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RPM Gauge does not work

So it doesn't work or it's not connected? two different issues.

If it's simply disconnected you should have either a grey wire or a purple wire at your control/shifter that will feed the signal to the tach (goes to the post marked "signal").

You also need to feed accessory power from your keyswitch to the tach itself (normally have a common red wire that powers all your gauges - simply piggy back off one of the other gauges if it's not disconnected. Likewise, you will have a common ground (black wire OR yellow wire if it's on a newer boat or it's been rewired by someone following the new recommendations) - the ground is usually connected to the bracket (or one of the bracket screws) on the gauge.

IF (if) you have an OEM tach (from Merc) that matches your control unit it has a simple plug in harness (the socket end is at the control) with two wires (power and signal I believe) that go up to the tach (wired as described above).

That's how you reconnect it at the dash end.

If it's disconnected on the motor end, the grey wire from the main harness goes to one of the posts on the rectifier (do not connect to the post that has the red wire that goes over to the starter solenoid, but either of the other two posts are fine). The tach signal is generated by the rectifier.
 
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