Mrkitkat

Just swapped clusters in my EG (had no tach) and now I have a tach but it reads incorrectly. When I start the car it reads 0. When I'm driving lightly, still 0. But as soon as I gun it it'll jump up to around 4 or 5k, then when I let off the gas, it drops back to 0. Just wondering if anyone else has encountered this, or knows how to fix it. Thanks!