Mercedes ML350 – From No Crank to a Burnt ECU

Mercedes ML350 – From No Crank to a Burnt ECU

The car was towed in to us from out of town. The first thing we did was connect our AUSCAN scan tool to look for codes.

We found faults stored in nearly every module, all showing a familiar pattern — low voltage. The battery measured just 10 volts. That’s too low to do any meaningful testing, so we removed it to put on charge overnight.

The battery itself was a nightmare to remove. Hidden under the driver’s seat, the seat had to be moved fully forward and raised to maximum height for clearance. The battery’s size made it nearly a two-person job to lift out.

Day 2 – A New Battery Doesn’t Fix It

The next day we tested the charged battery — it held some charge, but wasn’t ideal. The customer supplied a new battery, which we fitted. We cleared the codes and rescanned, but the same low-voltage codes came straight back. That meant there was still a hard fault somewhere in the system.

Time for Wiring Diagrams

We pulled up Autodata wiring diagrams, looking for any connection between the modules with faults — especially in the starter circuit. That’s when we spotted a fuse between the main relay and the starter relay.

The starter relay was missing power at terminal 85, which traced back to terminal 30 of the ECM main relay. The only thing in between was a fuse.

We located the fuse — blown. Replaced it, and it popped instantly. That’s a dead short to ground.

Narrowing It Down

We swapped the fuse for a 5-amp test light. It lit up bright, confirming the short. Back to the wiring diagram to see what components lived on that circuit.

The easiest to check was the engine control module (ECM). We unplugged it — the test light went out. The short was gone. That meant the problem was inside the ECM.

The Smoking Gun

Something had caused the ECU to short internally. Once removed, the familiar smell of burnt electronics hit — unmistakable.

At this stage, we can’t say for certain what caused the failure. The ECU on this model sits right on top of the engine, which exposes it to a lot of heat over time. It’s also possible the vehicle was jump-started before it reached us, or that a hidden wiring fault — like a rubbed-through cable — shorted to the ECU and could do it again once the new one is installed.

The Fix

The next step is to either repair the ECU or replace it. A repair is often the easiest path, as replacement means either coding a new unit or cloning a used one to match the car. We’ll also be inspecting the wiring harness closely before fitting the replacement, to ensure the root cause won’t repeat itself.

Takeaway:Even with a new battery, a modern car can hide a deeper electrical fault. This case shows why structured diagnosis beats guesswork — it saves time, prevents unnecessary parts, and gets straight to the root cause.