i would like to know where is the ground side of the ignition coli on a softail fxst in order to connect a digital tach

 

There is no ground side to an ignition coil. One side is 12 volts, typically from the IGN circuit breaker and through the RUN switch. The other side is the signal to fire from the ignition module or points. It does not matter which side is which, as long as you treat one side as 12 volts and the other as the signal.

If the tach needs to be grounded any convenient engine, frame or body ground point will suffice.

 

thank you.. the tach needs to be grounded but it also needs to be connected to the negative terminal of one ignition coil or to the corresponding terminal of the ignition box.. should i connect it to the ground black cable of the ignition module??

 

No. As IronMick stated in his post, assuming you have a stock dual fire coil, one of the small terminals has a wire that should have +12 volts on it whenever the ignition switch is on. That could be considered the "positive" terminal of the coil, but I don't know anybody who calls it that. The other small coil terminal is the one you want to connect the tach wire to.

(P.S. You may get better answers to your FXST questions if you post them in either the Softail or EVO Classic forums on this site. This is the Ironhead Sportster forum.)

 

The terminology +ve and -ve terminals of a coil, while correct, can easily be misinterpreted. It is more direct to say that one carries 12 volts to the ignition module [and to any other device attached to this terminal] and the other is the signal from the module to ignite the spark plugs. It is very important to be absolutely clear on which one has the 12 volts going to it.

If you are uncertain which is terminal is receiving the 12 volts you can test both with a voltmeter. On a Sportster this 12 volt wire is white [all the rest on the bike are orange] - it is probably the same on the FX.

The tach needs to get 12 volts in order to function, and any 12 volt wire on the bike will work just fine, but the usual wire for this is the white wire running from the IGN circuit breaker, thru the RUN switch, to a terminal of the coil, then on to the ignition module. So that specific terminal is where you attach the wire in question.

I am assuming, I believe correctly, that the FX wire system is virtually identical to that of the Sportster. No reason to think otherwise. But you could post this question in a ShovelHead forum.

 

Regardless of whether the bike has a points or an electronic ignition, the tach gets its 12 volts from the 12 volt terminal of the coil. But as I said above it quite simply does not matter where this 12 volts comes from - any 12 volt wire on the bike will be good. But best to follow the factory wire diagram to avoid confusion.

For electronic ignition [which this bike has] the tach gets its signal directly from the module, not from the coil. That is the way they are wired from the factory. But it quite simply does not matter - either end of the wire [module or coil] is the same.

For points ignition the tach gets its signal from the "not 12 volt" side of the coil, the side that receives signal from the points assembly. The points provide the signal to the coil which uses it and passes it on the the tach.