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Thread: HOW-TO: Replacing M104 Lower Wiring Harness

  1. #1
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    HOW-TO: Replacing M104 Lower Wiring Harness

    One of the items that I didn't do on my recent M104/W124 top-end rebuild was to replace the lower wiring harness. However, during that project, I did inspect the manufacturer's label on the lower harness, and it was confirmed to be original, having been manufactured during the 44th week of 1994 (October, 1994).

    The more critical upper wiring harness had been replaced during 2006, and this was confirmed during the top-end rebuild by examining the manufacturer's label on that harness (see this post for photos of the upper wiring harness label).

    The lower wiring harness on the M104 consists of two separate, self-contained wiring harnesses that are bound together by black cloth tape. The first and main portion of the harness is a straight cable that originates as the positive battery terminal, and terminates at the engine's starter solenoid in a large "L" shaped metal terminal, and a smaller ring-clamp terminal. Both of these connectors attach to the solenoid. A short lead "Y's" off from the positive battery terminal to a block near the battery, and provides power to several other wires in this area.

    The secondary, smaller portion of the wiring harness connects the three-pronged connector at the inner firewall (just ahead of the brake booster and master cylinder) and goes to the oil level sender and oil pressure sender. This three-pronged connector leads to the instrument cluster to provide power to the gauges, and also allows the car to be started (bypassing the starter lockout/neutral safety switch) if necessary by applying voltage to the third terminal.

    Quite often, the upper harnesses are replaced, but the lower ones are neglected because they are at the lower portion of the engine except where the main cable runs along the inner firewall directly behind the engine. Thus, given its placement, it does not experience quite the level of heat that the upper harness gets, given that heat tends to rise in the engine compartment. Nevertheless, after 15+ years, given the biodegradable nature of these harnesses' materials, it is prudent to replace the lower harness.

    The MB part number for the lower harness on the late (HFM injection) M104 cars is 124 540 42 30. Current MB list price for this harness is around $130; it can be obtained from parts.com for around $103. All connectors are supplied with the harness, so no additional items must be purchased to go with the harness.

    The job takes an average DIYer around 1.5-2 hours, taking one's time. It can likely be done in approximately 1 hour if focus is applied. It is best to do this job with a warm or cold engine; do not do it when the engine is hot. Ramps or jack-stands under the front of the car are required for under-car access.

    I would rate this job as a moderately difficult job for a beginning DIYer, but a fairly easy job for an intermediate DIYer. On a scale of 1-10, I would rate this job at 2.5-3.0 difficulty.

    The following hand tools are also required for this job:

    • 1/4" ratchet with long extension (or two), and 8mm and 10mm sockets
    • 3/8" ratchet with 10mm and 13mm sockets
    • medium-sized Philips screwdriver
    • 13mm open-end or box-end wrench
    • work-light
    • magnetic pick-up tool (in case hardware is dropped)


    Here is the procedure to do this job.

    First, a couple of views of the harness in & outside of its packaging.
    IMG_4744.JPG IMG_4745.JPG IMG_4746.JPG


    And some close-up views of the connectors at the ends of the branches of the new harness.
    IMG_4747.JPG IMG_4748.JPG IMG_4749.JPG IMG_4750.JPG IMG_4751.JPG


    You can see the poor condition of the lead that "Y's" off of the battery terminal and goes to the terminal near the battery. The outer portion of the insulation has flaked off with age.
    IMG_4752.JPG


    The first step is to remove the negative terminal from the battery. To secure it, I wrap it in a shop towel and move it out of the way, so that it cannot make direct contact with anything.
    IMG_4753.jpg IMG_4754.JPG


    Next, I loosen and remove the positive battery terminal from the battery. And then, I remove the "Y" piece that is held by a ring clamp to the power terminal just forward of the battery.
    IMG_4755.jpg IMG_4756.jpgIMG_4757.JPG IMG_4758.JPG IMG_4759.JPG


    And here are the two ends of the wiring harness, from the battery area.
    IMG_4760.JPG


    Then I remove the rubber seal across the top of the inner firewall, since I'll be needing to slightly move the firewall to route the old cable and new cable past it. Same thing with the driver's side.
    IMG_4761.JPG IMG_4762.jpg


    The next step is to disconnect the three-pronged plug at the inner firewall, and then to remove the connector from the firewall itself. Just tuck the connector out of the way; I just put it next to the brake lines. The male portion of the connector at the firewall is held on by two Philips head sheet-metal screws. Save these screws!!
    IMG_4763.JPG IMG_4764.JPG IMG_4765.JPG IMG_4766.JPG IMG_4767.JPG


