Rev Counter sensing from alternator - how does this work?

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Rafiki

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Having just got to grips with the alternator wiring etc I realise that one of the connections is for a Tachometer, which leads me to think I might fit one. However before going out and buying one I would like to understand how they work.

Although I appreciate that the Alternator can send out some sort of pulse or current every time it completes a revolution how does a tacho know how this relates to engine revs? After all the relative sizes of the (flywheel driven) pulley on the engine and the pulley on the Alternator determine the ratios between engine revs and alternator revs, so how does the tacho know that 2 alternator revs (or 2.5 or 3.7) equates to one engine rev?

In other words if I buy one of these bits of kit have I got to very carefully work out the diameter of each pulley, do some maths and calibrate the tacho. Measuring the diameter of each pulley could easily have a cumulative 5- 10% margin of error which would equate to the same in tacho reading.

Is this how they work and am I missing something here?

Regards
 
Onia said:
Having just got to grips with the alternator wiring etc I realise that one of the connections is for a Tachometer, which leads me to think I might fit one. However before going out and buying one I would like to understand how they work.

Although I appreciate that the Alternator can send out some sort of pulse or current every time it completes a revolution how does a tacho know how this relates to engine revs? After all the relative sizes of the (flywheel driven) pulley on the engine and the pulley on the Alternator determine the ratios between engine revs and alternator revs, so how does the tacho know that 2 alternator revs (or 2.5 or 3.7) equates to one engine rev?

In other words if I buy one of these bits of kit have I got to very carefully work out the diameter of each pulley, do some maths and calibrate the tacho. Measuring the diameter of each pulley could easily have a cumulative 5- 10% margin of error which would equate to the same in tacho reading.

Is this how they work and am I missing something here?

Regards
Click to expand...

my alternator had to be set / programmed to suit the engine type from a hand book provided by Nanni
 
The simplest circuits have a rectifier which converts the ac pulses into dc which then charges a capacitor which has a resistor across it. This dc voltage is fed to the meter. The resistor is usually variable and controls the discharge rate of the capacitor. In replacement Faria tachometers from ASAP there is a calibration leaflet which gives the resistor setting for different pulley ratios.
(If the tach was an original specification for the engine eg Volvo, it may not have this adjustment)
 
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sailorman said:
my alternator had to be set / programmed to suit the engine type from a hand book provided by Nanni
Click to expand...
Tachos usually come with simple instructions of how to adjust the reading to your engine, this is determined by the number of poles on your alternator and the diameter of the pulleys that drive it, no maths involved with the Faria I fitted to my boat, just a simple table of usual set ups, if you don't have a take off connection on your alternator, this too is a simple job. a wire needs soldering onto one of the windings inside the body of the alternator. The tacho senses the alternating current before it is rectified and converts this into RPM.
 
Thanks guys, so I think you have confirmed my fears that this wouldn't be a simple connect up and forget it job!

Although the engine side of the calculation should be OK (BUKH 24) the alternator is very much older than the engine - but does have a dedictaed connection for a tacho.

I think I will give it a try some time providing I don't ahve to spend too much on a tacho.

Regards
 
When I fitted my VDO tacho to a Bukh 20, I just calibrated on the basis of tickover revs for that engine being in region of 850-950 and max revs on full throttle with no load being c 3150,(figs supplied by the Bukh guys @ Poole), with idea of calibrating more accurately subsequently by borrowing a light tacho & measuring the actual revs on the flywheel, but to be honest , haven't got around to doing it yet as the info I am currently getting has proved more than sufficient for my needs-you would be pushed to read a difference of less than 100 rpm on my meter anyway!
 
I've fitted two new alternators recently (Citroen DS and Yanmar 1GM10) so I have been playing with alternator manuals. In both of these cases (and for all the other alternators made by whoever did the DS one) the tachometer output has simply been a connection to one phase of the armature. I presume, as others have said, that the tachometer instrument is calibrated to the installation as required.
 
