The voltages on the secondary side of an ignition system are too high to be measured directly. To safely measure these signals, the Ignition Pickup TP-IP250 can be used.
 
The Ignition Pickup TP-IP250 is a capacitive secondary ignition pickup, which can be clamped around a spark plug wire. The pickup has a ground wire with a large alligator clip. The pickup is fitted with a male BNC terminal, a BNC to 4 mm banana converter is included to connect measure leads with banana terminals.
 
Warning:
The Ignition Pickup TP-IP250 is designed to be used in combination with an instrument with a differential input, like the Automotive Test Scope ATS5004D. Using it in combination with an oscilloscope with single ended inputs may cause a short circuit that can damage the scope and the car.