My 1988 mercury mariner 75hp, I put a new stator and I

Ok since the tach doesn't work and the volt gauge don't show a charge the rectifier is bad.

Now have you checked the voltage at the battery before you start the engine and while engine is running?

What is your voltage reading before starting and when running at 1500 rpms?

Was this the stator off the other engine?

Test with your ohm meter between the 2 yellow wires disconnected from rectifier. You should have less than 1 ohm.

What is the amperage of the new stator?

Ok what terminal do you have the gray wire connected to? You should have the yellows connected to their own terminal and the gray connected with one of the yellows then a red on the 3 terminal. Is this how you have the rec wired?

Here is a diagram of the rec. The yellows should be connected to the terminals A and C then gray on one of the A or C then battery on + terminal. http://www.myoutboardengine.com/pics/rectifier/drawingPost.jpg

Here is the rectifier you should have. http://www.marineengine.com/newparts/part_details.php?pnum=MER8M0058226

Not likely being a new stator.

Have you checked the charge rate with engine at 1500 rpms? This is a very low output stator and has to have some rpms to produce any real amperage.

It doesn't have to be exactly 1500 just high rpms. What wires do you have connected to the rectifier?

Are they connected to the correct terminals? Are the terminals labeled?

Ok it's wired correctly. We need to verify that the engine is charging or not. If it is charging then we have to be sure the gray wire to the tach has continuity and we have voltage and ground to the tach. If we have all of these then the tach is bad.