no ground for the entire electrical system (i.e. it was an open circuit). Fixed the broken welds, reattached the battery ground and once again all is good.

What puzzles me is when the bike was stationary and troubleshooting the issue I took a voltage tester to the positive terminal on the starter and the other end of the tested to a ground (i.e., the frame). The tester lit up indicating I had 12v power going to the starter. Note this was observed without pushing the starter button or touching anything on the bike. Why/how would I have 12v hot going to the starter when the system was NOT grounded off the battery?

I'm confused why I'd have what sure appears to be a 12v closed circuit to the starter when the ground off the battery was not attached? I just can't understand why this wouldn't result in an open circuit to the starter?!?!?!

Rick

P.S. FWIW, the broken welds on the subframe were my fault, not a material failure or poor welds.

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6/12/2018 2:51 PM

The meter will only read a difference in potential energy. You'll have 12V to anything not connected to the negative. If you got another battery and connected them in series lets say, you would read 24V between the positive and metal.

In order for the starter to work, the (-) side of the starter needs to in some way return back to the negative lead in order to get 12V flowing. The 12V needs to return back to the power source, otherwise it won't work.

Pretty much all vehicles achieve this by grounding to the frame or other metal parts as well as the negative terminal grounding to the frame. Since the ground plate welds are broken, then there is no way for anything to work since it has no path back to the power source. Electrical circuits need to be complete and return to where they came. If not, it's open.

Photo

Replace L1 with (+) and N with (-) and you get the idea. Every circuit is connected back to neutral or (-) in DC circuits. If not, it's an open hot condition and in the instance of AC power, it can lead to damaged equipment if you lose a neutral.

In DC circuits, equipment won't be damaged; it just won't work since it has no where to go.

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If you're not mixing gas, you're not haulin ass.

6/12/2018 3:47 PM

It may have a negative cable running back to the battery for the starter just because it's a high amperage draw, maybe it's got a relay that won't let it crank over if the fuel pump doesn't have power.

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10' 200 XC-w
96' XR600r

6/12/2018 3:58 PM

TrailDigger wrote:

The meter will only read a difference in potential energy. You'll have 12V to anything not connected to the negative. If you ...more

Excellent explanation/tutorial. It's been driving me crazy trying to understand. You did a great job laying it out for me and I GREATLY appreciate you taking the time to teach, not ridicule my ignorance.

A bit embarrassing as I'm a Petroleum Engineer by degree. However, I took Electrical Engineering 401 about 35 years ago and that class has long been forgotten. As they say, what you don't use you forget.

Thanks.

Rick

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