zsmith3rd(at)earthlink.ne
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 Posted: Wed Oct 31, 2007 3:44 am Post subject: Tach problem |
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do not archive
You don't say what brand of tach, but if it is a WesTach brand you might try putting a 1K Ohm resistor across the tach leads to swamp the "ringing" that is present with each tach pulse from the engine.
Observed on a 'scope, this looks like several small pulses on either side of the main pulse.
The tach circuitry sees these as just more pulses, and results in erratic readings.
The "error factor" changes with engine speed as the amount/amplitude/shape of the ringing changes.
The resistor lowers the amplitude of the extra stuff so that the tach only sees the main pulse.
The 1K value may not be correct for each application. WesTach web site has application info which will probably apply to most any brand of electronic tach.
You can connect a 5K pot across the tach, using the pot center terminal and one end terminal, then vary the pot to determine at what setting the tach works best. Read the resistance between these two terminals with an ohmmeter to find the value you need.
Hope this helps.
Zed/701/R912/90+%/etc
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