External Tach

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penguin356

Cadet
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May 3, 2005
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8
Quick question...<br /><br />If I take a regular marine/automotive 12V Negative ground tachometer and connect the + side to the + side of the coil and ground it to the block, in essence I have created an external tach.<br /><br />Automotive 3 wire:<br />+ to the Battery <br />- to the block/battery<br />Signal + side of the coil or wrapped around a plug wire<br /><br />Right?
 
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rodbolt

Supreme Mariner
Joined
Sep 1, 2003
Messages
20,066
Re: External Tach

depends on the ign system and the tach<br /> the plug wire types are inductive pick up. the one that goes to the coil primary is an analog pulse type. some ign systems used a pulsed 5VDC from the ECM as a tach driver.<br /> need a bit more info on what you have, what your trying to accomplish and why.
 
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penguin356

Cadet
Joined
May 3, 2005
Messages
8
Re: External Tach

Going to be installing a new Carb on a 3.0L Merc that has had the P&C coverted to solid state ignition. Directions say to set the idle speed by using a tach at the motor.
 
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rodbolt

Supreme Mariner
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Sep 1, 2003
Messages
20,066
Re: External Tach

do you have a tach in the dash?<br /> even if you added a tach at the motor ,unless its an inductive type you still have to verify the tachs accuracy. most hull tachs are not very accurate at either end of the meter movement range. but its not super critical. just needs to be within spec plus or minus 50 RPM. with a merc not above 750 rpm. the 750 RPM spec is for 2 things. one it helps prevent dieseling at shut down and 2 it helps reduce shift clutch wear when going into gear and cable/moving parts wear when coming out.
 
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