Tacho question.

[ExUserGone]

Below is a piccies of my tacho's, they are smiths 3" dials using magnetic pickups triggered from 4 slots at 90 degrees on the flywheels, one of them has always jumped about a bit between 1500 and 2500 (where I always run).

Does anyone know if they are tweakable to stop this happening?

As the boat is now 90 miles away it's not so easy to experiment.

[quo vadis]

Have you tried cleaning the mag pick ups? Also check the distance from the flywheel, on a CAT you wind the pick up in till it touches and then back 180 degrees, dont know about your set up.

[ExUserGone]

I've tried that before but it always seems to mess about again after a short while, have considered building an optical pickup on the crank pulley.

[mjt]

Dave, have you tried swapping the pickups over to see if one is a bit 'iffy'? and/or swapping the dials to see if one of those is not responding properly to the signal?

[ExUserGone]

Not yet but it is on the cards for this weekend to see if it's the pickup or dial.

[Guest]

Dave, have you tried swapping the pickups over to see if one is a bit 'iffy'? and/or swapping the dials to see if one of those is not responding properly to the signal?

That's would I'd do first, then you instantly narrow down the fault location to the instrument or the wiring.

If it's only flicking at a restricted rev range, my money's on the gauge itself, internally.

I'd then dismantle that carefully, usually just a case of bending the crimps around the bezel and releasing the crews on the back. Then clean and tighten the connections inside. It could be high requency vibration at that rev range, maybe try unfastening the dial from the dash and holding it, and seeing if it still flicks.

[ExUserGone]

Annoyingly it doesn't always do it so I'll have to run it for a while to get a definate clue, probably at least as far as a pub...

[ExUserGone]

Final outcome, took tachos home to calibrate and dropped one breaking the glass bought a pair of pricol tachs from a guy on ebay selling thorneycroft stuff, modified alternators as didn't have tacho outputs and calibrated to engines as close as poss, then put a jumper across signal wires and run on one engine to match the two gauges together, mostly done outside ferry house at surlingham.

Now have two lovely smooth calibrated tachos and engines sound so much closer when revs matched, £100 well spent, me a happy bunny

[Hylander]

Posted also on the Bounty Site.  I will post my saga -    Having carried out all tests to establish what had gone wrong with the Tachometer, it became clear that the Tachometer generator was not giving an output.     On my Bounty the Tachometer generator is at the rear of the engine inboard and is positioned beneath the manifold driven from the camshaft via a speedo type drive through a long shaft to a dog tooth drive into the generator.    I then set about removing the generator, the nuts on the shaft are right hand thread but the main nut on the generator is left hand thread.    Therefore it soon became clear that in the attempt to remove the generator any movement on the nut would send the generator upwards and foul against the drain stud on the water jacket manifold.     The option to remove the manifold was out of the question as I thought this would lead to more difficulties and the removal of the drain stud was impossible as it was seized and I did not wish to do damage to the water manifold.      So upon persistence and a lot of blaspheming I manage to remove the generator but in so doing bent the drive into the speedo type drive.

 

Taking the whole thing home and straightening the drive in the vice I proceeded to establish what was wrong with the generator.  The whole thing is a sealed unit and to strip it down the ring around it had to be cut away.     Once inside I could see that the plastic drive inside it had shattered and therefore was not turning the actual generator.  

 

My only course was to replace the unit.     After extensive investigations and internet searching the only place I could get the unit was from America.  Here came the shock.     They wanted around 250 dollars for a new one.   However I managed to find a different supplier which unit was compatible and made in Czechoslovakia and who wanted around £60 but the delivery from America was to be up to six weeks.   I decided upon this option as I had gone so far down this route that I could not turn back.  

 

www.maxmarineelectronics.com

 

 

Upon its arrival which was in fact 3 weeks, (UPS tracked)   I went back to the boat to attempt to fit it.  What a nightmare!!!    It was impossible to locate both ends of the drive and keep it engaged in the generator because of the lack of space between the drain plug and the flywheel.    Eventually after having the threads cleaned on the shaft by a kindly engineer, I managed to get enough movement to get the generator in and out relatively easily.   However I still could not locate the drives.   My only answer was to raise the drive slightly by putting a small nut over the square speedo end of the drive to hold it up some 8^th of an inch.  I then managed to get enough movement downwards on the generator to locate it.    Finally upon starting for the umpteenth time the taco worked.

 

I am obviously concerned as to how long this fix will last,   hopefully forever.     In hindsight it would have been a much simpler job to have replaced the taco head and wired it to the alternator.  What put me off doing this in the first place was the prohibitive price of the Taco head.    At that time I did not realise the costs involved in my new generator.   One thing is for certain if it goes wrong again the option of a alternator driven tachometer will come to the fore.age of the replacement of a taco.

[bitingmidge]

I'll apologise up front if this post is out of order, but my tacho was (out of order) a couple of years ago, and it simply wasn't practical or for that matter economically feasible to wire in a traditional gauge to the alternator.

 

I ended up buying a TinyTach for less than $100 US including shipping, and it's been a perfect solution for me.

 

On another note, if economy is your aim, I have met some blokes in France who used a cheap cycle computer with the magnetic pickup mounted on the driveshaft, then calibrated to give a rough idea of engine revs.   It only works when the boat is in gear of course!

 

Cheerz,

 

P

[Hylander]

Thanks for the link Peter.