W124 - ECU or Coil?

[maxicab]

I seem to be going down a well worn path here, judging by others' threads here. I took my W124 95 E320 Cabriolet with a very intermittent misfire problem to a local but well regarded Bosch garage for a second opinion after the not so local Merc specialist quoted me £950 for a new engine wiring loom after what seemed a pretty cursory examination.
The car would run fine most of the time, but without warning, it seemed that 2 cylinders would cut out for a few minutes, then restart for no particular reason. It happened once when stone cold after a few days of non use and cold weather, but another time when well warmed up. To cut a long story short, they (Bosch guys) tried and failed to do a cheaper repair, and I ended up paying £900+ anyway for the diagnostics, new loom with fitting and VAT. They checked the cat after all was done and said it was fine. All was great for about 3 weeks, but has since then almost exactly done the same misfiring trick 2 further times now. I've taken it back to the local garage. They say they took it all apart, tested everything, checked all the coils and wiring, found nothing amiss, put it all back together again, fired it up and remarkably, the car was only firing on four cylinders (the front 4 I think). They decided to swap the coil from the failing cylinders to the front cylinders (and the front to the rear), expecting to prove the coil was at fault, but even more remarkably (considering how rarely the fault had occurred), the fault persisted in the rear 2 cylinders, implying to them that it was the ECU at fault, not the coil.
So, time for questions!
Is the ECU necessarily faulty or could it be shutting down those cylinders correctly because of, for example an injector/fuel supply problem?
If the ECU is to be replaced/repaired, should one or all of the coils be replaced as well, as some previous threads here suggest that the coils will likely have been damaged by the problems from the faulty wiring loom? Can such faults be detected, or should the coils be replaced just in case? (any sources other than GSF at £40 each coil).
Is there any way they could mis-wire the loom?
Googling E320, ECU leads me to several auto electronic specialists (eg East Anglia, Dorset) who offer to repair Merc ECUs for around £80 - £120 or so. And some even have no fix, no fee. Anyone any experience of these places? I saw a thread here about a couple of rip-off places I won't be trying. Did the legal action against one of them ever succeed? I respect Alan@ECUs opinions here, but £280+ for soldering in a couple of transistors seems a bit hefty. Someone a year or so back was going to try his own ECU repair with a couple of output transistors for £3.95. Anyone know if he succeeded? I do have the gear if I can get the parts, but only if I know what the fault is, and I can't test it, so this route isn't high on my preference list
And is buying ECUs from wreckers a problem with respect to ECUs being coded for specific vehicles really a problem for any 124s? Anyone have any other source of ECUs, new or old?
Thank you for your patience, David (I'm losing mine)

 

[Ellsy Tanners]

If you have swapped the coils about and the fault didnt switch cylinders then you would say that rules them out.

I belive the system works on the wasted spark system and the two rear cylinders arnt fired by the same coil. so if cylinders 5 and 6 arnt firing then it would inpile something other than one coil, as they arnt fired by the same coil. I hope i am right in saying this its been a while since ive been involved with the 104 engine.


is it possible its a fuel injection fault.

 

[maxicab]

E320 W124 Ecu or coil

Well thanks for your reply Ellsy.
I've done some more research since I posted, and it seems it could be the rubber connnector (no.8!) that connects from the ignition lead to the coil. Is this right?
The fact that the fault is so intermittent is significant, as either an ECU or Coil fault (or dare I say it, having just paid a fortune out for it, a wiring loom fault) should be more or less continuous (is that correct?).
You may well be right about the actual cylinders affected. I was going on what I was told over the phone, relayed not by the guy who did the testing but by the garage manager. But yes, they did swap the coils over, and the fault persisted in the same 2 cylinders regardless (whichever ones they were), implying it's not the coil at fault.

I don't know if it's a fuel fault or not, I was just hypothesising (and hoping it mightn't be the ECU!)

Cheers, david

If you have swapped the coils about and the fault didnt switch cylinders then you would say that rules them out.

I belive the system works on the wasted spark system and the two rear cylinders arnt fired by the same coil. so if cylinders 5 and 6 arnt firing then it would inpile something other than one coil, as they arnt fired by the same coil. I hope i am right in saying this its been a while since ive been involved with the 104 engine.


is it possible its a fuel injection fault.

 

[Ellsy Tanners]

Maybe David, a poor contact at any of the conections weather it be the plug lead or plug caps or the connector at the coil, could cause a intermitant misfire. To be honest its very hard to say without actualy looking at the car and seeing wots happerning as it is going wrong.

 

[Juddian]

If its the ecu faulty, please be careful who you send it to.

I will recommend without hesitation the chap on the Isle of Wight, no he isn't cheap but he can do the job (two 124 320's in my case).

One was the almost identical situation to yours, the other was a corroded connector that needed a new 48 (i think) pin connector soldering in, both good long term repairs.

