The single wire tach is most likely from a 6 cylinder car. That I am afraid that it just won’t work on a V8 without serious modifications. The 6 cylinder distributor puts out 6000 pulses/minute at 2000RPM (2000 / 2) * 6 It’s divided by 2 since the distributor turns half the RPMs of the motor, multiply by 6 because there are 6 pulses per rotation of the distributor), where a V8 puts out 8000 pulses/minute at 2000RPM. The difference being 8000 – 6000 = 2000 pulses/minute. Now reverse the math, (2000 / 8) * 2 = 500 RPM added to the reading. As for the 2 prong plug, id say one is the same tach pulse as the single wire tach and the other is a ground or possibly a light, but id be guessing. Good luck.
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TigerEFI.com (Classic Pontiac Fuel Injection)
2004 GTO Company car, 1994 TA 25th Anv, 1971 Grand Safari Wagon, 1968 GTO EFI, 1968 GTO Stock, 1968 GTO Convertible in desperate need of restoration.