I have tried with additional ground.
A friend of mine tested the first coil according to instructions found on this page. The test indicated a bad coil. That's why I have changed it.
Didn't test the new one before mounting. Guess I have to do that as well, but how likely is it that two brand new coils are both bad?
(Yes I know I know... The master cylinder I have is the third one. The two first ones were leaking past the piston...)