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40flash · 10-09-2012, 10:49 PM

While we are on the subject of gauge wiring, here is a post from 350sx NW build up.
Actually the blue/white wire is the shared 12v source for the entire ignition circuit so that wouldn't work. Each coil is 3 wires, Ground, Signal, and Pwr. Blue/White is power and black is obviously ground. The signal wire is going to be the only unique wire per each given coil. The signal is faint, but grapping it from that signal wire works for me. I'm using a Dakota Digital Tach Interface box and I adjusted accordingly to drive my tach. I'm pretty sure that the signal wires for the fuel injectors would be a better source because they run on a higher volt signal but I've got it working so I'm not going to mess with it unless it gives me trouble.

I posted this to suggest that he has it wired correctly but his suggestion that the injector signal wire would be a better and stronger source for the tach signal. I disagree. If you look at the wiring diagram for the injectors you will see that the every injectors is fed from a common 12v from the fuse box any time the ignition is turned on. When the ECU wants to energize a particular injector it closes a ground wire from the ECU to that injector. Since the they all get a constant 12 volts positive, the positive can't be used to pick up a pulse for the tach. Also you can't run the tach with the pulsed ground wire from the ECU.
The signal wire for Ignition coil number 1 is a better source for the tach signal. On the 2003 and maybe others, that wire is a yellow and red wire that feeds the coil from terminal 17 at the ECU. Don't cut the wire but splice into it and wire it to the Dakota Digital SGI-8 "signal in" terminal.

10-09-2012, 10:49 PM	#79
40flash Leaky Injector Join Date: Oct 2009 Location: Arizona Age: 72 Posts: 56 Trader Rating: (0)	While we are on the subject of gauge wiring, here is a post from 350sx NW build up. Actually the blue/white wire is the shared 12v source for the entire ignition circuit so that wouldn't work. Each coil is 3 wires, Ground, Signal, and Pwr. Blue/White is power and black is obviously ground. The signal wire is going to be the only unique wire per each given coil. The signal is faint, but grapping it from that signal wire works for me. I'm using a Dakota Digital Tach Interface box and I adjusted accordingly to drive my tach. I'm pretty sure that the signal wires for the fuel injectors would be a better source because they run on a higher volt signal but I've got it working so I'm not going to mess with it unless it gives me trouble. I posted this to suggest that he has it wired correctly but his suggestion that the injector signal wire would be a better and stronger source for the tach signal. I disagree. If you look at the wiring diagram for the injectors you will see that the every injectors is fed from a common 12v from the fuse box any time the ignition is turned on. When the ECU wants to energize a particular injector it closes a ground wire from the ECU to that injector. Since the they all get a constant 12 volts positive, the positive can't be used to pick up a pulse for the tach. Also you can't run the tach with the pulsed ground wire from the ECU. The signal wire for Ignition coil number 1 is a better source for the tach signal. On the 2003 and maybe others, that wire is a yellow and red wire that feeds the coil from terminal 17 at the ECU. Don't cut the wire but splice into it and wire it to the Dakota Digital SGI-8 "signal in" terminal.