W124 wiring loom rebuilt - BUT...!

[Williamwoo]

Hi all,

I have spent many evenings in the last few months rebuilding the engine wiring loom of my E320 and, after replacing it last week, much to my surprise it started first time!

However, there are a few problems:
Firstly, the car is not running 'right' - it feels like it is running on 5 cylinders (although this is significantly better than immediately beforehand when it felt like it was running on about 2).
Secondly, some electrical bits have stopped working: before I fired the car up the other day, the windows worked fine; now they don't (although, bizarrely, they move fractionally if you hold the switch as you switch off the ignition). The mirrors, seat heaters and rear wiper also no longer work. However, the front wiper is fine as are the sunroof and the seat adjustment controls.

My questions are: What would you recommend as the 1st port of call in tracking down the misfire? (I have taken the car for a quick blast to clear the cobwebs.)
And: What connection does the engine wiring loom have with the aforementioned peripheral electrical bits?

(I'm no electrician, but I replaced each wire in the loom individually and documented everything - took ages!)

Any thoughts gratefully received. Many thanks,

Simon

 

[kth286]

it is likely to be one of the coil extenders down to the spark plug.

they should be carefully twisted off as the wire inside go brittle with age and heat and can lose contact with spark plug.

about 10 UK pounds.

do you have a code reader to identify which cylinder is missing ?

 

[grober]

Sounds as if you may have a faulty convenience relay or blown the fuse that supplies it. On the other hand if the car has been sitting a while the fuse holders [ in the main box] may have developed poor contacts due to corrosion-often a quick clean up of fuse and holder mating surfaces is enough to clear up the problem.

In the lost spark ignition system you have each of 3 coils fires 2 plugs simultaneously Perhaps one coil is not firing or the high tension connectors/extenders that fit on top of the plugs has/have gone faulty. Take out the plugs and examine them. After that run you took the car, the plug/s that is/are not firing should be dirty compared to the OK ones. Don't run too long with a misfiring HIGH TENSION system as this can take out the engine ECU ignition output transistors!

 

[Williamwoo]

Hi,

Thanks for your replies. I will give the plug leads a thorough inspection when I get a chance and have a look at the relays and fuses (although I think the fuse for windows must be ok as they do work in very short bursts, as described).

kth286, what is a code reader?

Thanks again,

Simon

 

[EDZ649]

Well done for having a go. Hope you get it sorted

 

[Williamwoo]

Hi all again,

Well, I've managed to get all the peripheral electrical bits (windows etc) working by cleaning up the fuses (strange really as I didn't touch them when replacing the loom).

However, I still have my misfire, which I have tracked down to being one of the 3 coils. The plug on which the coil sits fires fine, but the output to its remote plug (cylinder 3) seems not to work. Each coil should of course fire 2 plugs simultaneously - is it possible for it to only fire one? Seems unlikely to me somehow...

 

[grober]

Nope should fire both simultaneously. Points to problem with the coil itself, the flying lead or the extender/plug connector or even the plug itself. You do get the odd dud.

 

[Williamwoo]

Thanks for your prompt reply.

I have swapped the plug and the leads around with others and proven that they are both fine, so I think it must be the coil - I was just surprised that it seems to be possible for the coil to fire just one of its two plugs.

 

[jonnyboy]

Thanks for your prompt reply.

I have swapped the plug and the leads around with others and proven that they are both fine, so I think it must be the coil - I was just surprised that it seems to be possible for the coil to fire just one of its two plugs.

Welcome to my world

 

[Ian B Walker]

I would seriously examine the coil pack extensions.

 

[Williamwoo]

Thanks for all your comments. I have now replaced the errant coil and she is running beautifully! (Might be worth filing away the fact that it is possible for a coil to only produce one of its two sparks.)

Total cost of rebuilt wiring loom? About £50. Oh, and several dozen hours with a soldering iron.

 

[grober]

Excellent result.