Mercedes M Klasse Terminal 30 Issue

My 2005 Mercedes M Klasse with a diesel engine is showing some bizarre behavior. I'm getting intermittent issues with terminal 30 continuous plus. Mileage is at 189256 km. The vehicle starts poorly, and my initial diagnosis points to a heavily discharged or defective battery. I've checked for parasitic drain, but so far, nothing obvious. Could this be a symptom of a failing starter motor or perhaps related to the charging system? I've also considered a voltage drop somewhere in the wiring harness affecting the power supply. Seen anything similar?

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I had a similar issue on my M Klasse (don't recall the exact year, but it's the same generation). Hard starts, weird electrical glitches. Check your alternator output. Even if it seems okay, put a multimeter on the battery while the engine is running and check for consistent voltage. Also, have you load tested the battery itself?

I appreciate the suggestion. When you say 'similar issue', did you also see problems specifically with terminal 30? And regarding the alternator, what voltage readings would you consider within spec and what would indicate a problem?

Yes, it manifested as a terminal 30 issue as well, triggering related fault codes. Regarding the alternator, you should be seeing around 13.5-14.5 volts at the battery with the engine running. Anything significantly below that indicates a problem. If you're not comfortable diagnosing it yourself, a workshop visit might be the best course of action.

Thanks for the confirmation and voltage specs. I took it to a workshop, and it turned out to be a defective battery and a corroded connection on the starter motor. New battery and cleaning up the connections sorted it out. Total cost: 290€

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