Alternator cutting tacho output

[tim]

The rev counter on my Volvo 2030 drops to and stays at about 800 RPM, regardless of actual RPM, after a period of running. I'm exploring various avenues why that could be. But I've read on CruisersForum that when the battery is fully charged the regulator will cut the alternator output (as it should) which will also cut the voltage to the tacho out put (W+ on the Volvo). That doesn't seem right to me??

 

[pvb]

The rev counter on my Volvo 2030 drops to and stays at about 800 RPM, regardless of actual RPM, after a period of running. I'm exploring various avenues why that could be. But I've read on CruisersForum that when the battery is fully charged the regulator will cut the alternator output (as it should) which will also cut the voltage to the tacho out put (W+ on the Volvo). That doesn't seem right to me??

It's not right. The rev counter is driven by a feed from the W terminal on the alternator. This terminal outputs an alternating current, the frequency of which is proportional to the alternator speed (and hence to the engine speed). If it works OK for a time, it's possible that your rev counter may be faulty.

 

[TangaraToo]

Sure the belt is not loose once all is warmed up?

 

[tim]

Thanks. I'll check the feed to the rev counter as well. I assume it'll be 12V AC?

 

[pvb]

I assume it'll be 12V AC?

Around that.

 

[nigelmercier]

Do you have an external regulator?

 

[<152587>]

Thanks. I'll check the feed to the rev counter as well. I assume it'll be 12V AC?

As PVB pointed out you may indicate 12v output but the tacho is a frequency indicator not voltmeter so do not assume healthy output on voltage alone.

 

[PaulRainbow]

Also worth checking the 12v DC connections too.

 

[VicS]

Thanks. I'll check the feed to the rev counter as well. I assume it'll be 12V AC?

Around that.

The Tacho will have a 12 volt DC power supply direct from the ignition switch


As far as the signal from the W terminal is concerned I may be less than 12volts. I beleive that some are only 8 volt

Although its taken diectly from one of the alternator stator windings the return path is via the common DC negative and the negative half of the alternator rectifier. ITYWF this effectively this makes the signal at the tachometer a series of half wave rectified positive pulses rather than full wave AC.

 

[pvb]

I may be less than 12volts I beleive that some are only 8 volt

Although its taken diectly from one of the alternator stator windings the return path is via the common DC negative and the negative half of the alternator rectifier. Effectively this make the signal at the tachometer a series of half wave rectified positive pulses rather than full wave AC.

True, but when the OP said "I assume it'll be 12V AC?", I thought he might be thinking of ordinary mains voltage as an alternative. Setting a meter on anything around 10-20v would be fine.

 

[Billjratt]

is it consistent and predictable -ie bang on time, or could it be temperature sensitive - and always 800revs regardless of actual?
There is a calibrator pot on the side of the tacho head (pull out the plastic plug) does it vary the reading in either condition?
A lot of good quality multimeters have a frequency range which will tell you what the alternator is doing, and also if the AC is reaching the tacho terminals. Are there any (other) issues with the alternator - do you have an ammeter in circuit, what does it do at the time of the error. etc, etc, etc.
We would like readings in error and non-error states, but it would probably be quicker and easier to get a new head from an online skip!

 

[VicS]

True, but when the OP said "I assume it'll be 12V AC?", I thought he might be thinking of ordinary mains voltage as an alternative. Setting a meter on anything around 10-20v would be fine.

But it may mean that you will have to set a voltmeter to the 20 volt DC range.

 

[pvb]

But it may mean that you will have to set a voltmeter to the 20 volt DC range.

I don't think so, all he's doing is checking whether there's some sort of output from the alternator's W terminal. The voltage itself is fairly meaningless, it's the frequency which the rev counter uses.

 

[VicS]

I don't think so, all he's doing is checking whether there's some sort of output from the alternator's W terminal. The voltage itself is fairly meaningless, it's the frequency which the rev counter uses.

One of my multimeters does not give a reading ( just 0.00) if AC volts is selected and its connected to a DC supply. (The other gives an unexpected high reading.)

 

[pvb]

One of my multimeters does not give a reading ( just 0.00) if AC volts is selected and its connected to a DC supply. (The other gives an unexpected high reading.)

Is there any positive advice you can offer the OP, rather than nitpicking other suggestions?

 

[VicS]

Is there any positive advice you can offer the OP, rather than nitpicking other suggestions?

Not nit picking at all

It was intended as warning that there might be a pitfall in checking the signal from the W terminal if one assumed it was full wave AC and therefore set the meter to an AC volts range

Take the person who suggested that the OP might think it was 240 volt mains voltage to task instead .... Oh that was you!

 

[William_H]

As said the output to the tacho is a series of pulses that come form the centre of the rectifier set. (not sure if thye are AC as in reversing polarity) The actual voltage will depend on the field current of the alternator. This of course depends on the demands of the regulator so a hard working alternator (flat battery or heavy load) will give more voltage to the tacho. However the tacho should be able to operate on the lowest expected voltage pulses from the alternator.
Suggest OP apply a large load to the alternator (fridge lights or what have you) and see if that brings tacho back to life. If not i suggest perhaps the tacho itself is failing when hot. good luck olewill

 

[nigelmercier]

Still no answer to my query, but I agree with Vic that a DC range should be used for pulsating DC voltages. An oscilloscope is of course the correct tool, and will show the frequency of rotation multiplied by the number of coils. Or possibly half that.

 

[earlybird]

FWIW, According to Beta, my W terminal is supposed to show 7.5 - 9 v. AC. Presumably as read by a DVM, ie. rms. not peak.

 

[Billjratt]

Without getting into debate, the "W" terminal will show the AC sine voltage before rectification to both the lamp and B+ terminals. In this case it really doesn't matter what you use to measure it and whether it's peak to peak or root mean square - ie how it's measured. If there is a voltage of somewhere between 12 and 15 volts on your 'scope, digimeter, analogue meter AT THE "W" terminal on the tacho head,,,,,, then there should be a response from the tacho needle. As already posted MANY TIMES it's the frequency that matters, so a reasonable quality digi meter or a 'scope (or a free program loaded to your laptop) will display the input that the tacho head is looking for.
I had a similar problem to the OP on a 4108 with a generator sender (which sends rev-dependent voltage) whereby one of the diodes in the head was intermittent and the reading dropped by a few hundred revs randomly before finally failing to the low reading. The thread is here in case anyone has the other type of tacho http://www.ybw.com/forums/showthrea...-with-a-multimeter/page3&highlight=tacho+4108 .