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?

 

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.

 

I need to update this as just changing the SAM did not fix it. The problem came back later the same day.

At the moment I'm focusing on a problem with the CAN network not being operational at the point when you're trying to start the car. I've replaced A 003 546 42 40 - part number for a distributor block for the CAN network. It's like a little hub that all the CAN connectors join up and talk. It is just under the carpet by the left front door sill. Have to take off the plastic tred plate / sill cover and pull back the carpet and then the plastic cover and you can see it. Not hard to access at all.

 

Replacing the CAN distribution block didn't fix it either. I tried two. One brand new from the dealer - only £10 so not a waste for the sake of eliminating it. It was mentioned on another thread as the fix for this issue.

While performing further diagnosis I found a few other odd things. The main power lead into the drivers side SAM (fuse box) was showing weird voltages like 3 or 4 or 6 volts... it should be battery voltage. It was dancing around a bit i.e. fluctuating. I thought that was odd.

Diagnosing inside the control units with the diagnostics tools (I actually have Xentry Mercedes diagnostics and another 3rd party tool) I found an error for 'terminal 61'. This is a small 3-pin connector which plugs into the top of the SAM. One wire goes straight to the ECU and that was fine, one wire goes to the alternator which I couldn't test as the alternator is tucked down under the turbo at the bottom of the engine bay.

After some more work on Google / checking forums I found quite a few people talking about the brush pack on the back of the alternator being a weak point.

Around the same sort of time the battery went flat, that was unexpected as it had been a very strong battery which had never shown any signs of weakness previously. I forgot to mention earlier that I had got to the point where if you turned the ignition on and off enough times it would eventually start. So I went out driving and did a couple of errands. It must have been running on the battery as it was after this that the battery became flat. Then I got a 'battery' warning light on the centre display and when it did crank over it was barely turning / would not turn... definitely a flat battery.

From here I thought as I've had the battery a while I'll replace it. It's always good to have a fresh battery so I did that. I also assumed the alternator would be the cause of the battery being flat. So I pre-ordered an alternator on the premise that I didn't have to buy it if I didn't need it.. just so it would be at the shop for me to collect it if necessary.

After replacing the battery it had the same problems but with a fresh battery eventually I got it started and moved it to my garage with a plan to change the alternator. So went and got the alternator and put it on.

Some of the other forums have said the wires could be bad between the alternator and 'terminal 61' so I made sure I checked those wires during the replacement of the alternator.

The other potential issue which was mentioned is that on dual-battery cars like mine that the relay which switches the auxiliary battery on and off can go faulty. So during the work I also tested the relay off the car, seemed fine. It's a very basic relay, just the 4-pins and nothing fancy going on.

Since changing the alternator it has started fine 3 times. First start it was a bit grumpy having had all the control units offline while the battery was disconnected but I turned it off and back on and was all sweet. I've started it again after leaving it overnight and then again a few hours after that. It has started straight away every time.

I'm hoping that is it now... i.e. the alternator was faulty. Will update again if anything changes.

 

It's a good discipline to check battery and alternator health as your first step. I'm always amazed as to how many issues are solved almost immediately due to low voltage coming out of either and the mistaken belief some have that a battery with 11.5v is OK or that with engine running that less than 13v is good. Some modules on these cars hate even a small drop in voltage, and thats most likely why you had the ABS/ESP warning, but I get you can't easily test the alternator if the damn thing won't start.

Might be good preventative maintenance to check the grounds on the alternator and starter as well to ensure good constant contact. Glad you found your solution.

 

There's no alternator ground for this car as it's not bushed. It is solidly fixed to the engine block with 4 screws so no need for a ground strap. I tested the main alternator cable to make sure I had contact between the alternator end and the positive terminal at the top of the engine bay. Some models have a fuse in that cable before it gets to the starter and sometimes it fails so made sure to check that.

What's misleading about this fault is that the alternator itself isn't faulty, it's the brush pack on the rear of the alternator which has given up and not performing reliably. In older cars where there was no 'intelligence' in the charging circuit the only thing which could really go wrong was for the alternator to stop charging and the red light would come on the dashboard... then you know you have an alternator fault, nice and simple.

