Tachometer Question

[Hartnerk]

Happy holidays all,

I have a tachometer question that I have not been able to solve myself and wanted to see if anybody had any experience with.

I’m working on a 31 Hunter(2000) 310 with a Yanmar 2GM20F engine. I decided to replace the broken tach with veethree gasoline engine tach. Now that I am looking at the Old and the new tachometer I see on the back different calibration settings. The new one uses cycles and pulses and the old uses teeth to calibrate. After a lot of trouble shooting I know am getting a variable AC voltage between 5 V and 12 V at the helm when throttling my engine. However, the new tachometer’s needle stays at zero . Did I buy the wrong tachometer / is there anyway to convert from pulses to teeth? Thanks for any help and if there’s anything I can clarify please let me know.

Kyle

[BobHorn]

Diesel engine tachs either use a mechanical drive from the engine, a pulse pickup from the alternator or a proximity sensor off the flywheel, hence counting teeth. Gas engine tachs generally use a signal from the ignition system although they could also use a signal from the alternator.

[GordMay]

It seems you've now got an Alternator Type Tach', whereas the old one was a Magnetic Pickup type.

Tachometers are available for magnetic sensor, alternator, or ignition signal(s).
For magnetic proximity sensor (pickup) driven models, the pulses are obtained from the ring gear of an engine flywheel (having from 50 to 304 teeth). Diagram at bottom.


☞ Alternator driven models function from pulses, with 4, 8, 12, 14 or 16 poles on the rotor (3 - 40 PPR), generated by the engine driven alternator that charges the battery. The alternator must have a AC tap terminal for the tachometer, usually marked “AC”, “R” or “W”.

Early original equipment Yanmars were equipped with magnetic flywheel pickup sensor tachometers.
For Yanmar models 2GM, 3GM (with a 97 tooth flywheel), the range selector must be set to the 90-120T tooth setting, and will be reasonably close to correct (to within about 50 - 100 RPM).


To use your new Alternator type Tach', you'll probably have to add an AC Tach Sense wire (to tach). Picture shows isolated ground, while you probably have case ground /w only 1 B- (no matter).


See video ➥ https :// www . Youtube.com/watch?v=kseFhAaVn4o

https://www.youtube.com/watch?v=kseFhAaVn4o

[Hartnerk]

Great this is starting me in the right direction thank you.

Is it possible to convert if I bought an alternator driven tach and have the magnetic proximity sensor?

The weird thing is I am pretty sure the “tap” is coming off the alternator. The “tap” I see does not have a label on but am I right to assume it could be an ac tap based on the variable ac voltage between 5-12vac it puts off ?

The below is the new tach I bought:
https://www.amazon.com/gp/aw/d/B004D...b_b_prod_image

[Hartnerk]

Yes looks like you are correct on all accounts thank you so much for the help.


-Kyle