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Message board > Engines > Tach / Rev counter not working

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Message 1 of 6
Posted by member Sandpiper on Tuesday 3 November 2015

I have a Hardy Pilot SE with a Volvo Penta TAMD22 Diesel engine. Recently my rev counter has stopped working correctly, although about every minute or so the hand swings from 0 to maximum on the dial (no change in engine revs), otherwise it sits at zero. It previously worked fine.
I have checked the ‘W’ output at the alternator and get an ac voltage range from about 7.28 volts to 7.6v as the revs increase – not as much change as I would have expected. At the rear of the rev/tacho gauge there are an awful lot of wires, a plug block of six and 3 wires on a spade connector at the top of the gauge. As yet I haven’t investigated which one, if any, could be the alternator tacho input. Before I delve into pulling things apart:-
1. Does anybody have or know how I can obtain a wiring diagram?
2. Does anybody have any idea what/why there are all these wires on the back of the rev indicator?
3. Any ideas what might be wrong or even what the voltage readings might be realistically?
Thanks - Dave

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Message 2 of 6
Posted by member John on Tuesday 3 November 2015

The tachometer usually measures the alternator a.c. frequency hence rotational speed and I would not expect the voltage to change very much.

Possible reasons - a dirty connector, try unplugging and re plugging those wires, that usually cures mine. One wire will be ground, positive supply, tachograph input, maybe instrument light. Others may not be anything to do with the tachograph.

This wiring diagram link may help (copy & paste)


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Message 3 of 6
Posted by member Peter on Tuesday 3 November 2015

Hi Dave,

Not sure if this will help but seems to be a wiring diagram for a TAMD22. You are measuring an AC voltage with your meter which is actually measuring the average voltage so as the frequency increases (with higher RPM) so the average voltage will rise slightly. (the wave form gets closer together as frequency rises so the average voltage will rise a bit). Your meter may have the facility to measure frequency and that's what you need to see changing but by the sounds of it that is happening.



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Message 4 of 6
Posted by member Dave on Sunday 22 November 2015

Thanks for your inputs - it looks like the tacho is at fault. I have the alternator signal registering at the tacho plug, although on the grey/black wire rather than the grey that leaves the alternator - looks like they have been swapped somewhere. It turns out that the engine hours lcd has never worked since we have had the boat, didn't even notice it was there!

The tacho is the original 1997 fitting and was made by VDO but the only replacement is a very very expensive Volvo tacho at £180 plus. I am currently searching for a more reasonably priced replacement for 85mm hole and revs from 4600 to 6000 - I don't think I will be lucky!

Correction to my original input - the plug has 4 pins, two for power input and the signal and return. The 3 wires at the top of the instrument were for the lamp.

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Message 5 of 6
Posted by member John on Sunday 22 November 2015

Are you SURE that there is a good 12v supply to the tacho, that it has a good reliable ground and that the signal from the alternator is connected to the correct pin. The hour meter just measures when the + supply is on. That flicking of the needle could well indicate a bad connection. It is just what I get on mine when the connections are dirty because if the needle can move it does not sound like a complete dud

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Message 6 of 6
Posted by member Anthony Purnell on Monday 16 October 2017

Failing that you can send the tacho off to Speedy cables Ltd which are based in Wales and can be found on internet search.

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