China tach doesnt work

Having a hard time measuring the frequency of my distributor lead to coil negative using a digital o-scope. Rigol 5120 ish 2 channel 1GHz rated scope. I hook up the lead to the negative side coil expecting to see a 40 Hz 12v square wave of the distributor sending the 12v to ground but I just see garbage on the screen even when I set it to "auto test" with no clear frequency defined. Any pointers? Maybe wrap a wire around the coil wire as a 20kv pickup? I have this digital Rigol and a tradutional vacuum tube HP scope. Any scope doctors here?