Out of the blue on Monday my 320 CDI 2004 refused to start. Turn the key = nothing. It has keyless start and that does not work either. So not the physical ignition switch.

The dash lights come on and all the other functions of the car are working. Just won't crank. Since the problem occurred it has started once and ran lumpy but after turning off it hasn't started since.

I used a diagnostics tool to check it out and communication with the ECU seemed to be a problem. The same problem was showing for the instrument cluster and ABS / ESP systems. They could not communicate with the ECU either.

So, I assumed defective ECU. Took it to a ECU technician and they said it was faulty and reflowed the solder joints / tested and it supposedly now has a years guarantee. So I've ruled out ECU fault at the beginning. I'm a bit hacked off about them diagnosing the ECU as faulty as it clearly wasn't if the exact same symptoms still remain. I have replaced the ECU once before to fix a problem with the glow plug circuit so still had the old one. Plugged in the old ECU and the symptoms are still the same.

I've replaced all the relays in the engine bay fuse box as I have a complete full set and I checked all the fuses in the engine bay and side of the dash and all seem good.

When you turn it to position 2 and try to start you get no response from the starter motor. The cooling fan starts to run at full speed and the dash warnings start coming on for ESP / ABS.

I have seen in various other threads that this is likely to be an issue of power to the ECU but I've not yet tested this. Will check tomorrow.

In the meantime, this seems to be a very common issue but none of the other threads I've read have stated fixes. Some of them said ECU but I've got two and neither of them work.. Does anyone have any ideas / knowledge of previously solving this issue?

 

quiet possibly in your country its a common problem, but in the goodol USA it aint-----what amazes me is that you assumed the engine ECU was faulty because it could not communicate with various modules within the vehicle. Did you ever think that the lack of communication was caused by the ESP and or ABS system and not the ECU.
The first thing I would do is change mechanics to someone who knows MB and has their diagnostic system, your man may be good on bicycles but not MB. The second thing is to find a wiring diagram for the ESP and ABS systems and have your MB mechanic start the diagnostics

 

This is symbolic of a dead battery. The low voltage is causing the other faults. Have you checked the voltage and or replaced the battery. Was there a fault for the cps? Have you tried shifting to neutral and back to park to see if it’s related to park interlock?

after a dead battery, you have to rotate the steering wheel all the way to one direction and then the other to reset the esp and ABS. The communication with the ECU is likely due to insufficient power as you suggested.

 

Hi, I don't use mechanics. I'm an ex-mechanic and I do all the work myself.

Think about what you're saying.... you're saying it's not 1 module (i.e. the ECU) it must be several others which are all coincidently faulty at the same time? Naa! Don't forget, ABS, ESP and Instrument Cluster are all separate modules. They all need to communicate with the ECU to be able to work. The ECU is the common denominator in all of this. As of right now I'm assuming since the ECU has already been given a clean bill of health the problem must be with the wiring in the car.

 

That was my first thought. I've got small children and they often get into the cars and turn on lights without telling anyone and you only find out when you try to start it next.
I measured strong voltage at the terminals in the engine bay and there aren't any other signs of a degraded battery. Wipers move fast, all other systems seem operational. Also... it did start once since the problem first occurred. Only once and while it was running it was lumpy as hell. The rev counter was bouncing on and off and other parts of the instrument cluster were flickering.

I think there's a power issue somewhere.

I've got a spare SAM (the fuse box) from a previous diagnostics process which wasn't faulty last time. So I'll swap that in. Doing that swap will involve disconnecting and reconnecting everything so I can look for trouble while doing that.

 

Changed the SAM (front fuse box) and now it's working.