erratic tach readings

[james roach]

The tach on my 1987 200 HP Johnson starts acting up after the motor warms up. It reads fine for the first 5 minutes and then starts dancing all over the place. After a few more minutes, it quits all together. The thing that is confusing me he is the volt reading. It is still charging. I have cleaned the grounds at the terminal block and at the rectifier itself. I have also cleaned the connections behind the tach. Is my rectifier fried? I performed the tests in the manual but I was not satisfied. Any suggestions?

 

[rickdb1boat]

Re: erratic tach readings

I would bet on the rectifier/regulator, as the tach will not heat up, but the recifier surely will somewhat after 5 minutes. Borrow a Tach and verify operation. This will narrow it down..

 

[james roach]

Re: erratic tach readings

anyone else?

 

[angus63]

Re: erratic tach readings

Swap tachs, if same thing happens, retest rectifier when warm. If the rectifier is water cooled, you may need to vent the block to get water up to the rectifier mating surface. I remember a thread where a guy idled the motor and heated the rectifier with a heat gun to dicover it breaking down at elevated temps.<br />Good Luck

 

[james roach]

Re: erratic tach readings

Thanks guys. This rectifier is 17 years old so I shouldn't mind replacing it. How much should I expect to pay for 1.