Tach doesn't work after engine swap.

[Head Doctor]

I had a 1992 75 HP Mercury 3 cyl. that I swapped out to a 1998 125 HP Mercury Offshore. Engine starts and run good although I haven't had it out on the water to fully test it. The only problem I have with it is the tach doesn't work now. It did work before the swap. Any help would be greatly appreciated. Thanks.

 

[Barnacle_Bill]

Test the charging system. A bad diode in the rectifier is the most common thing to cause tach problems.

 

[Head Doctor]

I just swapped out a 1992 75 HP 3 cylinder Mercury 2 stroke for a 1998 Mercury 125 offshore. Everything is fine except the tach does not work now. Any help would be greatly appreciated. Thanks

 

[mr 88]

On the back of many tachs there is a screw that you turn to change it from 2-3-4-etc cylinder. Maybe go on line and google the manufacter to see if you can come up with some instructions.

 

[alldodge]

I just swapped out a 1992 75 HP 3 cylinder Mercury 2 stroke for a 1998 Mercury 125 offshore. Everything is fine except the tach does not work now. Any help would be greatly appreciated. Thanks

I would suggest checking your charging circuit.

 

[Head Doctor]

The charging system appears to be ok just shy of 14 volts. Mr 88 may be on to something. The tach has a setting dial on the back. 1 2 3 4 are the settings. It was set on 2. I read somewhere in the past that this is not the number of cylinder setting but instead is the pulses of the stator. I've googled it to try and find the correct setting but have come up empty so far.

 

[Barnacle_Bill]

Both those motors have 12 pole, 6 pulse stators.

 

[Head Doctor]

So that means the tach setting of 2 should work on both engines. Correct?

 

[merc850]

I think it's set to 6p and attached to the Yellow wire from the stator. Are all the other wires connected to the tach and working + and - ?

 

[Head Doctor]

I only have the choice of 1, 2, 3, or 4 on the dial on the back of the tach. Yes, all the wires are connected and 12 volts present between + and -. It is connected to the grey wire at the rectifier/voltage regulator not the yellow. There are 2 yellow wires that run from the rectifier/voltage regulator to the stator under the flywheel.

 

[mr 88]

Try another setting. To the best of my knowledge it will not fry the tach if its on the wrong number. Reversing the positive/negative wires will.

 

[Head Doctor]

After checking the charging system and all wiring, trying all the setting on the back of the tach it still did no work. Finally I disconnected the grey wire from the rectifier/voltage regulator and connected it in with one of the yellow wires and it worked but was reading 1400 at idle which I knew was not correct. I started changing the settings on the back of the tach and the #3 setting brought it down to 700 which is about where it should be. Hopefully this will help someone else that may have the same problem. Thanks to everyone for their input. I really appreciate it.
Sincerely;
The Head Doctor

 

[b.gagnon]

This in not related to your tach....but was the 75hp a 20" shaft? offshore motors tend to be 25" ?

 

[Head Doctor]

Yes, you are correct. The 75 was a 20" shaft and the 125 is a 25". This is my first dealings with an outboard. I've always had IO's or straight inboards, so I didn't do enough homework. Sounds like you have experience with this issue. I have mounted the engine in the lowest holes which raises the engine 3 inches. If that doesn't work I guess I''ll have to drop back and punt. Do you have any suggestions?