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Old 30 November 2011, 17:59   #1
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Clamshell alternator voltages

Was trying out a new tacho tonight, and when nothing happened I thought "put the old one back in & check it still works". It didn't.

So, I poked about with the multimeter. Everything off (main switch)- battery voltage 12.3V. Engine at idle, 12.3V Revved to a guestimated 2K, still 12.3V. Most I got out the tacho signal wire with the meter set to AC was 1.3-ish variable. Got the same 12.3 out the switched +ve, which on this engine hangs off the rectifier out via a fuse that I proved OK.

Now, I am aware that the tacho signal at idle on a 6 pole alternator is in the region of 180 Hz, so the reading on the meter is nothing like accurate, but my memory of doing a similar test on my Yam 55 it showed something in the region of 5V.


Two questions:
1) based on that, thoughts? (I have a theory, but not going to bias your opinions with it yet, other than to say "pass me the flywheel puller"
2) could anyone else with a Clamshell 3 cyl 50 or 60 measure the batt voltage with the engine both idling and at 2K rpm. An AC volts reading on the tacho signal wire would also be much appreciated.

In summary I don't have a good feeling about this......
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Old 30 November 2011, 18:32   #2
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You might want to correct the title to "Clamshell dynamo voltages"
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Old 01 December 2011, 04:43   #3
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It's not that old - it does have a rectifier!

Now I'm awake again, my curent plan is to chase the charging voltage (or apparent lack thereof) out from the alternator. My thought is that if I amnot getting 5-ish VAC on the meter out the tacho sender then the coil is likely gubbed. No charging voltage might hint at a gubbed rectifier, but if there's nothing going in, then obviously nothing is going to come out.

If any of my usual suppliers are reading, anyone got a coil?
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Old 01 December 2011, 12:01   #4
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Quote:
Originally Posted by 9D280 View Post
Now, I am aware that the tacho signal at idle on a 6 pole alternator is in the region of 180 Hz
This is guesswork (haven't had to look at this stuff on any engine I've ever owned), but...

Isn't it more like rpm*6 (or maybe 3)? Taking the rectified-but-unsmoothed signal from the alternator coil after it passes the magnets?

DC Voltage you read is going to depend on a lot of factors: duty cycle of the signal, rpm of the motor, sensitivity and persistence of the filter in your meter, etc.

You'd probably do better to read AC volts, or better yet, borrow an oscilloscope.

But yeah; sounds like your signal has gone missing.

jky
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Old 01 December 2011, 15:05   #5
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Check the voltage regulator, the tacho takes its signal from this.
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Old 02 December 2011, 04:52   #6
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Originally Posted by jyasaki View Post
Isn't it more like rpm*6 (or maybe 3)? Taking the rectified-but-unsmoothed signal from the alternator coil after it passes the magnets?
Say 850rpm with a 6 pole (12 pulses per rev)
=5100 pulses/ min
=85 Pulses / sec

This is approx 150Hz at 1500 RPM

(and I've just noticed the "fat finger moment" on my original post! ) Anyhow, it's not 50Hz pure sine which is what the bog standard DVM is calibrated against. Bottom line is any other time I've been chasing tacho probs regardless of the # poles it;s always shown around 4.5 - 5 V at idle. rather then the 1-ishV I got this time.

Quote:
Originally Posted by jyasaki View Post
DC Voltage you read is going to depend on a lot of factors: duty cycle of the signal, rpm of the motor, sensitivity and persistence of the filter in your meter, etc.
Absolutely. But I would have expected to see a small rise to 14-ish V when I revved it.

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Originally Posted by jyasaki View Post
But yeah; sounds like your signal has gone missing.
Yup.
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