im lost on this one, i cant figure out what you mean,is it a used tach?..........and does the light do a self test when you turn on the ignition?.....if it doesnt lite up when you turn the car over than you can start your problem solving from there.......if im not mistaken the autometer tachs get there ground and there power from when you turn the light into the tach housing, you can also double check that to see if its touching the leads.......