Mercury outboard tachometer and speedometer not working?

Question

my boat has a 1995 mercury 90HP outboard. The speedometer has always worked until this year and the tachometer has never worked. Since they are not necessities I am not going to fix them unless i can fix them on my own. Can anyone get me going in the right direction on what to check or where to start. Thank you in advance.

Answer 1

The speedometer has a flexible tube on the back that is connected to the motor on the other end.  It is either cut, perforated or disconnected.  The tach problem can be caused by a bad sending unit on the motor, bad wiring or connections, or the tach itself could be shot.

Answer 2

Tach not working is a symptom of blown rectifier - but if the battery is charging then you know that's not the problem.  Try this:  use a jumper wire to apply 12V directly from the battery to the IGN terminal, the needle should go to zero.  If it doesn't, then use another jumper to connect battery (-) to GND.  If it still doesn't zero out, the tach is bad.  If you get it to zero connecting directly like this, you have a loose wire somewhere probably under the helm.

IGN is typically connected to a purple wire, GND to black, SND to gray (sometimes yellow), and LT to blue.

If you can get the tach to zero out with the key "on", but it still doesn't register RPMs with the engine running, measure the voltage on the wire connected to SND with the engine running, you need at least 4 volts AC.

Good voltage on the SND wire but no RPM's, you have a bad tach.  Bad voltage on SND, it's a wiring fault, or else the rectifier is bad (but as mentioned, a bad rectifier won't charge the battery).

I consider my tachometer a necessity - it's an important troubleshooting tool, and often the first clue that the motor needs attention.

Answer 3

i was going to tell you but jtexas hit it right on the money  listen to him  hes got you covered

but if you cant fix the speedometer dont get a gps there more accurate anyway

Answer 4

There is 2 tiny slots in the lower unit of the motor facing forward just below the ventilation plate.      They look like this ," - - " Clean those and your speedometer will work. They get clogged with mud and c**p.