Strange Tach Issue

[zarn02]

Basics: 2002 998 with a PC3, sidestand switch enabling stand-down idling, and integrated tail-light. None of which I installed, so I didn't do any of the wiring up. There's a whole heap of shit hooked up the battery, but I have no idea how much of it is OEM as I've never seen an unmolested 998 to compare.

The problem: Turning the high-beam on or off changes my tachometer reading. At idle, the tach will read 1000-1300, with low-beam only. Turning on the high-beam drops the reading to 0-1000, depending on how the bike is feeling at the moment.

At speed, the high-beam seems to have the opposite effect, raising the tach reading by as much as 1000 rpm.

The longer I ride (highway, anyway), the more the problem seems to sort itself out. After riding for twenty or thirty minutes, the difference in readings changes to only a few hundred, and sometimes it even ends up to where pressing the "light horn" button only makes the tach needle twitch a bit, then it settles back down to about where it was.

Since the actual rpm isn't changing, my assumption is that this is a problem with the voltage the tach (or gauge cluster as a whole) is getting. It doesn't take very long for the battery tender to give me a green light when charging the bike, and (most of the time) it starts right up, so it doesn't seem to be an issue with overall charging.

Thoughts?

[J5]

sounds like you might have a bad ground somewhere

[Thermite]

Stock configuration at the battery is two grounds and one positive leads.  The two grounds are one ECU and one frame/starter.  My 998 is an '04 but there should be little difference.

If I get stuck in heavy traffic and the battery isn't getting charged and I'm running the fans for cooling mine will do the same thing.  Sometimes it's bouncing off zero while I'm idling.