W124 - Engine Wiring Harness Replacement

Mercedes built from the year 1992 to 1997 was installed with an eco-friendly (degradable) wiring harness. The issue with this type of harness is that the insulation disintegrates and crumbles especially when exposed to heat. For the W124 model, it is safe to say that all cars built from the year 1992 onwards were marred with this issue, both petrol and diesel models.

What is baffling is that Mercedes is aware of the issue but a safety recall was never issued to have the wiring harness replaced with a non-eco version.

The intention of going green badly backfired as this issue introduces an array of problems on the car. The reason why this happens is that the insulation upon crumbling exposes the bare wire and when two or more of the bare wire touches one another, it messes up the communication to and from the ECU causing strange behaviors and in more serious cases, it causes fire.

This issue was covered in length in the following links:

• http://badmercedes.com/
• http://www.mercedesdefects.com/2008/04/wire-harness-defect.html
• A quick search on Youtube for "Mercedes biodegradable harness" loads a number of video on this issue

Update #1

As a recommended fix, many have suggested that to accurately diagnose problems on this car, there are three main wiring segment that needs to be replaced, especially if it is fitted with the biodegradable harness and they are:

1. Main harness that links sensors, fueling system, ETA, ECU, and other points. This is the main harness and perhaps the most expansive one
2. The lower harness which is from the positive terminal to starter and a harness that goes behind the firewall
3. The wiring harness that comes from the ETA connecting to the main harness

In my case, what I will be doing is to replace the main harness which I have managed to source new old stock. The lower harness will be removed from the car to be sent to a wireman to be replicated and the harness from the ETA has been addressed recently with the replacement of the ETA.

Once the harness has been properly installed, we will then have a better idea of what the issue could be and perhaps allowing narrower troubleshooting from there.

The focus is instead of solving the issue that I have on idling upon starting, cold or warm, I am more concerned about the fire hazard and malfunctions while driving which will put the occupants and the investments of the car in jeopardy.

Knowing that the wiring harness is a hazardous one, a good friend told me that regardless of the harness fixing the issue or otherwise, this is a known issue and we need to start a baseline fix from here with all the changes that have been done.

I used to tell my wife that I am rebuilding this car from scratch as almost everything needs some sort of work on and I think I would have jinxed myself as that is exactly what is happening.

It has come to a point that I will stop chasing the rabbit to not fall deeper into the rabbit hole. The harness may very much be the last that I change to address the issue. It runs fine, drives fine, idles fine, does not consume too much fuel, does not overheat, and performed pretty well, there isn't really a need to dive deep into this issue.

Update #2:

We got the lower harness removed, there are 2 sets of cables that were removed and sent for rebuilding:

• Cable 1, the battery positive terminal cable. This cable links the positive terminal of the battery to the starter (thick cable) and a thinner cable potentially to power the cabin. We replaced this with a thicker cable to promote better electricity flow.
• Cable 2, this cable links the 3 pin connector on one end and the other split into a lead for the oil pressure gauge, another for the oil level light indicator, and lastly a ground lead.
• **We re-used the sender connector from the old harness on the above cable 2 on the new wire and metal clips.
• The work was properly done by a car electrical garage and all cables were sleeved to protect from heat and wear due to rubbing.

The fuel pump relay was also sourced and will be installed soon. As for the main harness, the one that was sourced unfortunately was incorrect, so the correct version is being sourced. The harness that was sourced had an 8 pin ETA socket while the ETA has an 11 pin socket and another difference is that the temperature sender socket on the harness has 3 pins while the temperature sensor has 4. Progress is currently halted till a cable is sourced.

Update #3:

I was updated that the lower harness has been installed. There wasn't a noticeable difference in the starting and idling behavior on Rhino, but the oil pressure gauge is now working fine. It was not working prior to having the harness removed. 

Update #4:

The mechanic says we need to have the car wiring checked and he isn't an expert. The car was then sent to the wiring garage that worked on rebuilding the harnesses as per update #2 above. The main harness from the car was removed and sent to the spare part shop to be validated and confirmed. The correct harness for this car is A124 440 3433. A replacement was found and the wiring man said for the wiring to be done properly, he also asked for the AC relay. 

For this car, the throttle body is not connected to the engine wiring harness but rather a separate harness that goes to somewhere the ECU is, that too will have to be checked.

We managed to get that as well albeit a little pricy as it was the original ones, the correct part number for the AC relay of this car is 003 545 5605.

While assessing the wiring, he found a wire that has burned, at this moment we are unsure as to what wire that was and what caused it to burn.

So at this juncture, the wireman will start tracing the wires on the engine bay and thoroughly check on the issues. He needs a week or so for the work to be completed and I told him not to rush, do a proper job is my ask.

