2013 Mercedes C250 — Starter Relay Wiring

Whole car dead, no click?

The starter relay only gets power after the ignition circuit (circuit 15) switches on. If the dash won't light, fix power first:

1. Battery voltage at the posts (12.4V+ healthy) and whether it holds when the key turns.
2. Clean and tighten both battery terminals.
3. Battery ground point near the battery and the engine ground straps.
4. Front prefuse box (right rear of engine bay), including the 400A main fuse.
5. Try the spare key.

General diagnostic order, not from the Mercedes manual.

Starter relay (relay "M", front fuse box, left rear of engine bay)

• Power in: circuit 15 relay → fuse 7 (20A) → relay
• Output: connector 3M pin 4 → starter terminal 50
• Test: key held in START → 12V at starter terminal 50. 12V but no crank = starter/cable. No 12V = relay or its control.