Should I bite the bullet and install a new M104 wiring harness
 

Hi all,

Been reading all about the M104 wiring harness problem. Went to the dealer to try and get a goodwill fix before it fails and takes out a lot of other components. No luck as they said they would only look at it if the car was exhibiting signs of failure. My concern is that even if they do goodwill on this if there are other things that have failed that it will cost me as much as if I just replaced now at my own cost.

Am thinking of doing this myself and looked up the procedure on the CD rom. The procedure number is 01-0310 and it provides the replacement steps for models 202 and 210 but not the 124 which is what I have.

Has anybody done their own replacement on the 124? It looks like a rather easy DIY job based on the instructions for the 202 and 210 but would like confirmation from someone who had done it on a 124.

Any help would be appreciated. I am at the point where I will be checking harness cost and making a decision in the near future.

BTW, the car is a 95 E320 with 67K miles and exhibits no signs of a wiring harness problem now. Also, where would be the best place to peel back some outer covering and check the insulation?

TIA

philmartin

I am wondering why you.re thinking about doing this job.
Not ALL 320E will have problems with the harness. Not all e320 will need a complete replacement if any problems surface.
Why don,t you wait to have some sign of weakness in this area before looking for solution for a problem that may never exist.
jackD

Man, do I disagree. The insulation used on the wiring harness was defective. They are all bad since all wiring harnesses used the same insulation (between '93 and '95).

It is heat that breaks down the insulation, so age, not mileage, is going to determine the rate of failure of the insualation.

Further, if the problem hasn't developed on its own (driveability problems, CE light problems, misfiring, etc.), it is when the wiring harness gets handled, i.e. the head gasket is changed, that the insulation flakes off badly, causing the wires to short, and taking a lot of other components with it that cost much more than the wiring harness itself.

I say it all the time... go ahead and do a search with the words "wiring harness" or "engine wiring harness" and you'll read dozens and dozens of posts of unhappy owners with thousands of dollars in repairs because when the harness acted up, it took a lot of other parts with it.

It's not a harmless problem, like the leaky evaporators on early model W124 and early W140's. When and if the evaporators fail, the only thing that happens is your a/c stops working.

Re: Should I bite the bullet and install a new M104 wiring harness
 

usually, right at the the terminations on the thermostat housing you'll see the insulation at the ends coming off. i think it's even hotter further back by the injectors, but then you could be asking for trouble if you disturb the connections there. if you are not about to change the harness let sleeping dogs lie. BTW the harness costs ~$500...

Ive found that the wires in the plug to the intake cam advance magnet are the easiest to inspect for failing insulation. It may be necessary to remove the plastic connector from the wires to see the insulation. I agree with Suginami, its not a matter of if its going to fail, but when.It does seem to be related to heat. Having worked at dealers in the Atlanta area I relaced many of these harnesses on cars before the warranty period expired. It does seem that when the problems arise its after the harness has been disturbed, such as when plugs are replaced during routine service work.You should have no problem doing this repair, it is fairly easy on a 124 car.

Possible good news?
 

I checked the wires in several locations such as the intake cam advance connection, temp sensor, etc. These were locations where the outer covering does not quite reach the connector and the inner wires can be seen. In all cases the wires looked good so I gently abraded the insulation with my fingernail checking for cracking or disintegration and each one remained intact. This car was purchased late in 1995 as the 96s were already on the showroom floor and maybe this has an upgraded harness. Since it looks OK now probably won't do anything near term.

Also, when I changed oil the other day I did an inspection from under the car of the right rear head/ block interface and found no evidence of gasket leakage so perhaps this also has an upgraded head gasket. So far, this car exhibits no signs of the dreaded harness or head gasket problems.

Am planning on calling my old dealer in NY with the VIN and seeing if they can verify this car has upgraded parts.

philmartin

If one has seen the problem and seen those not having the problem the difference is striking. I believe their are many 95 cars that will never have the problem plus there is no real way to tell if it has already been replaced on used cars.

I have seen many of the problems and so far have not seen any consequences. Most MB controllers are current protected against output shorts. Software sometimes gets scrambled but is usually correctable.

If it ain't broke...don't fix it!

BTW, the job is very easy. I would think within the grasp of even the beginning level DIYer. Here are the problems: disconnecting proprietary connectors (simple if you have been there), dealing with codes or adaptations miscreated during failure and reassembly, and dealing with the concept that you didn't get it right if things don't work when back together. A lot of risk of decision but not much real risk.

Steve,

I agree completely with you that if it's not broken, don't fix it so I am not planning on doing anything except periodically checking both areas.

Thanks for the DIY input although I didn't think that codes would be generated if the battery was disconnected for work like this.

philmartin