Not sure that's 100% correct. Even if he were to take off the ground side he still has positive - can easily be proven by using a meter to ground. All it does is regulate the ground side to trigger the coil - hot is always hot and doesn't turn off because the trigger is off.

Twisting the wires together if they were run in the same loom may have eliminated EMI interference but I do suspect it would be a "noisy" circuit none the less.

However, tapping at the coil isn't ideal because you want that positive power lead to the coil alone - not adding circuits to it, so the advice of a different positive connection is good.

On my 30 year old Yamaha, I used the positive feed to the coil to activate a relay and ran fused battery power through the relay to the coil. So when the bike is keyed up, the coil has full battery voltage - not voltage through 30 year old wiring with mediocre connectors at best.

Breakdown goes like this (terminals 85 and 86 - coil 12v to one and the other to ground (nor trigger wire - good ground) - using resistor not diode relay). +12v from battery through a fuse to relay terminal 30. Terminal 87 from relay to coil 12v (where original wire used to go). When keyed on, old coil circuit activates 85/86 and closes pin 30 to 87 giving fused good new + circuit to coil.

You can also do this with lights, radios, fuel pumps when the OE wires are old or sketchy.

 

well mr. ed
in a way you are correct, there will be power there but it will not be a clean source due to the switching of the ignition module. as rpm rises so does the EMF back-flow and this EMF has a tendency to oppose current in the opposite direction. the thing about the coil is that when the field collapses, there is a size-able amount of energy that is left over. this energy is an ac component not dc and it will ride on top of the dc as ripple, when this ripple is of enough energy it will screw with electronics and can cause the tach to react negatively. there is an old trick that used to be used in days of old when tacks weren't as advanced and that was to put a filter capacitor to shunt the ac to ground.
a lot of people think that since you have 12vdc for the ignition is just plain ole dc or worst yet, pulsed dc. the ignition displays characteristics of both.
in actuality, this concept has been used to develop switch mode inverters.
another issue is running circuits parallel to high tension leads which will induce ripple into them, reason to stick to factory routing.

 

Ed kind of lost me when he started talking about yamahas and terminals that don't exist on my schematics but I still appreciate his input. I will run power to the tach from inside the nacelle so it should only be a short run of wire and to get it away from the trigger wire which of course runs up the top frame rail.Just have to wait to get home but it will be time to ride by then. Yahoo

 

Still waiting

 

Don't worry I haven't forgot ya. I still have my toes in the water and *** in the sand, as the song goes for another ten days before I get back home.Once there I will be right on it and yes "Riding season" will be there too.

 

Ok so here’s what I did. Took my power from the terminal board in the nacelle and ran to the tach. Noticed the oil was quite low and yes it had wet sumped so without adding more oil I fired it up and of course it puked out of the breather vent but I was ready and had some diapers down. So the tachometer works at home but I still need to road test to be sure but now the bike won’t shut off with the key. I disconnected the power wire to the tachometer but no change. One step forward and two back. Lol

 

Ok all seems fine bike shuts off not sure what was going on there mad at me for sitting for so long I guess. Oil level back up and in the tank. The tach is working fine for now but the real test will be after twenty minutes or so. This time I used a little bit older style tach but if it works I will try one of the newer styles. I have acquired a few different ones in my quest. If it craps out after a few miles I will let you know

 

Sounds like that was the issue with the tach, let us know how your further tests turn out.