It will not work on a 25th...
I needed 4 hours to sort it out, that's the reason why I don't tell it, because I'm selling the boxes. Well not the box but a ready made plug in solution which needs some hours to produce.

The stock power and ground has nothing to do with the tacho.
It works normal with the existing wiring, meanwhile 4 boxes from 1982 LP500s to 25th were installed and run great.

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RaymondQV
Countach QV Ex-Mimran #GLA12997, Urraco P300 #20530 sold in August 2008, Jalpa #FLA12203 sold in May 2006
www.countach.ch
www.jalpa.ch

When Frank Sinatra got his Miura S in 1970, he said: “If you wanna be someone, buy Ferrari - If you are someone, you own a Lamborghini”.