Make sure that the wires are connected correctly at the back of the tach. The red wire (light blue for early cars) goes to the + terminal. Check for + 12v at the tach + terminal with the key on and the engine off.
Next check is to disconnect the blue / black wire from the back of the tach. Connect a voltmeter to the disconnected blue / black wire and to ground. Put the voltmeter on the AC scale (NOT the DC scale). Start the engine. You should get a voltage reading that increases with engine rpm.
If all of the above checks out, then the tach may have a problem. Also make sure that the tach has a good ground.
John
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