Shorted battery on 2020 Mercedes A35, now car won't start

Question

I recently had a short happen to me when I was looking at a broken plastic clip on the ECU housing. I was just trying to get the part # to replace it, so I wasn't expecting to work on the car and need to disconnect the battery.

When I rotated the ECU to see the back of the cracked plastic part on it, a piece of the metal enclosing the ECU hit the positive terminal on the battery and caused an arc/short, and now the car won't start. I can get into accessory mode and see the display on the dash (speedometer/tach) and radio screen so I assume the ECU is OK, but no ignition when I hit the starter button. I checked the main fuse box and they're all fine, except for the starter fuses. There's no power going to them, so I thought it was either a linkable fuse blew, or something in the pre-fuse box.

I tested the continuity at various points on the battery, and the points indicated by either ends of the green line in the picture below. They were good (so the pyro fuse is fine), but the one in red was not.

That leads me to believe that the fuse in the charge sensor (if there is one there) is blown.

I tried to bypass it by attaching jumper cables from the positive terminal to the far end of the pyro fuse as indicated by the blue path in the above linked picture, and also below but the car still wouldn't start.

I'm thinking there could be something else keeping the car from starting apart from the possible dead fuse in the charge sensor. I also checked the continuity on the (125A and 80A) high amp pre-fuses coming up the side of the main fuse box pictured below), and they were OK.

I also tried pulling out the box to the left of the battery with the thick red cables coming out of it pictured below.

I opened it up to test the continuity on the 3 fuses inside (30A, 300A, 200A, pictured below) and they were OK too.

At this point I'm at a loss on what else could be keeping the car from starting. Any ideas on what else I could check to see why the car won't start?

Answer 1

I presume that is the ECU in the foreground of the first picture.

You said that the metal case touched directly on the battery positive terminal.

Since the case was not bolted to the chassis at this point, the current from the battery will have passed through the case and down the ground wires running to the ECU via the ECU connector.

It is highly likely that the ground wires in the connector go straight to the ECU PCB and that the case is grounded to the PCB.

This could have cause a ground track in the ECU to burn out due to the large current flowing through it when you shorted it.

I presume the functions you are seeing in the dash do not need the engine ECU to operate. You could verify this by disconnecting the ECU and seeing if the dash still works.

I would test the continuity of the ECU case to the battery negative. If there is now no continuity, this would back up my theory.

Do this test and report back.

Answer 2

It turns out that it was the ECU that was fried. You can see the part with the melted plastic in the picture below, where the current went through the circuit board