W124 Wiring Harness Woes...

[howeshouse]

Finally bit the bullet and decided I needed to replace the wiring harness in my 1995 E280 to try and fix the intermittent misfire. Looked on the web, and was seduced into thinking this was a task that could be managed myself, so bought the thing from the local dealership and got the bonnet open... actually, most of it wasn't too bad, but there are two connections that I can't reach at all - looking at the car from the front, both of these are on the right hand side, one about half way back buried in between the manifold and all sorts of other gubbins. I can actually touch that one, but can't get two fingers on it to release the clip. The second is at the back of the engine, and I think reachable from underneath the car, except that I am too fat to get under far enough to know for sure. I am planning on sending my neighbour's small child under there to see if they fare any better...

Thing is, the car won't fire at all now: the engine turns over, of course, but no sign of ignition. So what I need to know is: will the lack of one or other of these two last connections prevent the engine from starting, or have I caused some other damage during the installation, or was there a problem with the wiring harness in the first place? Whichever, I suspect trailering the car to the local garage/dealership to get it fixed is going to far outweigh any putative savings in fitting the thing myself in the first place, but at the moment the car is just so much drive-jewellery and I am going to have to decide what to do soon one way or another.

Any advice at all would be extremely welcome! Many thanks, Adrian

 

[kth286]

You are talking about the knock sensor/s (there are two) on the side of the engine, and it is the crank position sensor on the bell housing.

If it is just these two sensors you have to connect then jack the front of the car up on that side and remove the front wheel and gain access through the wheel arch recess areas.

The car will absolutely NOT start without the crank position sensor connected. It is the number one most important sensor which tells the ECU that the engine is in fact turning. the ECU will then do it's stuff.

You really should have the battery disconnected and you should not be trying to start the car with these two connections loose.

I trust you gave the VIN to the parts man and he asked you about the spec of your car and the options that were built into the car. I say this as there are several different looms.

So, in conclusion you are nearly there with just two connections to make. No need to take to garage.

However, at the start of your post you said the car was missing intermittently. That is not the normal problem with wiring harness faults.

It is the coil feeds shorting out and you will loose spark to two cylinders permanently.

You should normally have purchased a new coil with the loom, ready to replace the one which will have blown, as they all do when the connections are shorted.

I will be very interested in your outcome.

Please let us know it is the only way we learn more.

 

[howeshouse]

w124 Wiring Harness Woes

Hadn't thought of taking the wheel of to gain access, so thanks for that! Was just adamant I wasn't getting underneath the thing with just the jack to support it... Yes, I did give the dealer the VIN number, but I was still very reassured to find that the new harness had the same number of connectors as the one that was coming off as in the past I have ordered replacement keys using the same VIN number which didn't fit (dealer then tried to tell me I needed to buy a whole new set of locks because they must have been changed at some time in the past. Managed to persuade them to get another key using the same VIN which then worked perfectly, of course.) I've replaced all three coils over the last few months: the first one when the misfire started, which was definitely coil-related and there all the time so relatively easy to diagnose. The current problem is not there from a cold start, and seems to only start once the engine reaches a particular temperature - it is reliably reproducible within minutes of hitting traffic with a lot of idling. I am not at all sure that changing the wiring harness is going to fix this, unfortunately, but it had certainly perished/(decomposed) to a large degree in and around the coils. I'll report back - hopefully with good news - as soon as I have can coincide enough time with sufficient daylight to get the wheel off and those two remaining connectors on. Thanks again for the encouragement.

 

[howeshouse]

Well, spent an interesting couple of hours not quite under the car this afternoon following your advice. The plug at the back of the engine was really easy to reach with the wheel off. The one in the middle was still rather more of a challenge, and in the end I only got the old plug off by getting a screwdrier into the wire clip at arm's length and prising it off the plastic cover block. Then it was just 15 minutes or so to get the new one in, another 10 minutes to get the wheel back on and off the jack and it started first turn of the key! At the moment, running as smoothly as I have ever known it, but I don't know if the misfire itself is actually fixed because if was rather intermittent anyway, but so far so good. I was half hoping that it would fix the cruise control too but that is as dead as ever, I'm not about to complain, though. Thanks again for your help, Adrian

 

[Juddian]

Well done that man, we had the coupe loom replaced earlier this year, was a time when i would have done those jobs meself but i'd rather leave that sort of thing to my top indy.

