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monster tacho won't work... Help!!


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hi guys, i have an s13 with an RB20det. I cant seem to get the tacho ( silver face AUTOMETER pro comp) to work? it will only give me the slightest signal(1200rpm max) when i rev the crap out of my car.... does anyone know how i could wire it up so it would work. or of any distributor-less friendly tacho's?

any help would be greatly appreciated..

cheers!!! :P

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My understanding of how it all works.....this is not neccesarily completely correct

Tacho's work by picking up voltage pulses. Some work off high voltages (eg 200V tachos designed to work from ign coils), others work off 12V and some work off both. OEM tacho's work off a variety of voltages.

The RB20 ecu generates the tacho pulses by earthing pin #7 to ground. The std tacho works off a 5V square wave and there is some special electricketry in the tacho that provides 5V to pin 7 so this wave can be generated.

Connecting pin 7 straight to the pulse input of a non standard tacho will in many cases do nothing because the tacho doesn't provide any voltage to the ecu pin and therfore it is effectively switching between 0V and 0V.

A "pull up" resistor can be used to provide potential to pin 7. If you connect pin 7 to the tacho pulse input and then get a 10k ohm resistor and connect it between a 12V supply and pin 7, then the tacho will see 12V-0V input as pin 7 switches between open and ground. The 10k resistor will prevent too much current from flowing through the ecu and frying the tacho circuit. Cost ~$1.

If this doesn't work you can get tacho boosters from Sixworks in Newcastle. There are two types depending on which tacho you have and cost approx $50 /$100. I'll post up the contact details when I get home.

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