Rev counter

[Foss62]

Having a problem with my rev counter,was working ok but now goes up to 4000 rpm when the engine starts and stays there until i turn it off.

any ideas

[Tracy D'arth]

10 minutes ago, Foss62 said:

Having a problem with my rev counter,was working ok but now goes up to 4000 rpm when the engine starts and stays there until i turn it off.

any ideas

Make? Model? Engine? Make  Model?   Control panel Make?

Go on, give us a clue, the crystal balls are cloudy today.

[Foss62]

barrus shire engine and control panal,think the alternator is a mitzubishi

[Tracy D'arth]

The alternator "W" connection feeds the AC signal to the tacho.    It could be an alternator or wiring fault rather than the tacho itself.  Check the connections on the back of the alternator for a start.

Is the alternator charging the batteries properly as before the fault appeared?

 Is this the engine with 2 alternators? If so it is the smaller lower engine battery alternator that feeds the tacho.

There will be others with more experience of this on here soon.

[Tony Brooks]

8 minutes ago, Tracy D'arth said:

The alternator "W" connection feeds the AC signal to the tacho.    It could be an alternator or wiring fault rather than the tacho itself.  Check the connections on the back of the alternator for a start.

Is the alternator charging the batteries properly as before the fault appeared?

 Is this the engine with 2 alternators? If so it is the smaller lower engine battery alternator that feeds the tacho.

There will be others with more experience of this on here soon.

 

Not me. The given symptoms imply far too many pulses being delivered to the alternator and even with a loose connection I can't see that as being likely. If I was forced to guess I would suspect the taco electronics.

 

If the OP has a meter with a Hz (frequency) scale he could put that between the W terminal and any negative and gradually rev the engine. The frequency should rise and fall in time with the revs. If it jumps high the moment the alternator energizes then that would suggest an alternator fault. All theory, no practical experience.

 

PS.the AC volts  scale may serve if the OP has no Hz scale.

Edited May 23, 2022 by Tony Brooks

[Grassman]

A slightly different issue but how do I check the accuracy of the rev counter? My BMC 1.8 reads around 1800 rpm when cruising at around 3mph on a average narrow canal and about 2000 at 4mph, which is much higher than other boats I've driven in the past. But the engine pitch sounds about the same so I don't feel the engine is working any harder. Tickover is about 7 - 8 rpm if that is relevant.

 

It's been like this for the 9 years I've had the boat and now 4,000 engine hours later I don't worry about it any more like I used to, but I'd be intrigued to know why.

[Tony Brooks]

14 minutes ago, Grassman said:

A slightly different issue but how do I check the accuracy of the rev counter? My BMC 1.8 reads around 1800 rpm when cruising at around 3mph on a average narrow canal and about 2000 at 4mph, which is much higher than other boats I've driven in the past. But the engine pitch sounds about the same so I don't feel the engine is working any harder. Tickover is about 7 - 8 rpm if that is relevant.

 

It's been like this for the 9 years I've had the boat and now 4,000 engine hours later I don't worry about it any more like I used to, but I'd be intrigued to know why.

 

You use some type of tachometer on the bottom engine pulley.  It could be mechanical or stroboscopic. Somewhere in the forums there are instructions on how to use a cycle speedo for this duty. I suspect most smart phones now have access to a strobe rev counter app.

 

Most electric rev counters have a means of adjustment, so it is a question of looking in the manual.

[Grassman]

Thanks Tony, I will take a look.

[Jen-in-Wellies]

I've used an audio spectrum analyser app on a phone to carry out a boat tacho calibration. When you put the microphone near a running engine it gives a number of peaks, one of which should correspond to the engine speed. Divide frequency in Hz by 60 to give rpm. There will be harmonic peaks showing up too at double, quadruple the frequency. At low rpm, only the double frequency harmonic may appear. Been a few years since I did this, so details may be hazy. If when you do the sums, the rpm is obviously wrong, then halve, or double it and that is probably the right number.