 

[MotardMan]

If the loom was faulty, then there is a chance the coils, and the ECU may have been damaged. If you are certain the coils are OK, and the connections to and from the coils and plugs are OK, then I would be thinking the ECU has to be the issue here. Intermittent faults can be caused by heat cycling and so on, and are always a complete bazza to tie down. It seems you have gone as far as you can without spending more cash on another fix. I'd bank on the ECU being faulty, and follow the repair route.

Sorry it is not better news but your diagnosis and lengthy spells in the garage with car seem to point this way.

 

[Dave Chaucer]

Alan at the ecuclinic appears to offer good and reliable repairs to ECUs and complete with a 12 month guarantee. Does anyone who has used his services have any glowing reports?

Who this chap on the Isle of Wight?

 

[Juddian]

Hopefully this is the chap on the Isle of Wight.

http://www.carelect.demon.co.uk/index.html

 

[maxicab]

Or should I go for this second hand ECU offer from Dronsfield - Thanks to all who have contributed to date. Sadly it does seem like it's probably the ECU. The garage seem convinced that it's not the HT plugs or connectors or leads. They want to send the ECU off to a company called ATP for benchtesting. Anyone heard of them? I'm a bit inclined to pick up the car and take it on myself now.

YOU HAVE RECEIVED THE FOLLOWING QUOTE FROM: Dronsfields Mercedes Ltd
PHONE: 0701 312 4510 Please call between 8am and 6pm Mon-Sat
View feedback about this supplier - click the link below
http://www.breakeryard.com/view_supplier_feedback.aspx?count=SWUMBQFUJ
Your quote consists of 1 part(s):
----------------------------------------
Part Request Ref: 1079347
PartType : Other
Part(Details) : ECU (ENGINE MANAGMENT),  
Part Condition: USED
Price : £200
Part Warranty: 90days
----------------------------------------
Additional Information: CONTACT RICHARD IN PARTS 01457871717 OR richard.suthers@dronsfields.co.uk
Delivery : £10.5
VAT : £36.84
Total price : £247.34 (inc VAT and Delivery)


So it probably all works out about the same price either way.

Any comments

Cheers David

 

[Juddian]

David, i'm going by memory here, i think it depends on whether your 124 has the 'scorpion' type of alarm immobiliser or the infa red with the red and green lights on the drivers door and boot.

If you have the infa red, then the ecu is coded to your vehicle uniquely and if you require a replacement it gets extremely complicated and expensive, if you have the other sort of immobiliser, then that could possibly take a used ecu.

My repairs were one of each type:

When my coupe failed (infa red), that had almost identical symptoms to yours, if the ecu had not been repairable i was looking at a new unit supplied pre programmed by merc at somewhere in the 12 to 1400 squid bracket, thats why i am suggesting care in where you send a faulty unit to.

If you send your unit elsewhere, and they tell you it is scrap, insist it is returned to you, i dont need to elaborate i'm sure.

And do note the serial numbers of your unit, and if poss take pictures to make sure you get the correct one returned. I shall say no more.

 

[maxicab]

Thanks very much. That's exactly what I needed to know. Mine IS the infra-red type. I'll be very wary. Regards,

David

 

[maxicab]

All fixed now. It was the 3 long HT leads after all! Bought a new set from Euro Car Parts, cleaned everything up and its been purring beautifully ever since. Saved myself a bomb by not trusting the garage to send the ECU off.
Thanks for all your help, David

 

[Juddian]

Very pleased for you David.

Had a interesting conversation with my indy at the weekend, it seems now we've spent out many hundreds of pounds that merc have produced a repair kit fror the troublesome part of the wiring loom on our engines.

In some ways i wish he hadn't told me.

Regards

 

[turnipsock]

It's only the few wires that run along the top of the engine that get subjected to a bit more heat than the rest of the loom. The insulation then goes hard and breaks apart. I'm guessing this is what the not so local Merc specialist thought was going on...sometimes if the wires short out then it damages the ECU.

 

[Bolide]

The Cabrio has a unique ECU and they are seriously rare & expensive

There is an MB repair kit for the of the coil pack wiring. I don't know if it's for the LT or HT side

I am also told that there's a little rubber stand-off that the coil pack mounts to and these can cause a misfire

Nick Froome
www.w124.co.uk

 

[maxicab]

I spoke too soon!

Well, would you believe? After running very well for over 2 months (admittedly not that many miles), it's misfiring again!
Started it today after not run for 3 days, (hot days, sitting in sun) and it's misfiring from start-up.
Could an ECU be that intermittent?
Could it still be something (anything!) else? I'm not sure of the part that Nick is referring to in the last post - do you mean the little rubber (HT lead equivalent) plugs that goes from under each coil to the spark plug below? And the MB coil repair kit that you refer to - is that kit including the coils themselves, or just the connections (I assume it must be the LT side as the HT side comes as 3 HT leads you make up yourself, and 3 rubber like coil to spark plug connectors as I mentioned above.
I didn't realise the E320 Cabriolet ECU was different to any other late model 124 E320 ECU - that makes it sound like it will have to be repaired not replaced if at all possible.
Nick do you have a local MB auto electrical expert you can recommend as I'm very near you (in Lewes)? I could probably take the ECU out if I can work out how to do it and send it off to the IOW or somewhere, but I'd rather take the car somewhere not too far away and let them see it running (perfectly of course).
Thanks, David

 

[Juddian]

David, i feel for you matey, it galls even more when people say how good the 124's were.

