Rev Counter missreading?

[DaveW]

Hi All, This is my first post, although have been following the topics with interest over the last few months.

My problem lies with the rev counter, apparently over-reading by approx. 500-600 revs at tickover and this continues with power on. The boat, which belongs to a friend, is a Sadler 29 with a Volvo 2020 engine that has the standard instument panel layout.

Neither of us are in any way electrical experts - Must be room for the RYA to develope a Marine Electrics course here. Having noticed on previous posts that there are many 'novice' electricians out there.

I digress. The alternator has been removed and overhauled recently and put back by the owner. Could he have got the wiring on the wrong terminals - the small wires that is, all the lights and alarm etc appear to work O.K. on the instrument panel and engine runs fine.

Could the relay be faulty?
Have cleaned up the earth and all connections to the alternator!

All was O.K. before the alternator was overhauled the owner assures me.

Would appreciate any help.

Thanks, Dave

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[alan]

Hi Dave,

On some instruments there is a small screw on the back of the tacho head for calibration purposes - it depends on the make of tacho. If you can borrow a strobe light from a mechanic, you point the strobe at the engine pulley wheel (where there is a mark for this purpose on most engines) and then adjust the tacho head (instrument) screw to achieve the same reading. You need to check this over a range of revs as the error is likely to be non-linear. Then if the error is non-linear (i.e. different at various engine speeds) you adjust for a media; that is if you have an error of 100 revs at engine speed of 900 and an error of 75 revs at engine speed of 2000, then with the engine at 900 revs make error + 12 or so. Obviously this is much simpler with two people !!

Hope this helps.

Alan.
Nettuno, Italy.





<hr width=100% size=1>Alan Cloke

 

[halcyon]

If it worked before the alternator rebuild, and not after it's 99.9% to do with the rebuild or refit.
It's probable that the rev counter is run of the alternator field diodes, so check on how the connections should be with your local Volvo dealer. It may be the warning light / rev counter wires swopped. If they are ok get the alternator checked out for a faulty rebuild.

Brian

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[sonarbell]

Did it come back with the same pulley?????????

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[Heckler]

the rev counter takes its pulses off the alternator, he has connected it up wrongly.
stu

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[Alex_Blackwood]

If the rev counter is fed from the alternator the rev counter feed is taken before the diodes and is therefore Alternating Current (ac). The warning light and possibly the battery sensing, if fitted, are taken after the diodes and are Direct Current (DC). However the DC will not be "clean" and will have spikes or pulses formed by the peaks of the ac phases. The rev counter is taken from one phase of the alternator and works by counting the peaks of the ac. The DC output is taken from three phases of the alternator and therefore will have more peaks. The frequency is directly proportional to speed. to put it simply if you have a wrong connection 100rpm will be displayed as 300rpm
That is the simple version no guarrantees to accuracy of figures etc. but hope it helps.


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[DaveW]

Thanks Guys,

It seems to me that we have conected up the alternator wrongly.
I assume that trying the wires on different terminals until we get it right will do no harm, is that right?

DaveW

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[Alex_Blackwood]

If you don't have any external battery sensing then you will only have two small wires coming out of the alternator: One Warning Light (WL) and one for the tacho (W). The warning light (WL) wire should be connected to the common(DC) side of the field diodes and the phase (W) wire for the tacho should be connected to one stator winding leads that are connected to the ac side of the field and power diodes.
I would try and identify the wires to be on the safe side you can never be sure that you won't cause damage just by swapping connections.
Sorry can't be more helpful. As in most of these cases on the forum it would be simple if we could actually eyeball the problem.


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[Alex_Blackwood]

Add to my last: If you have had it connected up incorrectly then the damage is probably done! Sorry!

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