bench testing tacho?

hi all;
anyone ever found a way to bench test a tacho, i'm renovating my xj600, and was told by p.o. that the tacho didn't work, (was too busy checking other stuff on test ride)
I have the instrument cluster on the bench, and would like to get it sorted before replacing on bike.
there is a permanent live (brown)
and permanent ground (black) to it,
then the grey pulse wire.
ok, I can simulate the feed using a 12v battery , but how can I simulate the pulse, maybe flicking a torch battery on the grey and ground?.
or maybe I could feed it from the car and wrap the grey around a spark plug wire, anyone tried either way?
what kind of voltage would you expect on the grey wire, going to the tacho, when operating normally?
cheers
stu

 

You could probably put a test light on the tacho feed and see if there is signal to the tach. If there is, then the problem is internal to the tach.

 

you would not want to hook it to a spark plug wire but to the input side of a coil on a car not the spark plug side.
it is not going to read correctly on a car set up. the coil on a bike fires twice per rpm

your looking at 12 to 14 volts to the gray wire

how fast can you click a morse code key?

 

here's one that MIGHT work. One of your horns, a inductive load that's switched on and off. Try battery + to the horn, then battery - to the horn, while it's beeping touch the signal wire to the
battery-. the horn should beep and the tach should register something but no idea what. it might peg or barely move. cover the horn with a few rags

 

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You could probably put a test light on the tacho feed and see if there is signal to the tach. If there is, then the problem is internal to the tach.
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the complete bike is dismantled

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you would not want to hook it to a spark plug wire but to the input side of a coil on a car not the spark plug side.
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no, I didn't mean to hook it to the sparkplug wire, but to wrap the grey wire around the outer sheathing of the plug wire, similar to how the acewell tachos work, just to get a pulse.

polock's idea is worth a try though,
stu

 

the plug wire pumps out way more than 12 to 14 volts more like 20 to 30 thousand volts. your tach is measuring the signal to the input side of the coil not the output side

 

I think he means put some (wire) turns around the HT to induce a secondary voltage not use the HT itself - that would do bad things.

 

that makes a transformer , and it would still generate high voltage Polock method sounds safer

 

ok thanks guy's , I tried polock's idea with the horn, and I didn't get any deflection on the the tacho needle, so I have a spare tacho left over from the caferacer build
so I tried the horn trick ,and I got a small deflection, on the needle, but only once, i'll have to try the spare on the bike when I get it built.
thanks
stu