Update #5:

Took some time to check on the car today. The wireman has fitted the engine harness and started the car. His initial observation was that the cold engine start started with the RPM going up to below 2000 rpm and then gradually dropping to 700 RPM. He further says that this is the correct way the car is supposed to start. I did not witness it hence I am unsure if the engine starts as to how it should. However, based on his observation, the engine harness change has made a difference to the behavior of the car. Going back to an earlier post, W124 - Cold and Warm Engine Start, the starting behavior has now changed where it no longer starts and idles at about 800 rpm and remains there.

Subsequent to that, the throttle body wiring harness was also treated to new insulation and where segments of the cables that were replaced as the insulation have rotten.

There were 2 wires (so far at the time of writing), where some amateur soldering was done but we don't know why or what for. There could be more of this to be found as we go along further into the wiring work.

The earlier reported burned wire found was isolated and is a cable that connects to the fuel pump relay socket. Guessing from the condition of the wire, the fuel pump relay may not be in a good shape and I have already prepared a replacement for it. The fuel pump relay socket/connector was brittle and a replacement is being sought.

It was further observed that there was a bunch of wire from/to the fuel pump relay that had its insulation disintegrated which would have caused the wire to have burned.

The AC relay was also fitted and a Hella relay was used as makeshift where the pins were plugged into the relay ports. This was corrected with the installation of the correct AC relay.

Update #6:

More work was being done on the Rhino, more wires were being traced and repaired. The highlight for the day was sourcing the relay socket that was needed to replace the older brittle ones. I did not manage to get the exact replacement but Mercedes did use similar relay connectors as cable connectors as well.

What we needed was a connector with 4 pins for the fuel pump relay, but what I found was a similar socket with 6 pins and incorrect cable thickness.

To address this, the wireman has open the connector up and pulled out all the pins. He has traced the position of the pin to the corresponding pins on the relay and will solder proper wires with the correct thickness. This was not entirely a plug-and-play exercise, but some trimming needed to be done to the connector to allow it to be fitted back to the socket casing in the ECU bay. 

Update #7:

I received a call from the wireman saying that the car is ready. I made my way to check and found that the engine bay has completed.

Along with the engine bay work, I took the opportunity to also have a few other wiring items completed and they are

1. Fixing the switch to the rear sunshade - the sunshade now works
2. Added a door lock switch, this was to remediate opening of door to unlock doors
3. The rear headrest switch was connected to the switch at the AC panel
4. Added a light signal wire to the head unit
5. Corrected the wrong direction of power window (RR) when the key is inserted to the key barrel of the door and turned to the end, all windows will either come up or go down together. Previously, the one on the RL corner was going in the opposite direction

More importantly, how does the car starts now? On warm start, the engine revs to just below 1500 rpm and gradually drops further to about 600 rpm. On cold, I Notice that it no longer starts and idles at 900 rpm as it previously did but this will be further observed.

To ensure that we have the car at a good operating temperature, I also change the coolant temperature sensor from the 85c to 100c trigger. The previous setup does not allow the car to hit beyond 85c where the high-speed setting kicks in and cools the temperature to 80c. But Mercedes recommends the operating temperature to be in the range of 90-105 c temperature. Below 80c, the ECU will run the engine rich by allowing more fuel to be burned as it 'thinks' that the engine has not warmed up. This will cause the engine to run rich and the mileage will suffer. 

The throttle cable was also replaced making way for a better condition one from the junkyard. This smoothed the accelerator instead of the jerky heel it previously had.

All in all, the wiring has been completed and I will be monitoring the behavior of the car

 

Pictures:

Oil level and oil pressure harness missing the insulation and has some exposure in several places

Wire harness being rebuild - work in progress

The old positive to starter cable is old (top) and new (bottom). This cable was replaced with a larger cable to allow better current flow. They are measured to be of the same size

Oil level and oil pressure harness, old (bottom) and new (top). The cable was insulated with a sleeve to protect the cable from wear

Fuel pump relay to be replaced

Engine bay without the engine harness, battery, battery positive cable, battery, ECU, and a few others which have been removed

Unsure what wire is this but it is burned (Edit: This wire goes to the fuel pump relay socket)

 Amateur soldered wire connecting the aux fan resistor

Makeshift AC relay plugged into the AC relay connector

Wiring loom rectification in progress, all looms will then be placed in insulation tubes and tapes. Prevent damage due to heat and rubbing

Cables being placed in insulation tubes

Aux fan resistor (left replacement from a chop shop and right is the old one)

Comparison of cable thickness between the top from the car and bottom, from the replacement aux fan resistor. This cable will be replaced with a thicker cable to handle the current load better

Aircond relay socket, missing the above fuel pump socket

Cables for the fuel pump relay, missing the socket

Fuel pump relay and the faulty fuel pump socket that needs replacement

Replacement socket that has correct dimensions for the relay to be mount. The pins will need to be rewired with thicker cables