Fast302

I just bought an autogage 5" monster shift light tach. I installed it the other day, and followed the instructions given by autometer to the tee...i hooked up the ground, power, ignition and coil. But the tach doesn't read correct at all! When i start the car, it says i'm idling at 1500rpm, but i idle at around 900...then it just stays at 1500 until i'm at about 2300 and then jumps to about 2000, and from there on it's off by about 300rpm, until about 3000. It stays at 3000 even when my stock tach goes up to 4000...and the worst is that the shift light doesn't work!!! is this a common problem? how can i fix this...this is the 2nd tach i've gone through...the first one at least read the right rpms, but the light didn't work...please help!!!