This is the part of the program where "substitute known good unit" would be real handy (if you had an ignition module). The ignition modules are known for failure on that year model. The weird part is a "partial" failure. Usually they either work or they don't. I wouldn't suspect the rotor or cap unless there is a ton of carbon tracking. Coil failures are rare and the over-Voltage protection relay isn't even in that circuit. If you knew what Voltage the spark output was it would help. If I had to guess(man I hate to guess) I'd go for the module. If it helps, you can use a module from an early (1984-85) 190E 2.3. Really.