Test tach idea, please provide input

[dave37]

OK, tried taching my 50hp 1985 2 stroke Merc 4 500 with an inductive pickup and a Fluke 88 digital multimeter, couldn't get reliable readings.<br /><br />Now, here's my theory from everything I've read on the forums, count the number of poles in my stator. That is the tach setting number. Now, if I plug my multi meter in to the tach plug on the Mercontrol (front of the unit, 3 connections) between tach signal and ground, set the meter to Khz, take that reading, divide by the number of stator poles, and I will have my rpm.<br /><br />Am I correct??<br /><br />Thanks!

 

[Clams Canino]

Re: Test tach idea, please provide input

Correct. But remember that each pole of the stator is wound opposite - so you get a positive sine wave followed by a negative one. A 12 pole staotr will deliver 6 pos and 6 neg pulses per rev.<br /><br />-W

 

[dave37]

Re: Test tach idea, please provide input

So I should use 6 to divide the Khz if a 12 pole stator? Also, as it is a sine wave, am I going to be able to a digital meter on the Khz scale?<br /><br />Thank you!

 

[Clams Canino]

Re: Test tach idea, please provide input

I'm not sure... but if the reading is 1/2 or double - you outta know that quick enough to be able to compensate.<br /><br />-W

 

[gaugeguy]

Re: Test tach idea, please provide input

(# of poles/2)(RPM) / 60 = freq<br /><br />So RPM = (freq)(60)/(# poles/2)

 

[dave37]

Re: Test tach idea, please provide input

Thanks very much guys, will try it out this weekend.<br /><br />

 

[dave37]

Re: Test tach idea, please provide input

That worked great,using a Fluke 88 digital multimeter on a/c Hz scale, WOT RPM is 4200, way too low. Will check tonight, don't think the throttles are opening all the way--previous owner told me that after he had it tuned last, it didn't go as fast or rev as high as before. Otherwhise engine is running great, so I'll take a peak.<br /><br />Thanks!!