Measuring the RPM of an Onan P216, P218 or P220 using a digital Volt Ohm Meter with a Frequency Mode

[Handy Don 10,597]

This method uses the frequency of the AC electricity generated by the stator coils in the alternator to determine the engine's RPMs.
In these twin cylinder Onan's, there are eighteen (18) stator coils, nine (9) of them connected to each of the two inputs to the voltage regulator. That means that one revolution of the flywheel's embedded magnet passing the stator coils generates nine (9) current pulses at each voltage regulator terminal.
The frequency setting on a digital Volt Ohm Meter (VOM) counts the number of current pulses in one second. If we measure the frequency of current pulses at one of the voltage regulator terminals we can apply a simple formula to convert the measured frequency into RPM.

 

The formula for an Onan is:
     RPMs = pulses per second ÷ 9 pulses per revolution x 60 seconds per minute

 

Frequency    RPM    
       9                 60      One revolution per second = sixty per minute
   150             1000      A good minimum idle speed* per the Onan Service Manual
   165             1100      A good governed idle speed* per the Onan Service Manual that:

                                    a) overcomes hydro drag, and

                                    b) keeps a good amount of cooling air flowing

                                    c) doesn't charge the battery!
   300             2000    
   375             2500    
   450             3000    
   525             3500      Approaching max speed for the Onan
   540             3600      Normal max operating speed* for the Onan

 

To make the measurement:

 1. Set your VOM to measure frequency in the 0-1000 hz range for 0-50 VAC (see VOM manual, if you need help with this**)

 2. Connect the black or COM probe to a solid engine ground

 3. Set the engine throttle to the speed you want to measure

 4. Carefully touch the red or "V Ω" probe to one of the outer terminals on the engine's voltage regulator (see photo)

                    
        Be extremely careful to touch ONLY ONE terminal on the regulator--

        a short circuit caused by touching two terminals at once or grounding the terminal

        can cause serious electrical system damage. 

 5. Read the frequency on your meter's display and convert it to RPM using the table above

Frequency will likely fluctuate somewhat but on a smooth-running engine it won't be by more than 5-10 hz. If your VOM has a "HOLD" button, you can use it to "freeze" the reading at a point in time.

 

This technique will likely work on many engines used in WHs having an alternator and three-terminal voltage regulator. You must know the number of coils in the alternator stator (Google an image and count them!) and can use this more general formula:

          RPM = VOM frequency ÷ (number of alternator coils ÷ 2) x 60

A K321 stator, for example, also has 18 coils and a three-terminal voltage regulator so it uses the Onan table above.

 

*Refer to the Onan Service Manual for information on adjusting the carburetor mixture and idle limit screw as well as the linkages for the governor and throttle control to manage idle and maximum engine speeds.

**The frequency mode is normally labeled with "Hz" or "Hz Duty" or "Hz %" or "Freq".  Typically, a frequency-capable meter works where the voltage in the circuit is within the maximum AC voltage specification of the meter. Small engine alternators like the Onan typically can generate un-regulated voltages up to 50 volts. Again, check your meter's manual to confirm its suitability and how to use the frequency function.

 

(Thank you @lynnmor for checking my work and adding suggestions before I posted it!)

[WHX?? 46,772]

Thank you Don! I just happen to have one that measures frequency.

[squonk 38,925]

2 hours ago, WHX24 said:

Thank you Don! I just happen to have one that measures frequency.

Does it measure the frequency of how many times it is dropped? 

[Handy Don 10,597]

11 minutes ago, squonk said:

Does it measure the frequency of how many times it is dropped? 

I believe a VOM can measure drops.  In my experience, the display will become blank, internal parts will rattle, and that equals 1 drop. 

[Tuneup 1,416]

Great tip, Don. Thanks! When I get that 16 re-ringed, I'll apply your method. I'm kicking myself that I haven't thought of it. Off the tech bench for 28 years - could be a contributor.

[Charbs152 253]

i use a $20 tach from ebay

[Horse Newbie 6,859]

On 11/18/2020 at 11:42 AM, Charbs152 said:

i use a $20 tach from ebay

Do you have a link or source for the one you have ?

[R Scheer 499]

I've got the "Tiny Tach" mounted, seems to work well.  It has a lithium battery so the only connection is 1 wire wrapped around the spark plug wire.

[Charbs152 253]

19 hours ago, Horse Newbie said:

Do you have a link or source for the one you have ?

I put these on all my engines...

 

mowers, motorcycles etc..  they track hours as well... I have the middle and best

 

 cheapest - https://www.ebay.com/itm/Tiny-Tach-LCD-Digital-Hour-Meter-Tachometer-for-Marine-Spark-Mower-Engine-Motor/184009103702?hash=item2ad7cc1956:g:28sAAOSwj4RfIm51

 

Middle   https://ebay.us/1c1dKm  

 

BEST - https://www.ebay.com/itm/Tiny-Tach-TT2B-Standard-Gasoline-Hour-Meter/401588201990?hash=item5d8085aa06:g:hwcAAOSwri5bgGS5

Edited November 24, 2020 by Charbs152

[oliver2-44 8,852]

I bought a Tiny Tach 2 years ago but it died since it had a non replaceable battery

 @Charbs152 Do you know if the Middle and Best ones have replaceable batteries?

[Charbs152 253]

5 hours ago, oliver2-44 said:

I bought a Tiny Tach 2 years ago but it died since it had a non replaceable battery

 @Charbs152 Do you know if the Middle and Best ones have replaceable batteries?

Not sure. I know they make them though if you look

[R Scheer 499]

I believe they are potted, sealed, so the battery is not replacable.  Supposed to have a 5 year lifespan.

[onanparts.com 178]

I have been using a vibratory tach for many years. Measures RPM's and Frequency. Accurate, no batteries, no wires. Treysit Sirometer.

 

http://www.treysit.com/english/

 

Sold under rebranded names, B&S etc.

[Handy Don 10,597]

8 minutes ago, onanparts.com said:

I have been using a vibratory tach for many years. Measures RPM's and Frequency. Accurate, no batteries, no wires. Treysit Sirometer.

 

http://www.treysit.com/english/

 

Sold under rebranded names, B&S etc.

 

 

If I'd had to guess where such a device came from, it would have been Germany! Thanks for the link.