Typically the transistor that pulls the coil -, to ground, fails shorted, so the coil -, will be at about a volt or less with ignition on, for a failed unit. If the - coil terminal is stuck low, ( bad unit, wiring short, or tach wire short to ground), no spark happens. A simple test, remove tach wire and test again.
The short on transistor can also be measured as low Ohms between black lead and ground, not powered or connected to coil.
