Doubt if I can input anything that hasn't already crossed your mind, but here goes anyway:
- Battery is young, but....set VOM on 20 scale and test for 12.6 volts. That of course is optimum. In my experience, once it gets below 12.0, cranking fades out. I realize that a LOAD test is better, but I believe this simple test says alot about the batteries general state of health.
- Battery cables secure and posts/connections clean?
- When you crank with headlights on, do they dim at all? I believe this might indicate if juice is actually making it to the starter.
- I recently went thru a very similar experience. Turned out to be a situation with the anti-theft circuit.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.