rev counter

[westhinder]

We had our alternator replaced over the winter, and we now find that the rev counter does not give any indication when we start the engine. Only after a good burst of throttle does it spring to life, and only from that moment does the volt meter show the batteries are charging.
Any suggestions how we can solve this?

<hr width=100% size=1>Johan

 

[halcyon]

Alternator not giving output at low rpm, check feed from warning light, get the alternator supplier to run it up. Check the output to rpm, and check pulley is sutable to suit alternator.


Brian

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[toad]

You need to change the regulater or pully size i would think.

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[andyball]

ours does that....possibly nothing "wrong" with yours, just a higher "cut-in" speed for the new alternator.

eg. a lucas marine 40Amp 24V cuts in at 1800rpm, same case alternator at 12V is 1200, others vary from 700 up.

If you know the make/model of old/new alternators ,the supplier may be able to compare cut-in speeds.

The rev counter's driven from an alternator winding...so no output = no rev counter.



I got a slight improvement by increasing the warning lamp current from that provided by tiny,tiny vetus indicator lamps.....it prob doesn't cut in any earlier, but no longer totally drops out at tickover, stopping the revcounter & making me think the engine's packed in.



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[cliff]

I was always of the impression the rev counter was driven from the AC side of the regulator therefore would work at all speeds. You say your rev counter now does not operate until the ingition (charging light) goes out - mmmmm - Makes me think the rev counter is not correctly wired up to the alternator. The DC output has a ripple on the voltage - it is not pure dc that is the voltage is RMS voltage (Root Mean Square). Put an oscilliscope across the alternator output and you will see what I mean. Could be your alternator does not have a rev counter connection in which case you will need to open the alternator and connect one yourself. If it is a lucas alternator a trip to your local Lucas depot should confirm if the alternator has a connection for a rev counter and if not they can show you exactly where to take the rev counter feed from.

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[brianhumber]

Nothing is wrong, the alternator output in effect produces the voltage to work the rev counter, it just means your new diodes need higher output ie higher revs to 'kick on'.
I start my engine ( a 16yr old Ford 1.6) without any thottle, leave her for a minute or so to let the oil circulate and then blip the throttle to kick the diodes 'on' this starts the charging wakes the rev counter up and stops the alarm bleep. It does mean you have a second alternator alarm, no rev counter no alternator output !
Happy sailing

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[andyball]

the "w" or rev counter terminal is driven from the ac side....it's connected directly to a winding; this doesn't mean that power is available at all rotor speeds.

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[cliff]

Granted. Depends on the sensitivity of the pulse / rev counter - even at low speeds there should be enough AC voltage for the counter to count the pulses. Perhaps there is a fault in the wiring giving a higher than normal resistance. Still the fact that the counter does not start until the regulator kicks in still would make me check the counter is conected to the correct terminal (as you say normally identified as "W") before looking else where for the problem.

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[VO540]

I should not worry, my moody has a volvo MD22L (perkins really) and it always to be revved to 1200 rpm initially to bring rev counter on line and extinguish the warning lights therafter they stay on line even when engine is returned to idle. Alternator output is normal.

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[William_H]

Hello Westhinder my ideas come from theory rather than practical however obviously if you change the pulley size you might get the alternator on charge at lower RPM but your tacho reading will be incorrect. The usual system is that a 12 volt supply is fed through the warning lamp to the regulator through the field coil ie the rotating coil via the brushes. This provides some magnetic field to give the alternator some output. As soon as the alternator starts to provide some output a separate set of 3 diodes provides voltage to the regulator which then improves the output giving it more field coil current. The warnign light goes out because the current no longer flows through the lamp because it is provided by the alternator itself. The initial current is that which flows through the warning lamp. I woul;d suggest you try a larger wattage lamp. I t will be typically 3 watt so try a 6 watt globe or wire another 3 watt or even a 6 watt lamp in paralell with the original . This will give more initial current and hopefully bring the whole system on line at lower RPM. Now the tacho output usually comes from the connection of the centre of the 3 phase stator windings and gives the AC before it hits the rectifier diodes. You can not get any AC out until the alternator is charging or at least has a reasonable field coil current. Incidently the lamp will show an alternator failure if the belt falls off or breaks however a common failure mode of alternators is worn brushes giving bad connection to the slip rings which means no current through the lamp and no warning light. regards ole will

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[westhinder]

Thank you guys for your comments. Now at least I have an idea what to look for next weekend.

<hr width=100% size=1>Johan