tackometer check

[tblshur]

does anyone know if you can use a frequency meter to check how accurate your tach is. thanks for your replys:suspicion:

 

[UncleWillie]

Re: Tachometer Check

Re: Tachometer Check

In theory it should work just fine.

The signal to the Tach is just a 12 volt square wave taken from the ignition of the engine.
Depending on the number of cylinders you have, expect to see 1 to 20 pulses per revolution.
Some OB engines send the pulses from the alternator instead of the ignition/points/computer.

The back of the Tach may have a setting dial on it that sets the number of pulses it expects to see per revolution.

Don't forget to convert RPMs to RPSs.
A 4 cylinder, 4 stroke engine will fire 2 cylinders per revolution
At 900 RPM that would produce 1800 Pulses/Minute, or 30 Pulses/Second.
The meter would display 30Hz at 900 RPM in this case.
Your conversion factor will vary.

 

[gm280]

Re: tackometer check

Yes UncleWillie got it correct. Lots of quality DVM meters have frequency options. I've used them before to display pulsing outputs in electronic circuits and verified them with an O'scope. The meter was amazingly accurate too... So it would seem to work in place of the usual tachometer or to verify a tachometer as well!

 

[tblshur]

Re: tackometer check

thank you UncleWillie and gm280 i just got my sons fluke 87-111 it reads house hz . my tach on boat is suspect it is a j140tlcte the tach gets signal from stator , its 4cyl 2cycle not sure what the multiplyer would be.

 

[UncleWillie]

Re: Tachometer Check

Re: Tachometer Check

... it is a j140tlcte the Tach gets signal from stator , its 4cyl 2cycle not sure what the multiplier would be.

Johnsons, over 70Hp, commonly have 12 pole Stators. 6 pulses per rev.

Unless I miscalculated, Multiply the meter reading by 10 to get the RPMs.

 

[sam am I]

Re: Tachometer Check

Re: Tachometer Check

Johnsons, over 70Hp, commonly have 12 pole Stators. 6 pulses per rev.

Unless I miscalculated, Multiply the meter reading by 10 to get the RPMs.

Spot on U.W.

Using 100 RPM..and 6 Pulses per Rev

6 P/R * 100 R/M (RPM) = 600 P/M(R's cancel) or 10 P/S or a freq of 10Hz(cycle per second).

Knowing the reference is 100 R/M (RPM) and the freq meter now says "10hz", mult by 10 to get the R/M (RPM's)

 

[tblshur]

Re: Tachometer Check

Re: Tachometer Check

U.W., Sam thank you very much for making it so simple for me .(i needed that).that will be such a big help to me when i go to tenn tomorrow to fish.:thumb:

 

[tblshur]

Re: Tachometer Check

Re: Tachometer Check

just got back from tenn 2 days of fishing 1 small large mouth:embarassed: still was great. the test with the freq. meter was great and simple thanks to you guys. my meter read 75 hz and tach was reading 1050rpms so I think tach is off by 300rpms thanks again and good fishing:joyous:

 

[UncleWillie]

Re: Tachometer Check

Re: Tachometer Check

just got back from tenn 2 days of fishing 1 small large mouth:embarassed: still was great. the test with the freq. meter was great and simple thanks to you guys. my meter read 75 hz and tach was reading 1050rpms so I think tach is off by 300rpms thanks again and good fishing:joyous:

That would be correct if our assumption of a 12 pole Stator is correct. Which it likely is.
I am not saying we are wrong. Did it sound more like 750rpm or 1050/1200rpm?
Did you check the setting on the back of the Tachometer?
If it is set for an 8 pole stator then the multiplier would be 15, and 75hz would indicate 1200rpm.
If it is set incorrectly, it will read incorrectly, and that would not be any fault of the gauge.

 

[tblshur]

Re: Tachometer Check

Re: Tachometer Check

sorry it took so long to reply. the reason I wanted to check the tach the motor rpm sounds really slow sometimes dies if I just let it idle so I use throddle lever to set at about 1050 that rpm sounds right for idle to me.i have not looked at the setting on back of tach, will do when weather improves. what started all this was I did a decarb, new plugs and wires. sometime after that the rpms increased a little too much so not knowing how to adjust rpm, I thought you adj. it with the black knob on the throttle cable. since I have read these post I think that's not right. hopefully the service manual will help correct that, thanks again for all your help.stay warm:joyous: