90 horse mercury outboard tachometer quit working I've checked connections.

check for fuel leaking in engine area --intake gasket--line--water --or have a damaged water jacket /head unit

many mercury motors use a rubber mounted prop on the shaft which is a sacrificial idea to prevent gear box damage and prop damage in with under water objects
what you suggests indicates that at certain load situations that rubber has failed and allowing the shaft to turn but is not driving the prop as required
It is driving just enough to give you steerage to get home

Is trim motor turning, when switch is engaged? If not, can you hear the trim relays click with switch engauged? Do you have switch on engine as well? Did you check the fuses?

with out information as to size, hp, band color on the cover, serial number or photo , no one will be able to answer that question
you can go google and type in--- identifying mercury outboard models and types and power ranges --
there are sites that I found quite some time ago from mercury that was pretty good and if you have a serial number and horse power , you may get the answer
of course you could always take it in to a service dealer and have it identified

Hi Royscarrow,
This Could Be A Few Reasons Causing The Tachometer Not To Work.
The Voltage Regulator Mounted In The Engine Compartment Has A Grey Wire Coming Out Of It Together With Two Yellows,Two Red's and One Black. The Grey Wire Is The Sender For The Tachometer.
Check That It Is Connected.
Is The Battery Getting Charged When The engine Is Running?
Is The Potting In The Regulator Discolored Or Burnt Looking ?
Check The Plug For The Gauge Loom At The Control Box Is In Correctly.
Let Me Know What You Find.

look here
http://www.iboats.com/basics/serial_number_year.html

I have a problem with my 2 stroke 20hp mercury 1989,when I run 10-90% throttle it's works fine but when in 100% its only runs in 90% and doesn't sound normal like it's not burning right.

If it's a machanical tech check for a broen line from the engine. If electric check for + voltage to the tech if ok tech if bad