ML 320 CDI 4matic won't start

Hello.

"I'm having a problem starting my car."
The vehicle: a W164 ML 320 CDI 4matic.
As soon as I turn the ignition key to the start position, I only hear a single click from the engine, and then nothing happens. The battery, voltage regulator, alternator, and battery terminals themselves are all new. The diagnostic system also shows no error codes.
"This clicking sound when trying to start only happens once, and on the next attempt to turn the key, you only hear the actuator motor for the turbocharger."
Before the vehicle completely stopped starting, he experienced issues where he sometimes had to turn the key multiple times for the engine to ignite, causing the starter motor to engage. The battery voltage was 13.5 volts.
Recently, we jump-started the car, and it started right away, but that's no longer the case.
I'm really desperate and would appreciate any help.
Sincerely,

Good morning!

It sounds to me like the starter motor might be having problems.
I would start by taking measurements there. Does voltage appear when starting?

Regards,
pretzelman01

I will measure it later and get back to you .

Okay, I finally got around to taking measurements, but unfortunately, I can't access the starter motor. That's why I tried measuring the relay, but that didn't work either because there's a resistor built into the relay.
Voltage is 12.6 volts at the vehicle's electrical system.
Sincerely,

Hi,

It sounds like a faulty magnetic switch on the starter motor. Or a loose or corroded ground strap connecting the chassis to the engine. You could measure the (electrical) resistance from the engine to the chassis. If it's close to 0 (significantly less than 1 Ohm), then the magnetic switch (or some other relay along the way) is likely the problem.

Best regards from Berlin.
olli