You've been very lucky by the sounds of it as when our loom shorted out it spiked the ecu, thanks a bundle again to the designer!!

Luckily the chap on the Isle of Wight is jolly good at sorting out errant ecu's so it wasn't as drastic as it could have been.

Award yourself a cigar for a job well done.

 

[howeshouse]

Thanks! But... the original problem is still there. I'll post the symptoms in a new thread for some advice.

 

[J-Scho]

Mind if I ask how much these wiring looms cost? Presume you got it from the dealer?

 

[kth286]

In round figures it is £550 depending on which loom part number.

The correct loom is dependent on the options built into the car ie. auto gear box, ASR traction control etc etc. Merc only part.

If you are a DIYer it will take about a day to remove and replace and to ensure a neat job is done.

Should last more than 10/12 years of existing looms as new design is a better product.

Have you now got your 124 car ????????

If you have no history of it being done, then IT WILL need to be done.

I will make itself known by blowing one of the three coils over the spark plugs under the central engine top cover.

It is the feeds to the coils where the insulation breaks down because of twisting when the plugs are changed, and the feeds short on start up.

You can replace the coil and it may be OK again for sometime BUT it is likely to blow the ECU output driver, which is a load of hassle and cost you do not need.

 

[Legoman]

thankfully mine has the old coil style so that isnt an issue ,also the loom seems good in mine , i feel sorry for those with loom problems of later cars , definately a decline in quality back in the mid 90's

 

[kth286]

It was mercedes trying to be too clever by half. It was meant to be biodegradeable so the story goes.

Still prefer to have the all electronic engine though. Hate all that electro-mechanical stuff when it goes wrong. Stuff has to be within a fraction of an inch in tolerance or you get misfires etc etc and it gets gummed up etc.

With electronic, it usually works or does not work. And with the ease of checking the diagnostic trouble codes with a home made tester (which I carry in the boot with a list of 50 codes) then its all generally very simple.

 

[Legoman]

its not that hard to diagnose the older coil setup tbh , id rather have to replace that than the loom!. and if you saw the state of the dizzy cap in mine before i replaced it , tolerances are huge! , how on earth it still ran is beyond me to this day lol.

 

[Apial]

With electronic, it usually works or does not work. And with the ease of checking the diagnostic trouble codes with a home made tester (which I carry in the boot with a list of 50 codes) then its all generally very simple.

If its so reliable why would one need to build such a device??

 

[rmt6668]

Hi

I have this Engine Wire Harness for W202 C220 1994 model. I am unable to reach these 2 wire. Can any 1 share some light what to connects to?

Do I need to hoist vehicle to access these connectors?

Thanks

 

[allan l]

Howeshaus,

I did not fully cure the ignition problems with my '94 E320 Coupe until I also changed the Cable Plugs, (item 8) shown on this spares list :-

http://www.detali.ru/cat/oem_mb2.as...GM=717.450&CT=M&cat=520&SID=15&SGR=060&SGN=03

Have you tried this ?

 

[allan l]

Howeshouse,

It should say item "eight".

 

[howeshouse]

Next on my list of things to try, thanks! Was expecting to be able to get a "full set" of the 6 ignition leads without any difficulty, but nothing so straightforward: still can't quite believe the main dealers have to make up a set of the three leads other than the "item 8"s from the component parts... did they not make very many of these particular models?!?

 

[kth286]

Do not bother with the leads at this point as unlikely to be a problem.

It is the links under each coil that causes the problems.

I have changed the coil links, but never changed the leads in 161,000 miles and everything is fine.

 

[rmt6668]

Hi

Attached the Engine Wire Harness 202 540 3132. The insulation falling apart after 10 yr, some exposed are touching each other.

What type wire should I use? Is it by the gauge or the amps? I went to autobarn, repco & supercheap auto their automotive wire are firmer unlike dicksmith, tandy, jaycar, more electronic. May not be heat resistant.

I notice & confirm a seperate harness (much easier to repair) which connects to the 2 coil packs are touching on both. Saw attempt to sticky tape the exposed section but hardly done properly. This may be also the cause. But might as well overhaul the whole wiring issue & no issue further down the track.

 

[howeshouse]

Well, it may have been a combination of the wiring harness, the coils, the direct links between the coils and the plugs, but it seems to be finally fixed now that the three leads from the coils to other three plugs have been replaced. Very glad to report that after a week of cautious optimism, my wife is now sufficiently reassured to drive it again! Many thanks for your help and advice over the past few months.