Coil troubleshooting with a test light?

[JonnyQuest]

Hey Gang,
My A4 recently stopped running, and I believe I traced the problem down to an old coil--corrosion on contacts, rust on housing, and stripped threads on positive lead resulting in a questionable contact. Knocking the rust off the positive lead seems to have been what got the engine running again--started up like a champ on first bump, with no choke either!

Trying to educate myself a bit about the coil by monkeying around with a test light now. With the engine running I expected that the test light on the positive and negative leads of the coil would produce a flickering light as the electronic ignition system pulsed current through the system. Instead the test light on coil (while the engine running) lit up solid and normal full brightness instead of flickering. Is this what is expected? Also, the test light apparently robbed enough electrons from the coil to cause the engine to lose power dramatically and shut off. Remove test light and fired engine back up without any problems. Is this normal for the test light to reduce current enough to stall the engine? I wouldn't expect that. --Then again, if I were someone else I wouldn't be asking the likes of me for assistance!

I will be ordering a new coil from Don soon but wanted to inquire about the test light stalling my engine.

Thanks for the input,

Jonny

[smosher]

If you had the test light across the + and - of the coil I would think the flow would be across the light and not the ignition. To use the test light place 1 lead on the + and the other lead to the engine block, this should be a steady light, Move the + lead to the - side of the coil and this would flicker, with the engine turning over or running

Steve

[ndutton]

And it doesn't directly test the coil but rather the ignition system switching. Further, placing the light between the - terminal and ground negates the switching function at least in part, perhaps to the point of degraded performance. It really depends on the internal circuitry of the test light.

Coil testing needs to be done over a full temperature curve. We've seen coils function cool only to fail at higher temperatures.