There are testing techniques for most everything. The test needed here for the coil will look at available voltage. With your scondary ignition scope hooked up pull a plug wire and evaluate the open circuit voltage. It should see 25-30kv. If it only makes 10-15kv one needs to see if the reason is lack of capability or a high voltage short. The common hi voltage short is the rotor. To separate the posibilities one then takes the coil wire to ground and then opens the circuit to ground increasing the spark length while monitoring the voltage.

Without tools one tests by substitution.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician