Odd electrical question - voltage between grounds

[trepetti]

So i am working on my project (remember my questions about brake light modulation?) and I have been stuck at a certain point. When I test on the bench with a stable power supply, and when I test on the motorcycle with the key on / engine off, it works fine,

Start the bike and lots of odd behaviors, mostly the brake light firing spuriously.

So here is what I am stuck on. When I take my vom and connect neg to battery ground and the red to the tail light ground, meter shows 130 mv between them. Resistance shows 1meg ohm between the 2.

I believe that the only to get voltage between grounds is if one of them is a bad ground, floating / semi floating. If that is true, I will need to look at the chassis ground connections.

Can anyone weigh in on my theory? Does anybody have the locations where the wiring loom connects to the chassis?

Thanks

 

[Motogiro]

I think the key is the engine is running. How is the on/off logic for the light functioning? I would tend to think that your device is digital and an IC is seeing noise/ ripple when the bike is running. Maybe a brute line filter is in order as well as a capacitor from its supply to ground at it's location in the circuit. Would be nice if you could confirm( O scope) possible ripple (stator) or even a data signal present.

Great that your building a modulator!

Goop (Bryan) installed one on his Speed Triple and when we rode in a group my eye was always drawn to his brake light as it was modulating. I then installed one. I think I like the flash rate on his better than mine. Mine is a slower rate but I think a little faster would be better.

 

[trepetti]

No scope yet, I have been dealing with the noise for a while, As you likely guessed, I am trying to tame the brake light circuit so I can use it as a digital input to trigger the logic of the MCU. I originally thought that I could use a plain old voltage divider. Like I said, worked well with clean power. Never realized how unstable the environment would be on the bike.

So this test was done by trying to saturate a 2n3904 with >.65v whenever the brakes were applied. Used a voltage divider of 4k7 and 250, and this was the best results of all the other attempts, but the logic is still a little erratic. Since I am using the taillight ground as the dc ground for the circuit, this unaccounted for voltage cannot be good.

As to my original question, do you think I have a high impedance frame ground causing the float?

One final odd measurement.....

Setting up my vom with neg on battery ground and pos on tail light ground, I get 0v with the key off. Key on without starting the engine, I get 182mv while the fuel pump is priming, settles to 23mv when pump shuts off. Start the engine and jumps to 132mv.

Thanks for your help, I probably should be posting on an electronics forum, but I'd rather be an EE dummy on a motorcycle forum .

 

[Motogiro]

I haven't ever looked at differential voltage regarding ground on a vehicle. Given physical distance through the frame and resistance through frame contacts there is bound to be small voltage differential.

You might go through any frame grounds you can track down and redo them using a stainless steel star washer between the the ring connector and unpainted frame surface which pretty much guarantees a great bond. Then see if those differential voltages are reduced.