Tach question again

[HobbyMan]

A question for the electronics gurus here. I am having a tach problem. Works fine and smooth when plugged in and holding the unit in my hand. The second I bolt it up the tach starts going haywire. I have tried several tach wiring harnesses. They aren't broken. Not sure if this a vibration or grounding problem. I have also tried several tachs with the same problem. Any ideas. ?

[BA80]

It's the engine vibration, mine does the same thing. If I reach up and hold it still with my hand it doesn't bounce around so much

[MadHatter4119]

Hobby Man...are you saying your tach isn't jumping when your hold the tach
in your hands when its off the bike...Not mounted/installed...??? I'm not sure
your issue is the same as Greg's...If your tach's wiring harness has a black ground wire in it...why not tap into it and run it to frame...you might be right on point about it being a ground issue...any other electrical things seem weak
to you...Not charging as good...lights and blinkers aren't as bright as they use to be...??? All signs of a weak ground...Easy to overlook the importance of a clean ground system on an old bike...Good Luck

[HobbyMan]

Did some more troubleshooting today

Yes, when u hold the gauge in my hand, it is smooth as silk and no jumping. When the gauge is actually bolted to the bike, it jumps so bad at times it is unreadable. Starting to get frustrated. Never had so much trouble. Time to build a whole gauge cluster from the forks up.

[MadHatter4119]

Strange to say the least...you don't have any other issues with the bike...not misfiring or other running issues...did you try the ground ??? What year and model bike is it...??? Are you using the stock OEM tach ???

[jwhughes3]

I bet you have a few to choose from!

It seems to me that by grounding it to the bike you may be getting interference. Maybe a bad diode which allows tiny current to mess it up? Anyone know exactly what to look for? Jerry?

John

[DiverRay]

Do you have an analog volt meter? If so, set it to AC Volt and put one lead on the Tach and one lead on the Ground wire of the harness. With the engine running, see if you measure any AC between the two. You should have about 7V. AC, or 14V peak to peak.
IF you have 14AC, or 28 peak to peak the problem MAY be the diode in the wire harness is shorted. Yamaha has it deep in the tape, but I'm not sure exactly where in the harness it is.
The other question, and what you may want to look at first, is do you have all the isolation rubbers installed? if they are hard, and they probably are, that could be part of the problem. The bottom of the Tach is held in with two 4mm(I think) studs, and has rubber grommets on either side of the cup. there should also be a soft rubber seal around the top of the tach, between it and the cup.

[HobbyMan]

Good suggestions all. I will put the meter on it as you suggested. After thinking on this for a bit, there seems to be a calibration potentiometer on the driver board for the tach. Didn't notice it before. As it happens on ships, these things get dirty. We usually just turn them back and forth a bit to clean them off. Perhaps this thing is dirty and the vibration is causing an intermittentancy. We will see. Haven't been out to work on the bike for a day or so. After adjusting this, perhaps will need to get a hand held tach to calibrate unless someone can tell me the what RPM's give a certain speed on a standard model.

Ray, I don't have my books in hand but is not the diode for the lighting system, not the tach?