Author Topic: RPMs with a dwell tach? (Read 4815 times)

silvertdi · « **on:** November 20, 2007, 07:19:54 pm »

I've read in this forum that you can get an RPM signal off the alternator. I've got an old dwell/tach instrument. Can this be used to count RPMs off the alternator? Anybody try this?

somolovitch3 · « **Reply #1 on:** November 21, 2007, 04:19:21 am »

Probably................................~ maybe.

W teminal supplies a voltage referenced to alt speed and via belt. engine speed.

Tach/Dwell count the pulses? Or do they convert (?) the pulses to a voltage ?

WTH... give it a try, I would not expect to see a "true" RPM reading in any case.

Now if one could combine a points type distributor with a vane type vacume pump.............

Some way to get four pulses per ...hummm........ okay........ Two pulses per revelution of the engine (crankshaft).

Ooooooooooooooooh lots of fun things to try!!!!!!!!! :roll:

Or search for the digital tach setup....I've several web sites that I look at and try to see if the buget will let me play. :twisted:

jimfoo · « **Reply #2 on:** November 21, 2007, 08:40:54 am »

IF it worked, which I doubt, the RPM's would be so far off as to be meningless. Tach/dwell meters aren't meant to take AC, only DC in the correct polarity, so first off you'd need a doide. Second, it's possible the slow rise and fall times of the rectified sign wave would cause further inaccuracies.
Third, the pulses per revolution and pulley ratios would have to be calculated to get anything close to meaningful. A mechanical tach on the crank would be the best way to get an accurate result.

Vincent Waldon · « **Reply #3 on:** November 21, 2007, 08:49:01 am »

Assuming that the tach isn't confused by the different shape of the waveform (could work, depending on how sensitive the tach is) the pulse count is out by a factor of 12... ie the frequency of the W signal is about 12X the frequency from a 4-cylinder ignition system at a given RPM. Hence converters like the Dakota Digital adapter.

You could open the tach, figure out how its calibrated, and then adjust the components accordingly... this is how some people convert gasser clusters over to diesel, for instance.

I like the VW "special" adapter... clamps to the block and uses engine vibrations to convert to a tach-looking signal.

clbanman · « **Reply #4 on:** November 21, 2007, 09:03:45 am »

http://cgi.ebay.com/ebaymotors/SmartTach-Digital-Tachometer-Diesel-Gas-Engines_W0QQcmdZViewItemQQcategoryZ33679QQihZ002QQitemZ120184970937

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200326528_200326528

http://www.next.gr/shop/shop.php?c=9&x=Digital_Tachometer

http://www.4crawler.com/Diesel/CheapTricks/Tachometer/index.shtml

The third link is in my price range :lol: I'll have to see if they ship to Canada.

somolovitch3 · « **Reply #5 on:** November 21, 2007, 04:49:36 pm »

All great ideas, but we might be missing the point, maybe. :roll:

If a tach/dwell meter would work..great way to set the idle, no?

That line three item does look..................... interesting. :twisted:

Of course just because I am never wrong, does not mean that I am right all the time! :oops:

jimfoo · « **Reply #6 on:** November 21, 2007, 05:07:42 pm »

Quote from: "Vincent Waldon"

Assuming that the tach isn't confused by the different shape of the waveform (could work, depending on how sensitive the tach is) the pulse count is out by a factor of 12... ie the frequency of the W signal is about 12X the frequency from a 4-cylinder ignition system at a given RPM. Hence converters like the Dakota Digital adapter.

You could open the tach, figure out how its calibrated, and then adjust the components accordingly... this is how some people convert gasser clusters over to diesel, for instance.

I like the VW "special" adapter... clamps to the block and uses engine vibrations to convert to a tach-looking signal.

Or maybe just use a multi-meter that shows ac frequency and then divide by 12.