Tach topout

[shrubberman]

I have just bought an '81 XS1100 special. I have only ridden it home from where I bought it { 5 miles}. I noticed right away that as soon as I turn the ignition on the tach tops out, turn it off and it goes to zero. I have been reading other messages on here about tachs, and alternators and their problems going hand in hand. But nobody has mentioned that their tach tops out like mine does. Do you think that there is also a charging problem, Or is there maybe another reason my tach is doing this?

[shrubberman]

Well after some testing, I have decided the alternator is charging. But the tach stays topped out as long as the key is on. I have pretty much decided that at least one wire is going to the wrong place, or that maybe a wire is pinched and grounding when it shouldn't or that maybe it has lost a ground. I am not sure what tells it when it has made a revolution. I have looked at the wiring diagram and still undecided. does anyone have any thoughts on this? And can anyone tell me what activates the tach?

[sixtysix]

Never heard of this one before. I think you on the right track with the wiring problem. The tach (I think) uses regulator/rectifier output, at least I know if the tach quits working usually the regulator/rectifier is bad.

[shrubberman]

Well I have got to tell you. I sure have learned a lot about these tach's of ours, mine especially. It is shorted out somewhere in the circuit board. As soon as you hook up the brown wire [12v] with the ground wire hooked up, but not the white [the signal wire] it just jumps right up to full rpm's. took it all apart but couldn't find the culprit. I don't have enought knowledge to know how to test each individual resistor, connector etc... But the board itself was not shorted nor none of the wires. So I guess I will someday have to replace it. I didn't learn much from the wiring diagram, but I did by just digging in. lol

[sksxs1100]

after months of looking at my old tach sitting on my desk, i hooked it up to the 12vdc power supply and the signal generator on the shop test bench.
with a perfect signal(4vac and correct freq) the tach is steady as a rock no matter how you beat it around, it does not fluctuate any. the input signal to the white wire is between 3 to 5 vac (will get correct freq spread tommorow)
The circuit board outputs around 0 to 5 vdc(will double check tommorow) to the actual needle assy. SO... this gives me good starting points to try to find the cause of the tach wobble on my new tach. I hope this helps everyone who has the same problem.

[sksxs1100]

I double checked the measurments and they are as follows:
3 to 5 VAC trigger at between 0 and 1000Hz (100Hz per 1000rpm)
the output of the circuit board is between 0 to 2.5 VDC. I hope this helps someone with their tach problems.