testing ignition coil (starting problem)

in response to an earlier post for a similar problem, someone posted the following message regarding testing an ignition coil using a VOM:

If you don't get a spark as tested above, check the coil windings' resistance with your multimeter. The primary should measure between 0.4 and 0.6 Ohms, or slightly higher due to test lead resistance or calibration of the meter ... most meters are not very accurate below 1 Ohm. The secondary should measure between 650 and 790 Ohms. If coil is OK check all connections for corrosion. If test generates a spark, then look at distributor for problems!

i am OK testing the primary. how do you test the secondary? specifically, to what do you connect the test leads?

Well, those values are for a CDI-system coil, not for a Motronic engine, just so we are clear ... since you didn't mention your year! Terminal 1 (ground) and terminal 4 (center output terminal) are used to measure the secondary resistance.

One very important note while you are doing troubleshooting: Never disconnect the coil output terminal and lesve it 'open' while cranking the engine! The coil can breakdown and arc internally, which burns out the coil ... which is relatively expensive! If you want to test the coil output visually, take a spark plug lead, and plug into the coil. Then connect a spare spark plug, and grond the body with test clips to the engine block or sheet metal. Then crank the engine (SC or Turbo) or take the trigger lead ('69-'77) loose from the distributor, and ground and release the trigger lead repeatedly ... and look for a fat, blue & loud spark at the test sparkplug.

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Warren Hall
1973 911S Targa

Does anyone out there know what the resistance readings should be on the coil for the 3.2L engines? They accidentally left that info out of the Bentley book. I'd like to hook it up to the Fluke & test mine. ('85 Carrera)
regards,
jlex.

good point about which car i have......i have a '73 911 E.

Primary resistance between terminals 1 and 15 on the Carrera should be between .4 and .7 ohm.
Secondary risistance between terminals 1 and 4 should be 5.0 to 8.7 kohms. Terminal 4 is the center terminal, the high tension lead to the distributor.
Marshall

Thanks, Marshall. I'll enter it into the Bentley book.
regards,
jlex.