Yes they are lovely when they're running right, but by heck they can be a pain.

The ECU is dead easy to take out, i should know, i can do it in my sleep now!!!

Remove battery, remove plastic waterproof shield behind battery.
Undo and remove abs module, then the carrier plate for the abs module.
Undo two bolts/large philips bolts that hold the ecu, and fold back the clamps that hold the multi connectors to the ecu and withdraw it.

I hope one of the lads can diagnose this for you in your area, but i still have a feeling that the ecu will have suffered, quite why merc couldn't have put something in the wiring to protect the ecu in the event of coil failure or wiring fault is beyond me.

One other thing, my E320 estate wiring had been hacked about by a merc fitter at some point (only ever been dealer serviced before i bought it), and the waterproof rubber seals over the 2 multiconnectors on the ecu had been cut open, allowing water to enter the connector.
This eventually meant that 1 of the connector pins on the ecu (there's about a hundred) had corroded away and was making an arcing connection only.
Chap on the IOW replaced no problem, but it could be worth having a look to see if the connectors are well and fine.
Only takes about 15 minutes to get the ecu out anyway.

Good luck with it.

 

[tricky100]

Hi, sounds like you have had a right time of it with your 124 . I brought mine a few weeks ago , E320 coupe , my first Merc . I had heard that there was a known fault in the engine loom , but i didn't realise to such an extent untill i read this thread .
Anybody know if this repair section of loom is actually available from a dealer and if so where does it run between as i wouldn't mind buying and fitting it myself as a preventative measure ? or would it be simpler to locate the wires that get damaged and shield them from the heat better if this is the cause of the problem ?

 

[rayhennig]

Start simple ...

David, i feel for you matey, it galls even more when people say how good the 124's were.

Yes they are lovely when they're running right, but by heck they can be a pain.

The ECU is dead easy to take out, i should know, i can do it in my sleep now!!!

Remove battery, remove plastic waterproof shield behind battery.
Undo and remove abs module, then the carrier plate for the abs module.
Undo two bolts/large philips bolts that hold the ecu, and fold back the clamps that hold the multi connectors to the ecu and withdraw it.

I hope one of the lads can diagnose this for you in your area, but i still have a feeling that the ecu will have suffered, quite why merc couldn't have put something in the wiring to protect the ecu in the event of coil failure or wiring fault is beyond me.

One other thing, my E320 estate wiring had been hacked about by a merc fitter at some point (only ever been dealer serviced before i bought it), and the waterproof rubber seals over the 2 multiconnectors on the ecu had been cut open, allowing water to enter the connector.
This eventually meant that 1 of the connector pins on the ecu (there's about a hundred) had corroded away and was making an arcing connection only.
Chap on the IOW replaced no problem, but it could be worth having a look to see if the connectors are well and fine.
Only takes about 15 minutes to get the ecu out anyway.

Good luck with it.

Hello Max,

I haven't read your tale in detail but my impression is that you're looking at the exotic and expensive issues before the cheaper ones.

My old 300 idles and generally purrs along following replacement of spark plugs (Bosch, less than £30 as set), rotor arm, distributor cap and complete HT lead set.

Are you sure that all these parts are ok?

The single greatest step I made to smooth running was to remove the £70 quid's worth of exotic spark plugs supplied my my MB dealer and replace them with Bosch ordinary plugs.

The very best of luck to you.

RayH

 

[maxicab]

Hi thanks, yes, I have done all those things. But you might be on the right track and I might not have a faulty ECU. I think I may have found the problem........With the lovely weather and the car pretty much misfiring continuously for once, I took the coil cover off and ran it as I jiggled everything I could find. The ECU jiggles caused no problems, but as I jiggled everything under the coil cover, I could eventually turn on and off the misfire by wiggling the rear coil/connections. Eventually the penny dropped after checking everything electrical connection wise that I could find was tight. The little grub screw for want of a better word that holds each of the coils in place in one corner of each coil, is also the electrical connector for the coil to the earth of the engine. And the rear screw was a little bit loose! Apart from a jolt from the HT plug, I was delighted. It all made sense and the grounding obviously wasn't reliably occurring through the other 2 screws which don't necessarily touch on the metal of the coil body. Amazing this isn't a more common problem, as these screws are pretty fragile and in fact I've ordered 3 new screws from MB as the screwdriver slots in mine were all damaged. Here's hoping I've solved it.

David