REV Counter

[jonlaw]

My rev counter for my volvo 2003 has just started reading around 1000 rpm, no matter what the revs.
Where does it take a reading from ?
What should I do to investigate ?
Thanks.

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[redhot]

My rev counter takes its reading from the alternator, I am not sure about yours, but take a look there and see what you can see.

Mike.

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[Alex_Blackwood]

If your REV Counter takes its reading from the alternator this will be from an ALTERNATING CURRENT WAVEFORM (SINE WAVE). This is picked up on a PHASE connection normally marked W on the alternator. As it is very unlikely that you will have anything on this terminal with the engine stopped it is possible your meter is duff. You should be able to see if you have an output from this terminal by checking with a multimeter on the ac voltage setting, between the terminal and Negative.
You don't say if the reading is constant irrespective of engine stopped/running/speed?
The other commonly used pick up point for for a rev counter is from the FLYWHEEL. This is achieved via a magnetic pickup positioned close to the flywheel teeth and producing a square waveform output. I suspect this may be your situation. The usual problem with these is that they pickup a bit of muck and give strange readings. Have a check around the flywheel area. If you move the pickup make absolutely certain it goes back the correct distance from the flywheel, usually about 2mm. You can turn it in until it hits a tooth and then back it off half a turn, make sure it hits a tooth, not a gap!
Fault finding tip: IF ALL ELSE FAILS READ THE HANDBOOK
It is worth noting that irrespective of where the pickup point is it is the frequency of the waveform that is important as this is directely proportional to speed. The voltage level is immaterial.
I have assumed in all of the above that you have not recently installed anything new and have another instrument cable, or one carrying alternating current running close to the rev. counter cable?

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[Gordonmc]

Alex,

Apologies for jumping in on this, but previous inquiries on the forum failed to help. You may be able to.
My engine originally had a mechanical rev. counter which has been replaced by an electronic one, fed by a wire from the alternator. The alternator does not have a "W" terminal. The cable disappears inside, I assume connected to a field winding.
The multimeter shows a pulse voltage which increases with revs.
My problem is I don't know which terminal on the counter I should connect to. Does the counter need a live feed, other than for the light? I would have thought an earth (neg.) would do.
Any thoughts?

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[andyball]

Think a rev counter wd need "+ & -" as well as a w or phase connection.

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[DeeGee]

This may sound stupid, but you haven't accidentally turned your starter key back to the rest position? The engine will run, but the rev-counter will indeed indicate some fixed value. You may also screw up your alternator. I suppose the same symptoms may happen if your starter switch has dropped a wire off the back.

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[Birdseye]

following on from this post, i want to fit a revcounter to my 2GM but not at the prices charged for spares. Has anyone found a way of using / suitable donor motor car tachos?

Or alternatively, anything from Maplin using a frequency counter and, say, an led and a flywheel mark?

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[Alex_Blackwood]

F.A.O. Gordonmc.
Gordon,
Hope this is of some help to you and anyone else!
Yes you will need a 12volt feed to the electronic counter(+ & -). As andyball has said. You need some thing to power the electronics as well as the lighting in the beast. The pulsing voltage you are getting is The ac pulse from the alternator output before rectification to dc. This will be a maximum of around 12 volts but the frequency of the pulse will increase with the speed of the alternator. The electronics in the counter convert this frequency to a voltage directly proportional in value to the frequency. Therefore the the meter on the rev counter is in fact a volt meter.
At least that's how the ones I have come across work!
Without seeing your set up it is difficult to say exactly which terminals you should use. I can hazard a guess that you will have a + &- for the 12volt dc and possibly a symbol like an S lying on it's side, that will be the pulse input from the alternator, the symbol represents a sine wave (ac).
I hope the above is of some use. If not I would consider taking the electronic bit to the local marine electrcian and seek advice.
Good luck, let me know the outcome.

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[andyball]

Frequency counter way works easily enough, but why bother at <A target="_blank" HREF=http://www.netcomuk.co.uk/~marvelec/clearance.html>these</A> prices ?. If they've run out there's a guy does some boat jumbles selling cheap new vdo guages-my pal's bought some & is v.happy.

If you're stuck,pm me & when he's back from cruising (2 weeks) I'll try to find a name/number for the supplier.

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[paulrossall]

Yes I used a VDO tacho connected to a "W" terminal which I had put on my altenator. I have sent you a PM.

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[Alex_Blackwood]

Consider my wrist slapped! Didn't mention one obvious thing yesterday. Have you got a ZERO ADJUST on your system, could be it has drifted off and needs reseting. Be careful to check readings are correct over the RPM range if you do adjust.

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