    With the cable loosened, it is much easier to see the original manufacturer's tag attached to it. Yep ... it's original. The "44 94" moniker means it was made in October of 1994. Definitely overdue for replacement !
    IMG_4769.JPG IMG_4768.JPG


    After that, I removed the plastic piece that holds the wiring harness and computer cables to the inner firewall, protecting them from direct contact with the back of the cylinder head and cam cover. If you remember, I had replaced both this inner firewall piece and the plastic connector; it is a VERY common thing for both of these pieces to fail due to age and engine heat, allowing the cables to touch the back of the engine. It's probably not a hazard, but IMHO it looks like crap, so it's a nice cosmetic upgrade to set things back to being as MB intended them. The plastic piece that retains the cables easily unclasps by pressing the two prongs at the ends of the plastic piece.
    IMG_4772.JPG IMG_4771.JPG


    Then, I lifted the end of the inner firewall piece and removed the cable from the bundle, and moved it toward the center of the car. Here you can see me moving the large battery terminal cable.
    IMG_4773.JPG IMG_4774.JPG IMG_4775.JPG


    The next step is to get underneath the car. You need to use the 8mm socket on the 1/4" ratchet to remove the six bolts that hold the under-engine encapsulation panel to the car, and set the panel aside. Then, get under the car and find the oil level connector in the side of the oilpan. Pull this connector straight off and set it aside.
    IMG_4776.jpg IMG_4777.jpg


    Then, remove the wire leading to the oil level sender from the plastic connector that holds it in place. The connector just presses out of the hole in the bracket. It should be in there together with a black vacuum line that goes from the intake manifold to the transmission's modulator a bit further back.
    IMG_4779.jpg


    If you look straight up, you can see the wiring harness and where the lead that goes to the oil level sender branches out from the main part of the harness. You can also see, in the photo below, the black fabric tape unraveling from the harness. This unraveled as a result of the harness being moved aside and tweaked during oil changes, as it runs in very close proximity to the oil filter housing and is brushed aside and disturbed when unscrewing the oil filter housing cap to remove the filter from the housing.
    IMG_4780.JPG


    You can see in the photo below, the plastic circular connector that retains the green and gray vacuum lines to the oil filter housing, to keep them away from sources of heat in the area that might melt the lines. This retaining connector presses straight into a metal piece that is directly attached to the wiring harness; this metal piece is screwed directly into the oil filter housing. In the case of my harness, the zip-tie that had held the harness to this metal plate had evidently come loose or had disintegrated from heat, as my harness was not held to this plate. However, the circular connector was pressed into its hole in the plate, so I released it.
    IMG_4781.jpg



    CONTINUED IN NEXT POST

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    The next step in the removal of the harness, is to remove the two ring connectors that attach the harness to the starter solenoid. This is probably the toughest part of the job, because the connectors are way up there (requiring you to reach upward) and it is easy to lose the nut and washer that hold the harness to the terminals of the solenoid. Use your 3/8" ratchet with a 13mm socket (for the larger ring connector) and a 10mm socket (for the smaller ring connector). Set the nuts and washers aside in a safe place. If you are leery of losing them, have your magnetic pickup tool handy when the nuts come loose, so that they attract the hardware straightaway and do not allow it to drop away from the terminals.

    Here you can see the large and small terminals, as attached to the starter solenoid, which are actually the opposite ends of the wiring harness from its beginning point at the positive battery terminal.
    IMG_4782.jpg IMG_4783.jpg IMG_4784.jpg


    After removing the two starter solenoid connections, you need to go back up top and remove the metal plate from the oil filter housing. You will need your 1/4" ratchet with a 10mm socket; I used two long extensions to get down in there. Here are photos of its removal.
    IMG_4785.jpg IMG_4786.JPG IMG_4787.JPG


    Here are a couple of views of the metal piece from the oil filter housing, removed from the car. You can see that it is pretty caked up with grime. The slot at the top of the piece is where the zip-tie that holds it to the harness once was; it is long gone, so this piece came off the car by itself. Normally it would be attached to the harness.
    IMG_4788.JPG IMG_4789.JPG


    At this point, the harness is essentially out of the car. All you have to do is reach up, directly behind the oil filter housing, to the oil pressure sensor. Pull the plastic connector straight off of the sensor. This is the final connection, and now you can carefully pull the harness out of the engine compartment from the top of the engine.