Jen

[Scholar Gypsy]

23 hours ago, Jen-in-Wellies said:

I've used an audio spectrum analyser app on a phone to carry out a boat tacho calibration. When you put the microphone near a running engine it gives a number of peaks, one of which should correspond to the engine speed. Divide frequency in Hz by 60 to give rpm. There will be harmonic peaks showing up too at double, quadruple the frequency. At low rpm, only the double frequency harmonic may appear. Been a few years since I did this, so details may be hazy. If when you do the sums, the rpm is obviously wrong, then halve, or double it and that is probably the right number.

Jen

There's a free app call "G string" (no not that sort of a G string) that does this. It does work, my tacho hasn't worked for over 20 years now so I do it all by listening.  For a four cylinder engine 1500 rpm is 3000 bangs a minute (each cylinder goes bang once every two revolutions) which is 50 Hz.  So you divide by 30 not 60 (for a four cylinder four stroke engine). 

[Jen-in-Wellies]

8 minutes ago, Scholar Gypsy said:

There's a free app call "G string" (no not that sort of a G string) that does this. It does work, my tacho hasn't worked for over 20 years now so I do it all by listening.  For a four cylinder engine 1500 rpm is 3000 bangs a minute (each cylinder goes bang once every two revolutions) which is 50 Hz.  So you divide by 30 not 60 (for a four cylinder four stroke engine). 

Yes. Thirty, not sixty. It was a few years ago I did this! At some point at low engine speeds, you fall off the lower end of a mobile microphones frequency response, especially a cheap phone like mine, so you end up seeing the lowest peak actually being the second harmonic.

Edited May 26, 2022 by Jen-in-Wellies

[Opener]

Another thread hijack coming up. Another Barrus Shire - standard setup as far as I'm aware. I don't really need an answer - this is just an anecdote. 

My tacho is generally OK but on occasion - revving or changing speed - it will indicate silly number eg 15mph when I can obviously hear I'm in tickover. A quick tap on the glass usually restores a more sensible reading.

Ain't boating fun!

[Foss62]

I’ve just tried the wire  back to the alternator and that’s fine,I’ve fixed a small one to the wire and that one works so maybe it’s the Rev counter itself playing up.just one of those small jiggly jobs I’d like to sort whilst we’re out and about

[Alan de Enfield]

1 hour ago, Opener said:

My tacho is generally OK but on occasion - revving or changing speed - it will indicate silly number eg 15mph

 

Your tacho registers speed in mph rather than revs ?

[magnetman]

All these gauges they attach to engines don't really do anything. 

 

At the end of the day the eyes, ears and nose can work out what is happening with a boat and the engine installed therein. 

 

If boat is moving then you are going along. Use eyes for this. 

 

Engine running? Use ears. 

 

Smells a bit like burning then could overheating. Nose. 

 

Back in the day certain engine makers were clever enough to paint parts of the engine with a type of paint which would start to smell at the point where the engine was beginning to exceed it's designed operating temperature. 

 

I wonder if there was an app for it. 

 

 

 

 

 

[Opener]

5 hours ago, Alan de Enfield said:

 

Your tacho registers speed in mph rather than revs ?

Yeah, yeah, yeah - I knew that would be questioned as soon as I posted it. Tacho indicates an engine speed that would be somewhere near 15mpm...... 🤪

[blackrose]

5 hours ago, magnetman said:

Back in the day certain engine makers were clever enough to paint parts of the engine with a type of paint which would start to smell at the point where the engine was beginning to exceed it's designed operating temperature. 

 

Then someone else was clever enough to design a temperature gauge. Smelly paint was just an earlier development on that journey. 

 

You say that gauges don't do anything but I think I'd rather have a gauge tell me that the coolant in my engine was starting to overheat, than waiting until I actually smelled the coolant overheating! It happened to me on the tidal Thames and fortunately thanks to the gauge and alarm I became aware early enough to do something about it. 

[Jen-in-Wellies]

You don't need an oil pressure gauge, or warning light either. Just listen out for the engine sounding like a bag of spanners, then stopping and you'll know it had (note past tense) an oil pressure problem.