Measuring RPMs with non-contact tachometer

[bsangs]

Alright, my apologies if this is a dumb question. Changed all my belts and impeller today, and put reflective tape on the three wheels of my Yanmar 3GM30F, for use with a non-contact, optical tachometer. Which wheel should I be measuring for the most accurate RPMs? I only ask because each wheel is giving me a different read out.

The alternator wheel is registering much higher than the other two. The lowest wheel seems to be giving me the closest readout to what’s on the analog cockpit tach, so I’m guessing that’s the one. Just wanted to check with the more seasoned crowd.

Also my apologies if I’m using the wrong terminology for the “wheels.”

 

[Loren Beach]

Just wanted to check with the more seasoned crowd.

Yup. That's us!
I checked the tach accuracy on our diesel several years ago. So I bought an inexpensive optical laser tach readout and put the little piece of while tape on the large pully on the front of the engine. On my engine this drives the belt for the water pump and alternator. This is the one on the front of the engine and should be the front of the crank. Having owned only Kubota-based engines, my advice may be marginally helpful, tho.

 

[peaman]

The alternator wheel is registering much higher than the other two. The lowest wheel seems to be giving me the closest readout to what’s on the analog cockpit tach, so I’m guessing that’s the one.

You have it right. The bottom one is the crankshaft and is driving the other two: the water pump above the crankshaft, and the alternator.

 

[Alan Gomes]

Yes, you want to measure the rpm at the crank, not at the alternator. And that will be the lower one.

The Yanmar tach on my 1GM is not especially accurate. It is typically off by about 250 rpm in the middle ranges, less at the low and high end. Make a deviation table at, say, 500 rpm increments for future reference.

 

[bsangs]

Thanks guys. Had no doubt you’d know.

 

[Kenneth K]

It's an interesting question though because most (all ?) tachometers use the alternator as the RPM source.

The only pulley that will turn when the engine is running with no belt is the lowest, largest one connected directly to the crankshaft. With the belt installed, the other two pulleys will turn, but at a faster RPM equal to the crankshaft pulley diameter divided by the smaller pulley's diameter.

It's a selectable switch on the back of the tachometer that adjusts for the "smaller pulley=greater RPM" difference that the alternator sees, but the switches usually only offer a few (versus infinitely variable) settings, so there is some RPM error introduced here.

 

[Alan Gomes]

It's an interesting question though because most (all ?) tachometers use the alternator as the RPM source.

The only pulley that will turn when the engine is running with no belt is the lowest, largest one connected directly to the crankshaft. With the belt installed, the other two pulleys will turn, but at a faster RPM equal to the crankshaft pulley diameter divided by the smaller pulley's diameter.

It's a selectable switch on the back of the tachometer that adjusts for the "smaller pulley=greater RPM" difference that the alternator sees, but the switches usually only offer a few (versus infinitely variable) settings, so there is some RPM error introduced here.

The Yanmars--or at least the GM-series--has an optical sensor that reads the revolutions at the flywheel.

 

[bsangs]

It's a selectable switch on the back of the tachometer that adjusts for the "smaller pulley=greater RPM" difference that the alternator sees, but the switches usually only offer a few (versus infinitely variable) settings, so there is some RPM error introduced here.

This is a pretty basic tach - only $30 or so. Doesn’t have any adjustment switches.

 

[JSM]

This is a pretty basic tach - only $30 or so. Doesn’t have any adjustment switches

I installed a new Sierra tach from West Marine on our boat last year. The tach has a small screw on the back that adjusts with a jewelers screw driver. With the engine idling get a reading with the hand held tach off of the engine then adjust the tach to match that reading.

 

[Alan Gomes]

I installed a new Sierra tach from West Marine on our boat last year. The tach has a small screw on the back that adjusts with a jewelers screw driver. With the engine idling get a reading with the hand held tach off of the engine then adjust the tach to match that reading.

I wonder if there is a bit of confusion concerning which tach we are talking about. If bsangs was referring to the handheld optical tach in post #8, that of course has no switches, since it is just measuring straight rpm of whatever spinning object it's pointed at. On-engine tachs, though, often have such switches.