70hp Yamaha tach inaccurate

[Profittakr]

I have a '88 70hp Yamaha and the tach is inaccurate. I suspect the rectifier but need expert advise to know for sure.
The tach functions but registers about 2700 rpm at WOT (motor is reasonable wide open based on speed and sound) and registers about 1200 at idle. So, seems to read about 1/2 the correct rpm at WOT and about 2x at idle...give or take.
Battery seems to be charging ok.
I tested the rectifier per the manual and nothing seemed to be as it should. As far as I was concerned, every reading I took was not correct except for one (not sure which).
My questions are:
1) could a rectifier that is bad like this continue to charge the battery acceptably?
2) could this be the cause of the tachometer problems? The poles are correctly set at 6.
3) I read about a 'full wave rectifier' from radio shack that the guys in the Mercury forum have used as a replacement for their motors.
(25A, 50V Full-Wave Bridge Rectifiers Model: 276-1185 | Catalog #: 276-1185 ) The price is under $4 compared to $55 to $75 for a yamaha rectifier. Has anyone tried this on a Yamaha and will it work? Will it cause damage to anything else if it does not?
4) any other suggestions. Times are tight and replacing things without a plan is not in the budget.

 

[robert graham]

Re: 70hp Yamaha tach inaccurate

try moving your pole setting screw one way and then back the other to see if it will reset it to the correct reading. My tach will act wierd sometimes and they are very sensitive to moisture, good wire connections, etc. Just move the wires on the back a little. Also these tachs are supposed to be waterproof but mine gets condensation in it during rainy weather so how can it be waterproof? Good Luck!

 

[Profittakr]

Re: 70hp Yamaha tach inaccurate

I have basically tried that. I took the whole unit apart because the pole selection dial on the back of the unit was just 'spinning' on the actual selector knob inside the unit and it was not changing positions. I have changed the pole knob and back to 6 poles several times and there is no difference. I also tried 2 new 'out of the box' tachs and have the same issue. Never any sign of water or condensation inside the unit either.
Regarding wire connections; what/where can I tap into the engine to run a test wire for the tach signal direct to the tach to be sure that the wire is not the problem?

 

[Profittakr]

Re: 70hp Yamaha tach inaccurate - Radio Shack Rectifier?

Re: 70hp Yamaha tach inaccurate - Radio Shack Rectifier?

Any other suggestions here? Wondering if I can use the Radio Shack rectifier or other generic rectifier that are under $4 rather than the aftermarket or OEM outboard products for 1/15th the cost.