s13 blacktop ecu spits out 0-5v square wave on tach output. i needed that info bad when wiring in my AIM dash which expects 0-12v square. a 47k ohm pullup resistor worked perfect for me. note: i've never tried using a dash other than the AIM so ymmv. If you use a pullup, i would recommend using a switched 12v power so you don't pull current through the ecu 24/7.
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