Snap on something?

What RRR said. We used those long before computers. You could analyze each cylinder, kill a individual cylinder. Rev the engine and watch for spark drop, misses and even how much voltage was needed to see if a plug or coil was dropping out or shorting internally. The one I learned on was about 10 times bigger and hung from the ceiling on a track so every bay could use it. Tons of harness when to it.