In the case of the W211 (especially with two batteries) because the alternator communicates through the ECU and the front SAM there is a layer between you and the alternator which confuses the standard signals. The car didn't even know it was experiencing a charging problem, it just ran out of battery power 60% of the way through the diagnosis. The only thing which made me realise I had an alternator fault was the battery going flat. It did show the red battery symbol occasionally but not when the engine was running and I was driving about in it.

Some people in some other cases just replace the 'brush pack' (it's more than a simple brush pack on this car) but I thought I'd rather not do the job twice so would be a good idea to do the whole alternator. If you prefer being cheap you could probably just replace the brush pack and that would also fix it.

Edit: I suppose the other thing clouding the waters is the fact that I had already opened up the ECU on the car last year and did some soldering on the board myself... so I had wondered whether or not my soldering had been substandard and caused the ECU to fail... hence getting the ECU reconditioned as the first knee-jerk reaction. The fault actually is that the ECU doesn't wake up because it relies on the alternator to tell the car that it is receiving a signal from the SAM that there is the intention to start the engine. i.e. the signal comes from the ignition control unit to tell the SAM to tell the alternator to tell the ECU to wake up. You can see the problem!

 

The brush pack is also known as a voltage regulator and it quite easily replaceable. If you have charging issues, also check the BCM (Battery Control Module) and the AUX relay in the front (if you have a 2 battery setup). The BCM in the E-class is known for water ingress. In the end I bought a bluetooth battery monitor for both my batteries, so I can keep an eye on the situation from the ease of my phone. They can be found on amazon for a couple tenners :
I made a simple how to video for replacing the voltage regulator on W219 chassis

 

It does not seem to be a problem with the alternator either. So I've spent quite a lot on parts which weren't the cause. Around £800 so far.

Next suspect is the battery control module. It's pretty frustrating that this fault is intermittent. I think it's unlikely to be a broken wire as the car will run fine if you get it started in the morning then it will start every time after with no problem for the rest of the day. Normally broken wires work the other way around. i.e. when things get hot the resistance goes up and the weak connection fails - I'm not getting that kind of behaviour with this fault. It is difficult to start first thing but after that it will keep going absolutely fine for the rest of the day.

 

Just plug-in the voltage monitor into cigarette lighter so you can monitor the battery voltage in real-time. at least you are sure is not charging related issue. Normal reading when running on my CDI is 13.9V- 14V since my main battery is old and weak (I have new AUX battery).

 

The essence of the problem by this stage is that turning the key to position 2 activates much of the circuity that it should. Lights come on, radio, etc, etc... but the glow plug light isn't on and when you turn to position 3 to start nothing happens.

If you cycle the ignition switch a lot of times... 20 to 40 times (easiest is to use the Keyless Go button on the gear lever) you will see the glow plug symbol pop up and then you know it will start.

The problem with it not charging is part of the symptoms but not the cause as I've done more than 700 miles in it since this problem started and although one battery did go flat I think it was more to do with the age of the battery and the new battery hasn't gone flat.

 

Replaced the battery control module yesterday and tried it again this morning and the fault is still there.

I guess the next place to check is the fuse / terminal block behind the brake pedal where the main battery lead connects to the rest of the circuitry. I think that's a junction point and some damage or corrosion there could be causing this.

It did seem to start quite easily this morning but that's still within the nature of this fault and it definitely didn't start first time.

 

It's been just over a month since last update on here and I've been using the car daily as my other Mercedes got hit by some fool with a dodgy trailer and I'll have to wait to do the repair on that.

What I've noticed over time is that this fault only occurs when it has been raining. I've no idea where to even start in terms of figuring out why it is affected by moisture. I guess there's some marginal connection somewhere which is affected by humidity. As far as I can tell there are no vital electronics which are bathing in water.

If it is experiencing the issue it'll start if I keep hitting the Start/Stop button on the gear lever for about 5 minutes.