    Now, we get into some close-up comparisons of the wiring of the old harness. First, a comparo of the old and new battery terminals. Then, some close-up comparisons of the flaking, "bio-degradable" wiring insulation from the harness itself. Wow -- the more you get into it, the scarier it gets !! See the pieces of insulation flaked off of the harness onto the cardboard?!?
    IMG_4790.JPG IMG_4791.JPG IMG_4792.JPG IMG_4793.JPG IMG_4794.JPG IMG_4795.JPG IMG_4797.JPG IMG_4798.JPG IMG_4799.JPG


    Now, it's time to begin installing the routing the new harness. First, you place the main (thick) portion of the new harness across the back of the engine, with the battery terminal draped over the inner firewall in the general area of the battery. Then, you route the three-prong connector from the inner engine compartment through the inner firewall to the compartment where the brake booster and master cylinder is.
    IMG_4800.JPG IMG_4801.JPG


    After doing that, push the main part of the harness downward along the back of the engine, on the driver's side, and put the metal plate into place. It has a notch in it, so it only goes one way on the flange at the oil filter housing. Use your hand or your magnetic pickup tool to "start" the bolt that holds the metal plate to the flange, and then use your 10mm socket with extension to tighten it into place. Then press the plastic clip holding the two vacuum lines into its hole in the side of the metal piece.
    IMG_4802.JPG IMG_4804.JPG IMG_4805.JPG


    Get underneath the car, and reach up and route the cable that goes to the oil level sender (the longest wire) to that general direction. Be sure to insert it into the plastic retaining clip along with the vacuum line to the transmission modulator, and press this retaining clip back into its hole. After routing the connector, press it onto its electrical connector on the outside of the oil level sender.
    IMG_4806.jpg IMG_4807.jpg IMG_4808.jpg


    Then it's time to attach the large and small ring connectors to their respective posts on the starter solenoid. This is difficult because space is very limited to start the nuts while not losing the washers. I recommend attaching the smaller connector first, and then after that attaching the larger one. Use your 3/8" ratchet with appropriate sockets to tighten down the nuts. Then replace the plastic cover that covers these connections from dirt and corrosion.
    IMG_4809.jpg IMG_4810.jpg IMG_4811.jpg IMG_4812.jpg
    IMG_4809.jpg


    After those connections are made, you need to also press the small plastic connector onto the oil pressure sender at the rear of the oil filter housing. You can reach up with your right hand and do this by feel, as it is largely out of sight.

    Then, go back up top and you route the new battery terminal cable along the back of the engine, and through the passenger side inner firewall (alongside the computer cables) and into the battery area. It's pretty much the reverse of removal.
    IMG_4813.JPG IMG_4814.jpg IMG_4815.JPG IMG_4816.JPG IMG_4817.JPG IMG_4818.JPG


    CONTINUED IN NEXT POST

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Then attach the positive battery terminal and tighten it down.
    IMG_4819.JPG IMG_4823.JPG


    Re-attach the short ring-connector lead to the plastic power terminal near the battery.
    IMG_4821.JPG


    Replace the rubber on top of the inner firewall, and reconnect the three-pronged connector at the driver's side inner firewall. Re-attach the MAF connector, if you removed it at the beginning of the job.
    IMG_4820.JPG IMG_4824.JPG IMG_4825.JPG IMG_4826.JPG


    Then connect the negative battery terminal and tighten it down, and test out the system. The car should start right up and run as normal. Enjoy another job well done !!

    Cheers,
    Gerry

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    Senior Member Allgonquin's Avatar
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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Is there a similar DIY for the 500E lower harness? (looked but did not find one) I did the upper a couple years ago, but my conscience tells me I should do the lower as well. Any differences in harness between the model years?

    Rgds,
    Chris
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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Quote Originally Posted by Allgonquin View Post
    Is there a similar DIY for the 500E lower harness? (looked but did not find one) I did the upper a couple years ago, but my conscience tells me I should do the lower as well. Any differences in harness between the model years?

    Rgds,
    Chris
    There is a DIY for the lower harness for V-8 W124 models over at PeachParts. The link to that DIY Wiki article is here.

    I believe the lower harness is substantially more difficult on the V-8 models than it is on the I-6 W124s.

    Cheers,
    Gerry

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Gerry you are definitely skilled at putting these DIY's together. I'm impressed with your patience in documenting each step. You should have a TV show on Velocity.

