By Northcap – Patrol Mechanic & New Owner of Rustyautos.com
You get used to weird breakdowns in this job, but this week, I got called out to one of those head-scratchers that reminds you how picky modern cars can be.
I was called out to a 2019 Mercedes-Benz. The owner, who runs a small limo service, said it simply wouldn’t start. Key was recognized, dash lit up, but no crank.
No immobilizer issue—so I popped the hood and took a look.
First Clue: The Headlamp
Right away, something caught my eye: the right-side headlamp lens was cracked and slightly fogged. The DRL (Daytime Running Light) on that side wasn’t working. That alone wouldn’t normally stop a Benz from starting, but it’s always worth asking questions.
I turned to the owner and asked, “That headlamp—new damage?”
He nodded. “Yeah, broke it two nights ago, but it was working fine.”
“Drive in the rain since then?”
“No.”
“Wash it?”
“Yeah, yesterday—but the light worked fine after.”
“Still,” I said, “it’s not working now, and you’ve got a no-crank issue.”
Second Clue: Low Voltage
Next step: battery voltage. Hooked up the multimeter—10.5 volts. Way too low for the girl from Stuttgart to even consider waking up the starter. These newer Mercs are extremely voltage-sensitive.
I hooked up the jump pack and she fired right up—no hesitation. With the engine running, I checked the charging system. All good. Did the usual checks—no interior lights or exterior lights left on. After a few minutes on charge, the battery looked healthier. Just to be sure, I ran a battery test—it passed.
So what’s going on?
Suspect: Water Ingress
Back to that broken headlamp. Even though the DRL, main and dip beam were not working, the bigger issue might’ve been what I couldn’t see: moisture inside the unit. Water ingress is likely to have caused an internal short; that’s a prime suspect for a parasitic drain when the car is parked.
To play it safe, I disconnected the headlamp entirely and told the customer to head straight to the dealer and sort the unit before it turns into a recurring issue.
He nodded—but winced when he told me the price. Dealer quoted $3,000+ fitting for the replacement lamp. Need deep pockets to drive a car with a star.
My Takeaway:
Modern vehicles are more computer than car, and a cracked headlamp can do more than mess up your night driving—it can cripple your electrical system. When in doubt, always investigate visible damage and never rule out how something seemingly small (like water in a light housing) can lead to something big (like a dead car).
If you’re dealing with odd electrical problems or no-start conditions after a recent repair—or car wash—it’s worth checking what got wet.
Related Reads:
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Meet Northcap, a seasoned mechanic and roadside assistance expert with over 20 years of hands-on experience. At Rustyautos.com, Northcap shares exclusive insights into what really breaks down in cars, offering practical tips and real-world advice to keep drivers informed and their cars on the road.