Just got my GLS450 back from the dealership after warranty work. Mileage at 46k, so I asked for a price quote on changing spark plugs (will be due at 50k). Price came back at $545. Not surprising...

What did surprise me was the note on the quote: PLEASE NOTE ENGINE WIRING HARNESS MAY ALSO NEED REPLACING FOR SPARK PLUG JOB TO BE SUCCESSFULLY COMPLETED, DEPENDING ON CONDITION OF HARNESS."

I asked my SA about the note, and he said "Oh, this engine has a known issue with the wiring harnesses; they often disintegrate during the spark plug job."

Normally, this wouldn't bother me, since my car is still under warranty... what do I care if they have to replace the wiring harness, right?

EXCEPT, I was toying with the idea of replacing the spark plugs myself, as an opportunity to teach my teen son some basic under-the-hood skills. Grrrr...

Anyone else with the 450 engine had a wiring harness replaced?

Thanks...

I wonder if there's a special factory tool that saves the harness.

First I’ve ever heard of this. I had mine replaced a while ago by my trusted indi and there were no problems whatsoever. Who designs a wiring harness for spark plugs that would disintegrate upon changing the spark plugs, especially when you call for the spark plugs to be replaced in less than 100k miles in general? If true (which I highly doubt), then that has to be the most pathetic example of quality since the cloth interiors of a 1980 Mazda 626, which would react to the sunlight by turning to dust.. I call BS on your dealer. If they damage the wiring harness in the process, then it should be on their dime to fix it. The service manual doesn’t say to change the plugs and rewire the ignition system. It says change the plugs.

Yeah, and who would design wiring harnesses fabricated with vegetable based insulation that rodents find tasty?

….or “indestructible” fake leather seats that randomly split apart if sat in too long from 2010’s E class to 2020’s GLS class…. I think MB’s slogan should be:

”Mercedes Benz…..you’re welcome Lexus”.

or

”Mercedes Benz….so good, you buy it twice!”

That used to be said of BMW but no one's laughing now!
https://www.consumerreports.org/cars...s-a6159221985/

Merdedes Benz - "the best or nothing" -- I'm having a tough time accepting the "nothing".

So...um.... is it true the harness needs to be replaced and there is no way to prevent it while doing the spark plugs?

If this is true, then my Indi must have the magical touch, as we had our plugs replaced 15k miles ago without a single problem, and certainly no mention of needing to replace the wiring harness. Perhaps I have the unicorn GLS450....

Thanks for confirming it is indeed NOT true.

UPDATE: I just got my GLS450 back from the dealership after spark plug replacement.

They also replaced (under warranty):

- ENGINE WIRING HARNESS 256-150-60-00
- CONNECTOR (x2) 256-159-08-00

The technician notes stated: "Wire harness causing misfire; Found wiring to be cracking." And two spark plug connectors apparently disintegrated while doing the job.

Glad I didn't find that out the hard way, in my garage on a Saturday...

This should be a TSB if not a recall!

Thanks so much for the update OP!

We've seen a couple other posts that the technician damage the plug wires while replacing the plugs.
Makes me think this might be a CYA, not a factory defect.
Except that this may have corrected a misfire.

I hope we hear the outcome, to see if this fixed the misfire.

If it didn't, then I'll attribute the failure to ham-fistedness.

Changing the spark plugs on a GLS450 isn't too complicated. But if the dealership suggests you might also need a new wiring harness, that could be a bit concerning.

This is absolutely ridiculous and Mercedes Banz should be embarrassed and ashamed for it. Never in my life and 18 cars have I ever seen a situation whereby servicing the car causes even more damage. That’s just absolute junk. When we bought this GLS450 for $86,000 in September 2020, I told my wife to pick a car that when we incur a $5,000 repair bill that you will want to fix it instead of replacing it. A 10+ year car, whatever you want. As these things come up, from ripped seats, faulty battery grounds, fuel impeller, door buttons falling out, and so on, we’re both in agreement that this car is not a 10 year car and is a throw-away Mercedes. They don’t make them like they used to anymore (at all). What a shame.

I shake my head every time I recall multiple dealers that wanted $15-20k ADM over MSRP for a GLS last year!
Feel really bad for whoever fell for the "limited supply" line!

All dealers were doing it! I’m waiting on my Ford Bronco to be delivered at MSRP, but two years ago, even last year, heck even still in certain areas of the country, they’re still getting $10k over for them, for a friggin’ Ford!! I tell those folks at least it was for a brand new car that you don’t have to worry about previous abuse and have full warranty coverage. I sold my 7 year old VW GTI with 70k miles and in need of about $4k of service to Carvana for nearly $21k in 2022! I feel sorry for whomever had to buy that car now.

Funny thing... there was no misfire, at least not when I dropped the car off for the spark plug replacement. Ran smoothly... No CEL. Nothing.

I assumed their reference to the "misfire" was to ensure Mercedes corporate paid them for doing the warranty work on the harness. [insert shrug]

Likely.
Insert "Customer states...."
Done.

Does fixing this require removal of engine? I had a yellow engine light and did notice occasional knocking before that. Whatever harness they are replacing requires removal of engine and many days repair time. ‘21 GLS 580.

New here and I am thankful for this post. Thank you to all that have commented as it has made me feel...sane...and like Im not the only one.
I have a 2021 GLE 450 and have just got taken from the dealership on the wire harness repair. My fault for taking it to them so I guess I have them a softball. Took it in for the A8 service.....big mistake.
They also found the wire harness issue on mine. ust the wire harness repair is $2345. They also told me that the needed to do that harness repair if they are also going to do the spark plugs. They also wanted to replace my cabin air filter, for $203.11 as well as my front and rear wiper blades for $153.78. What, in the actual hell?!

I have asked them to put extra heat shielding on that harness. Who knows if they will do that but if I am able to locate it, I plan to try the liquid electrical tape someone mentioned earlier. They got me this time, but Im not going back to the stealership. I have finally learned my lesson.
Good luck to everyone out there

My GLS 580 was repaired for the wire harness (under original warranty, but I also have extended) and there were casualties. The brake line broke during the harness replacement. The car was in the shop for over 2 weeks. I didn't need a loaner, I suspect that put me at lower priority.
Later after getting the car back, we noticed an oil leak, so back it went. That time I did get a loaner which they ok'd to take on our summer vacation when it couldn't be repaired in time.

I still worry that there could be other issues found later by removing the engine.

Now knowing it's a known issue, I am disappointed that they aren't taking responsibility.

ps. my problem was identified when I took it in for an engine light, that had gone away by the time I got it in, but I had them check it anyway. I think then, in hindsight, I had noticed the engine sounded different at times.

I installed this shield fabric to prevent burning the wires and connectors from the hot turbocharger while idling. This will happen over time and for owners intending to keep the vehicle beyond the factory warranty, wire harness replacement is expensive. This is a $10 preventative measure.

Option 2 is

Turbo Heat Shield

Many models available. Used it a lot on th V12's BT