Forget alternator tachos - go for Tinytach

It all seems to be quite complex! I fitted Tinytach in half an hour, a tiny clamp around the fuel delivery pipe from the pump, a dc supply and fit the indicator. Simple!
 
Onia said:
Thanks guys, so I think you have confirmed my fears that this wouldn't be a simple connect up and forget it job!

Although the engine side of the calculation should be OK (BUKH 24) the alternator is very much older than the engine - but does have a dedictaed connection for a tacho.

I think I will give it a try some time providing I don't ahve to spend too much on a tacho.

Regards
Click to expand...

If you fit a VDO tacho they have an adjuster on the rear, connect it let the engine idle and adjust it to 800 read rpm. that will be near-enough.
 
tilmansailing said:
It all seems to be quite complex! I fitted Tinytach in half an hour, a tiny clamp around the fuel delivery pipe from the pump, a dc supply and fit the indicator. Simple!
Click to expand...
Another vote for the TinyTach - simple to fit and does more than just indicate RPM.
 
looking at a friends freeman , the rev counter reads backwards. is that likely to be an easy fix looking at the back of the rev counter?
 
I think Shug has it all wrong re how tacho on alternator works. AC Voltage out for a tacho will depend on charging demands from regulator so should be same fromn idle to full revs.
As I think it works the centre connection of 3 phase star rectifier of alternator will produce 6 pulses per revolution of alternator. These pulses can be converted to a meter reading by using them to trigger a multivibrator circuit which produces a finite pulse length of fixed voltage. This output is rectified and fed to DC meter.
hence for OP yes tacho readout is dependent on pulley sizes and so alternator speed. ol'will
 
Stemar said:
I had a Tiny Tach on Jissel, and wanted them for Jazzcat, but the price of the diesel one's gone mad. I think I paid under £50 a few years ago, but they're closer to £180 now.

OK, I know it's pure Chinese engineering, but any thoughts on these?
2'' 52mm 12V Tachometer Gauge Diesel Motor Engine 0-6000RPM Meter Rev Counter | eBay
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2" is rather small and you don't need 6000rpm for a diesel.
I have a Durite one with a mechanical hour meter like this Durite
You can even get waterproof ones Kus which might be a good idea as my Durite bezel is starting to rust after 20 years on the boat.
 
Stemar said:
I had a Tiny Tach on Jissel, and wanted them for Jazzcat, but the price of the diesel one's gone mad. I think I paid under £50 a few years ago, but they're closer to £180 now.

OK, I know it's pure Chinese engineering, but any thoughts on these?
2'' 52mm 12V Tachometer Gauge Diesel Motor Engine 0-6000RPM Meter Rev Counter | eBay
Click to expand...

I have the Fuel Gauge from similar source matching that ... 2 years fitted in cockpit ... works a treat.

The add on Tacho sensor I linked to earlier ... I have asked seller if it should work with that meter but I would be looking for 0 - 4000 range though ..... together a lot cheaper than many of the items seen around !!

TBH - I usually set my engtine by sound and 'feel' of the boat - aiming for 4.5 - 5kts. If I want to check RPM ... I have a cheap LED 'laser' tacho which is surprisingly accurate ....

s-l200.jpg
 
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With twin engines, it would be nice to be able to match them fairly accurately, but the boat's nearly 40 years old, and has survived without so far, so it's no more than a nice to have. I'm not sure about spending £60 quid on a pair of the ones I referred to, I certainly don't want to spend £120 on the Kus.

I guess I'll carry on without!
 
Stemar said:
With twin engines, it would be nice to be able to match them fairly accurately, but the boat's nearly 40 years old, and has survived without so far, so it's no more than a nice to have. I'm not sure about spending £60 quid on a pair of the ones I referred to, I certainly don't want to spend £120 on the Kus.

I guess I'll carry on without!
Click to expand...

Less than 20 quid on the handheld is not so bad .......

Your twin installation ... imagine some of my models with 4 engines ..... all done via 'ear' ..... here's my Lanc with multiple motors and sound modules ...

 
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