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zsmith3rd(at)earthlink.ne
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 Posted: Fri Jul 28, 2006 4:56 am Post subject: 912 tach problem |
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One problem that has surfaced concerning the 912 and electronic tachs is nearly what you described as "erratic".
The tach pickup coil on the 912 puts out an AC waveform that is not a pure sinewave....it has "ringing" riding on the AC which, viewed on a 'scope, looks like a lot of spikes or bumps. If a "load" is put on the tach pickup leads this ringing disappears.
These newer tachs are "counters" which count the number of oscillations and display it as RPM.
Having a "high input impedance" results in the tach counting each little spike as a revolution.
Some spikes are counted, some are not. Erratic display. There may even be a maximum signal input voltage spec, but I'm not aware of it.
Put a 150 Ohm resistor across the tach leads to swamp the pickup output. This will reduce the ringing/spikes and allow the tach to read/count only the revs.
WesTach has included this in the installation instructions in recent shipments, unfortunately mine was purchased prior to that so I spent hours messing with it. Other brands probably suffer the same symptoms.
Hope this is helpful,
Zed/701/R912/90+%/do not archive
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David X
Joined: 10 Jan 2006
Posts: 154
Location: Princeton, NJ, USA
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| Posted: Fri Jul 28, 2006 5:09 am Post subject: Re: 912 tach problem |
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Thank you, I'll give it a try.
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