OBD0 B16 tach issues

okay so first off, thanks for looking at this thread. to the main problem, after my b16 swap in my 91 civic sedan, i converted from auto to manual and installed a tach. i bought a s**t one from ebay only cause it could read up to 9,500 rpm. the problem is, it reads perfect at idle, but from about 1,500 rpm and up, it goes erratic. i just went to autozone and got a little 35 dollar tach since i thought ebay failed me once again, only to find out the rpm would read the same! so i think i connected it to the wrong wire or something... i used plug 12 in the gauge cluster which is for the tach, which is why i dont understand this madness!!! any ideas what it could be?

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