Tach drops off but no change in motor rpm's

[willxuout2]

Good morning to everyone!<br /><br />I've got another question for you guys. Last time out the tach needle dropped about 2000 rpm while I was cruising about 30mph (4800 rpm)...there was obviously no drop in the motor and the volt meter on the dash showed that the charging system was charging about 14-15 volts.<br /><br />Anyway, the tach would drop to 0, then jump up to 1500-2000, back to zero...then about five minutes later the tach revived itself and showed the normal rpm. Then volt meter stayed steady the whole time.<br /><br />Does anyone have any ideas?

 

[Mikedg]

Re: Tach drops off but no change in motor rpm's

Check for a loose ground on the tach.

 

[willxuout2]

Re: Tach drops off but no change in motor rpm's

Thanks, I'm going to check the charging voltage at lunch today.

 

[willxuout2]

Re: Tach drops off but no change in motor rpm's

Hey, Looked around for a loose ground and couldn't find one...the voltage checked out as follows<br /><br />everything off - 12.3 volts<br /><br />engine running at 1600 rpm - 14.2 volts<br /><br />I'll have to check the voltage when I'm actually having the tach problem.<br /><br />Will the boat run if the rectifier goes bad?

 

[mannyq]

Re: Tach drops off but no change in motor rpm's

I have the same problem. My tach will drop to 0 when RPM's reach between 1500 and 2000. Charging circuit works and also indicates 14-15 volts even after tach drops off. If I test using directions in Clymer manual, everything appears to work properly. Tried on a second Tach with same results. 2000 J115 Ocean Pro

 

[Solittle]

Re: Tach drops off but no change in motor rpm's

Could you beg or borrow another tach that you know is good & swap? That would tell you if it is the tach or wiring.

 

[VistaMan]

Re: Tach drops off but no change in motor rpm's

Two possible causes:<br /><br />1) There is a problem with the rectifier/regulator. I had this on my 120 looper it would drop off between 1500 and 3000 RPM. Turned out to be a bad module.<br /><br />2) The Teleflex site mentions that with some Johnson motors you would need to put a 1/2 Watt 2000 ohm resistor accross it.