    1992 500E
    1994 E500
    2011 E550 4Matic

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Thank you very much ! My 320E (1993) have the same problem (only 101.000km but 12 years !). The upper wiring harness is new since 2004. Now it's time to change the lower !

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Quote Originally Posted by Frederick View Post
    Thank you very much ! My 320E (1993) have the same problem (only 101.000km but 12 years !). The upper wiring harness is new since 2004. Now it's time to change the lower !
    Good luck with the replacement. It's not too difficult of a job.

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Hi Gerry,

    Well done ! The replacement is made. Not very difficult with your perfect tutorial, thanks a lot !

    Mine was a little bit different (european car vs american ?) but all is perfect now after 2 hours of work

    Here it is my little contribution with some pictures about my old harness.

    320E (1993 but first registering is march 1994) No problem with +12V cable, many problems with censor cables


    IMG_2370.jpg IMG_2373.jpg IMG_2371.jpg

    IMG_2374.jpg IMG_2372.jpg

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Is this he same part number (124 540 42 30) for 1993 300TE lower engine harness?

    1992 500E
    1994 E500
    2011 E550 4Matic

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Quote Originally Posted by Maui View Post
    Is this he same part number (124 540 42 30) for 1993 300TE lower engine harness?
    Ken, I'm 99.8% sure it's the same part, but I'd need your VIN to confirm. As long as your wagon is a 124.092 (which it should be) and uses an M104, which the 1993 models did, then this should be the correct part.

    The earlier cars that used an M103 engine, built up to model year 1992 for the US market, used a different harness part (though same function) which was 124 540 14 30.

    It's not a horrible DIY job at all.....not nearly as bad as the lower harness I just got on Friday for my E500 is going to be.....

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Quote Originally Posted by gerryvz View Post
    Ken, I'm 99.8% sure it's the same part, but I'd need your VIN to confirm. As long as your wagon is a 124.092 (which it should be) and uses an M104, which the 1993 models did, then this should be the correct part.

    The earlier cars that used an M103 engine, built up to model year 1992 for the US market, used a different harness part (though same function) which was 124 540 14 30.

    It's not a horrible DIY job at all.....not nearly as bad as the lower harness I just got on Friday for my E500 is going to be.....
    Looking at the two harnesses I can tell the M104 is much simpler. I've seen it described as a positive battery cable. I whimped out on my 500E lower harness and had MBI install it. I can only spend so much time laying on my back in the garage throwing wrenches. I'm not a young whipper snapper like most of you guys.

    1992 500E
    1994 E500
    2011 E550 4Matic

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Kudos to Gerry for posting this. I replaced the harness today on my 1995 cab. The starter connections are a PITA. My harness was flaking in chunks by the inner firewall and bare wire could be seen. No adverse effects other than I believe my oil pressure gauge was not accurate it was always pinned at 3. Iíve now done all three harnesses on the cab; engine, lower and throttle body and the lower on my former E420. The M119 lower is the worst but today was not fun. Hopefully Iím done with starter bolts and washers for a long time.

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Top Job ! Thanks for sharing !

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    A124 540 42 30 harness is NLA (at least in Europe)

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    E500E Guru captruff's Avatar
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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Quote Originally Posted by mercepor View Post
    A124 540 42 30 harness is NLA (at least in Europe)
    Correct the lower harness aka starter harness A 124 540 42 30 is now NLA in the USA as well...I was able to source one about 2 months ago via a dealership.

    I will have a friend rebuild the one coming off my wagon for future projects....they do not seen too complicated.

    Jeff
    "Nice Things Cost Money" - Oskar Schindler

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    Smile Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Quote Originally Posted by captruff View Post
    Correct the lower harness aka starter harness A 124 540 42 30 is now NLA in the USA as well...I was able to source one about 2 months ago via a dealership.

    I will have a friend rebuild the one coming off my wagon for future projects....they do not seen too complicated.

    Jeff
    Jeff, let me know if you need the leadwire. I have some stuff that is 10000% overbuilt for the purpose and is oil resistant.

    Edit: Didn't mean to insult you by insinuating that there would be any unwanted oil anywhere in the engine bay of your car, any of your cars. Ever.
    Last edited by kwontumspeed; 11-30-2017 at 12:01 PM.

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    Re: HOW-TO: Replacing M104 Lower Wiring Harness

    Quote Originally Posted by gerryvz View Post
    It's not a horrible DIY job at all.....not nearly as bad as the lower harness I just got on Friday for my E500 is going to be.....
    Thank you Gerry - this write up is a God send

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