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jimmcc jimmcc is offline
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Join Date: Jun 2005
Location: Birmingham, Alabama
Posts: 274
Originally Posted by fredmeister View Post
what is the voltage range that one should see at the terminal on a functioning tach as it idles Thur to 6800rpm?
I assume the voltage changes from one value to another at both ends of the rpm spectrum in order to move the gage needle to show the various rpms?
For example at idle it is 4.5 volts and drops to 3 volts at 7000rpm.
Or does it function some other way?
It's not actually a voltage from the CDI to the tach such as the output of a MSD or DME provides. It is a negative transition from what could be best described as a neutral state to negative. It simulates the action of an old point based system.

The tach provides a pull up voltage of about 6 volts which is then pulled to ground (low state) when the CDi triggers. Watch it on a scope with the tach connected and it appears to be a positive pulse. Disconnect the tach and the scope gets interesting. Then you spend time with the schematic and go ah. The first time I saw it on a scope it was weird to say the least and not what I was expecting. I have the scope images somewhere and will post if I can find them.

That's why pulsing the tach input terminal to ground will semi-simulate a working CDI.

Old 08-24-2009, 12:19 PM
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