Measuring Diesel RPM??

[pwilletts]

How does the RPM gauge measure RPM on a diesel? I know it is connected to the Voltage Reg...but how?

[four winds]

Two common methods are:

Senses AC pulses in the alternator, before rectification.

Sensor near flywheel detects magnet field from a number of magnets imbedded in the edge of the flywheel.

[Cavalier]

Originally Posted by pwilletts

How does the RPM gauge measure RPM on a diesel? I know it is connected to the Voltage Reg...but how?

Some diesels have a gauge mounted on the flywheel housing that takes a reading (pulled) off magnets in the rim of the flywheel (hall effect sensors or simple coils). However, the majority of modern diesels have a pair of field pulse outputs from the alternator which feeds the instrument - as the alternator rpm runs faster, so does the frequency of output pulses from the coils on the stator. Note that the RPM pulse output wires are different to the instrument power outputs. If you look at the output harness from your alternator you'll notice a number of conductors...







Sent from mTalk

[StuM]

Alternator, not regulator.

Engine speed is determined by the frequency from the alternator tachometer output. This is from a special connection called an "AC tap" which is a connection to one of the stator's coil output, before the rectifier.

The signal is a half-wave rectified output which has an amplitude about one-half the DC output voltage.

The tachometer has to be calibrated for the alternator/engine pulley diameters and the number of poles in the alternator.

[Emmalina]

There is also the fluid movement sensor employed by Tiny Tach that gives a more precise reading !

[model 10]

The Perkins 107 and it's offspring have a tach generator.

[Argyle38]

Originally Posted by Guy

The Perkins 107 and it's offspring have a tach generator.

What is this? I have a 108 and have never heard of it.

[StuM]

A tachometer generator is a small generator which is attached in line with the shaft. Its output voltage varies with speed. The readout is essentially a voltmeter.

[a64pilot]

Originally Posted by StuM

A tachometer generator is a small generator which is attached in line with the shaft. Its output voltage varies with speed. The readout is essentially a voltmeter.

You may be right on a Perkins, however an aircraft tach gen is an AC generator and the tachometer is driven based on frequency, reason is voltage is just too inaccurate, a dirty or imperfect connection will vary reading greatly, but frequency will stay stable.
I would hope a Marine tach would be the same.

[Bill O]

The tachometer has to be calibrated for the alternator/engine pulley diameters and the number of poles in the alternator.[/QUOTE]


With the alternator being the source for most engine panel tach gauges, will agree w/StuM's statement above. Once the original alternator is changed/swapped out, all bets are off on the accuracy of the rpm reading.

As a side question, how would one accurately calibrate/measure the tach error after the alternator is changed?

Bill O.

[Privleoplag]

Originally Posted by Bill O

The tachometer has to be calibrated for the alternator/engine pulley diameters and the number of poles in the alternator.

......
As a side question, how would one accurately calibrate/measure the tach error after the alternator is changed?

Bill O.[/QUOTE]

with one of these hand held tachometers, you just hold them against either the crankshaft or a pulley shaft:
https://static.summitracing.com/glob...82682_w_ml.jpg

[Bill O]

Originally Posted by Privleoplag

......
As a side question, how would one accurately calibrate/measure the tach error after the alternator is changed?

Bill O.

with one of these hand held tachometers, you just hold them against either the crankshaft or a pulley shaft:
https://static.summitracing.com/glob...82682_w_ml.jpg[/QUOTE]

That could work but per the manufacturer for some of these it needs a light strobe attached to the flywheel "Fabrication is required. Modifications to balancer or flywheel are required".
Seems like a lot of work for an aux. sailboat motor if all you want to know is the offset from the tach reading and real rpms. Maybe good for a powerboat attempting to synchronize multiple motors, but then again they probably have factory stock alternators, so the tach gauges are somewhat accurate.

Bill O.

Bill O.

[Snore]

There is an app for that!

Seriously, there is an app that converts the light on you iPhone to a strobe tachometer.

[Bill O]

With a quick look I found an answer/product that would be easy cheap/quick to calibrate the rpm on a diesel motor.

A Digital Photo Laser Tachometer Non Contact Tach RPM Meter

Something like this https://www.amazon.com/Digital-Photo.../dp/B001N4QY66.

It wasn't a burning question for me but thanks CF folks for making me think about it.

Cheers
Bill O.

[model 10]

A fluorescent trouble light works great as a strobe. Hold it in front of the pulley at an rpm that matches the frequency of your power grid. 1800 rpm for 60 cycle as an example.