Tachometer Calibration

[ontherocks83]

I have a Universal M25 with a New Balmar 6 series alternator, external regulator, and Balmar serpentine pulley kit. My old tachometer is not working and I am getting signal to it correctly. I purchased a new alternator but there is an adjustment knob that you can change depending on how many poles the alternator has and what your pulley ratio is. Does anyone know how to figure out my pulley ratio and to figure out how many poles my alternator has so I can select the correct setting? Here is a pic of the selection table from the instructions.

 

[Rich Stidger]

Well this chart is confusing to me because I interpret it to have two input variables: pulley ratio and number of generator poles. The switch position is clear IF for the number of poles one of the listed pulley ratios applies. If the actual ratio for a given number of poles is not listed, then I am not sure what to do. Pick the closest?
Anyway, the pulley ratio is easy. Measure the diameter of the engine drive pulley and the diameter of the alternator pulley. Divide the diameter of the engine pulley by the diameter of the alternator pulley. That will be your ratio.
As for the number of poles, that specification should be in your alternator specs.
Perhaps an easier way to determine the best switch setting is to compare your engine tachometer reading with an optical tachometer and pick the switch setting that allows the closest agreement. I would run the engine in neutral about 1/2 WOT to make this adjustment. That was the midpoint of your tachometer will be the closest point calibrated.

 

[Maine Sail]

Questions like these are why you should always buy from a good Balmar dealer! Oh wait, you did.. (wink)

The 6-Series is 12 Pole or 12 pulses per revolution alternator. If you have the AM-306 crank pulley, the OD is 5.65". The AM-106 alternator pulley is a 2.475" OD. To calculate this it would be; 5.65" / 2.475" = 2.28:1

You will need to confirm/measure the crank pulley OD, re-calculate the ratio, set it for 12 pole "C" or "D" then use a digitial photo tachometer to set the fine tuning. Start with either dial position "C" or "D" and see which one gets you closest. The one that gets you closest, most likely "C" is the one you want to fine tune with.

 

[ontherocks83]

I was able to calibrate the tachometer today with a digital photo tachometer that I picked up from harbor freight for $40. I started at 1200 rpm and found the "C" setting to be the closest. I then fine tuned it with the adjustment knob to match. I then upped it to 1600 and then 1800 and upon checking back and forth between the tach and the digital photo tach, I figure I am within about 50 rpms of true on the tach. Figured that was good enough for me. I like having the digital photo tach now, I plan on once or twice a year double checking the accuracy. Great and convenient little tool to have.

 

[d-marko]

I was able to calibrate the tachometer today with a digital photo tachometer that I picked up from harbor freight for $40. I started at 1200 rpm and found the "C" setting to be the closest. I then fine tuned it with the adjustment knob to match. I then upped it to 1600 and then 1800 and upon checking back and forth between the tach and the digital photo tach, I figure I am within about 50 rpms of true on the tach. Figured that was good enough for me. I like having the digital photo tach now, I plan on once or twice a year double checking the accuracy. Great and convenient little tool to have.

How do you measure RPM's with the digital tach tool? Do you just put tape on the belt and the tool displays the rpm? Is there any conversion/calculation involved?

 

[ontherocks83]

you put the tape on the main crank pulley and it just counts how many times it goes by.