Need help with Tacho install

[Dust]

Car - 1981 4wd hilux, 18r engine

Bought a cheap a/m tacho from ebay as the ute doesn't have one.
Connected it as per instructions and it reads too high rpms and needle does not sit steady.
I connected an old tacho from another hilux and it worked fine.
Then for some reason I now forget, I connected both tachos at the same time and they both worked fine.

Typically I have turned a 20 minute job into an unfinished one that took half a day.

Any help appreciated

[Kedderz]

the tacho should have an adjuster on it if its a modern electronic unit, check your instructions?

[Dust]

the tacho should have an adjuster on it if its a modern electronic unit, check your instructions?

all it has on it is the 4,6,8 switch, yes I have it on the right one

The instructions are just 1 sheet of paper with some roughly drawn pictures and 4 words in total, 2 of which have incorrect spelling lol.

Can anyone explain why it wont work properly by itself, but it will work fine when another tacho is installed at the same time.

[Vios-GT_07]

sounds like a cheap tacho ...

all the tacho does is read pulses.. autometer and most common aftermarket tachos can read the sine / square wave emitted by the coil, to indicate RPM.. even if you connect 3 or 4 tachos it should all read the same afaik..

so me thinks ur tacho is a cheapie that doesn't like the signal from the source? try it on another engine.. am curious to know if it works properly with other cars as well

[ViPeR_NiPPleX]

could be noise. How good was your earth?