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Yeah, the VSS output is a pulsed 12v where the engine uses a pull-up type circuit thus it needs a pulsed ground. The speedo has a read switch built into it and pulses a ground. The later cars that had electronic speedo's did this signal coversion/conditioning within itself.
I do thing thing that the two inputs to the Transistor should be diode protected. (i.e. the VSS to Base should be diode protected and the ECU to collector should also be diode protected.)... I forgot to mention that in the previous post. Probably not fully necessarly, but wont hurt either.
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