Primary and secondary resistance are the only passive tests, other than apperance. If it is an external frame coil like late model GMs, the presence of "wiskers" of iron growing out of the frame indicates serious secondary voltage leakage, time to toss. Any signs of carbon tracking around the secondary wire tower or "burn" marks to ground indicate broken down insulation and hence a current leak.

Other than that, you will need an ignition analyzer, good wires, and good plugs to check performance, and it's getting hard to find ignition analyzers anymore, no one uses them.

The other "quick" test is to replace the coil. Not cheap, just quick.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!