1995 90hp mercury ob (speedometer, tachometer problem)

[kevinpops]

I have a 1995 mercury 90 hp outboard on my bowrider. It runs well enough.
Last year the speedometer worked just fine but since I have had the boat 2 years now, the tacometer has not worked. I have put a lot of money in repairs on the boat and since these are not necessitites I want to try to fix them myself.
Can anyone walk me though where to begin with either or both problems.
Thank you in advance.
Kevin

 

[Silvertip]

Re: 1995 90hp mercury ob (speedometer, tachometer problem)

Let's do the speedometer first. This system has three parts 1) the speedometer head 2) the plastic tube that runs to the back of the boat, and 3) a pitot (pronounced "pit-oh"). The pitot is either built into the motor, or its the white plastic deal that hangs down at the back of the boat. The pitot has a small hole in it and it faces forward. As the boat moves through the water, water pressure acting on the air in the tube, compresses that air making the speedo needle move. No water actually flows through the tube. So - 99 time out of 100 the speedo head is not at fault. The tube is plugged, kinked, or disconnected at the back. But -- 99 times out of 100 the tiny hole in the pitot is plugged. Clean it and you are back in business. Just follow the tube back to the pitot or the engine.

As for the tach, they require +12V when the key is on, ground, and a sense or pulse line from the engine. A fourth wire is for the internal light if it has one. Check those connections. If you don't have +12V, ground, and the sense or pulse line the tach won't work. Nor will the tach work if the voltage regulator/rectifier is fried. If your charging system is working, then that's not the issue.