Tachometers for dummies!

i bought this tach brand new. hooked it up worked great for the first few months. gradually the needle started sticking at the RPMs the engine was turning when i shut the car off. it eventually stopped reading properly all together. now it doesnt work at all. its a POS in my opinion, but maybe i got a bad one. i do know i wont get another one. it lasted about 8 months. this pic is after i shut the key off LOL