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Unplug each grip, obtain a multimeter, each grip will be fed by two wires, with the multimeter set to ohms measure across the two wires of each grip and compare, ideally each grip should measure much the same, if you have a grip problem I would expect the right grip ( throttle) to have a higher...
I took a look at the diagram myself, as you have found it's not obvious, It does seem the tacho could be driven by a general dash control chip, this also means that the actual signal could be in the form of what I know ( from my car repairing experience) a bus or can message, you won't get a...
I can send you the official shop manual pdf if you give me your email, it has a wiring diagram in,
Your in an area I don't think anyone has messed with so far and I doubt anyone knows the answer.
Some time back on my son's little 125 I used an oscilloscope to back probe the dash wires to...