hey I have a 72 in dash tach, it has a two prong plug at the rear, it came out of a 72 gto with the unitized ignition, the other tachs pre 72 without unitized ign. have only one wire for the tach, when i hook it up ( the one wire to the neg. side of the coil) on my 70 gto it works but is reading about 500 rpms too high, I`m not sure what to do with the other wire (pos. or neg hook-up?), I have all the assembly manuals for 70`s and up but they all only show for the single wire tachs even the 72 books with schematics, what to do? please help, can i get this 72 tach that was for a untized distributor to work on a points type distributor? all the other tachs I`ve seen only have the one wire type tach? thanx

hey I have a 72 in dash tach, it has a two prong plug at the rear, it came out of a 72 gto with the unitized ignition, the other tachs pre 72 without unitized ign. have only one wire for the tach, when i hook it up ( the one wire to the neg. side of the coil) on my 70 gto it works but is reading about 500 rpms too high, I`m not sure what to do with the other wire (pos. or neg hook-up?), I have all the assembly manuals for 70`s and up but they all only show for the single wire tachs even the 72 books with schematics, what to do? please help, can i get this 72 tach that was for a untized distributor to work on a points type distributor? all the other tachs I`ve seen only have the one wire type tach? thanx

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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