Mid-90s Wiring Harnesses - PeachParts Mercedes-Benz Forum

MB-Dude · #1 07-26-2007, 11:05 PM

Does anyone have specific information on the infamous wiring harness failures that seem to plague MB's made in the mid-90s?

My '94 SL500 has been diagnosed by the MB dealer as having a bad engine wiring harness and throttle slide. This is in response to a Check Engine light. Car runs acceptably (perhaps a bit anemic) with not a significant change in idle speed, gas mileage, etc. According to the Service Advisor, the error codes point to these two items as being bad.

Considering they want $3,200 (parts & labor) to make repairs, I'm very curious as to the details of the failure.

I'm told the harnesses in the mid-90’s are bad as there is an insulation breakdown. But considering the thickness of the wiring insulation and the low voltage in use, I find that a bit tough to accept on face value.

Because the dealer wants to change 2 components, I am wondering if it's really the connector(s) that fail, and if so, can they be cleaned sufficiently for longer life.

I’m not looking for confirmation of the wiring harness saga, but instead am hoping someone has more detailed information.

Larry Delor · #2 07-26-2007, 11:22 PM

This link is to a fellow member's site.http://v12uberalles.com/throttle_actuator_rewire.htm On it, he shows how he re-wired his V12. The wiring that you see on his page, is plagued with the problem that the 6 & 8 cyls have. Many fellow members have had to replace their harness because of cracking/missing insulation. Myself, I had to not only replace the harness, but the throttle actuator as well (same crappy wiring) - there was no insulation at all, left in the actuator...it had all turned to dust. The connectors, btw, were all still good. You can get the parts and do it yourself for a lot less than your quote. If you wish to do your own wiring, you can save even more dough.

deanyel · #3 07-26-2007, 11:34 PM

I wouldn't trust these people. It sounds like they are trying to sell you a new car. There are no fault codes that say "bad wiring harness" and "bad throttle actuator". Learn how to read the fault codes yourself (search) and post them here. I wouldn't be surprised if neither one was bad. Furthermore $3200 is way too high.

nissanzx1 · #4 07-26-2007, 11:38 PM

It really has nothing to do with the connectors at all. Imagine 12 different wires in one insulation wrap. Then, imagine 8 of the individual wires with "rotten" insulation. The potential for short(s) and even engine fire exists.

I would try an independent shop in your area, and maybe you can save ~$1000. Selling the car with a major problem like that is unthinkable. I should know, my '95 E320 needs a harness right now

Part is $900, I am saving...

-GH

forp · #5 07-27-2007, 01:20 AM

If you have one of the problem harnesses and it's 13 years old, you shouldn't have any problem seeing the deterioration yourself. You're looking for cracking in the wire insulation at the connectors. Pull back some of the wrapping to get a good look.
Dan

MB-Dude · #6 07-27-2007, 01:52 AM

First, thanks gang for the feedback! As I mentioned, I am skeptical of the wiring harness as root cause of the fault. As a retired electrical engineer, the whole tale just didn't sound right. However, I did have a '94 S320 a couple years back that required an engine harness and after the work was completed, the car ran very well.

Larry Delor - Thanks for the link. Although the discussion there surrounds a V12, the concept is the same. Also, the pictures showing the cracked insulation give me an excellent idea of what to look for as I want to open up the harness and inspect the wiring.

deanyel - Yeah, I am a bit leery of any MB dealership. According to my discussion today, MB wants $875 for the engine harness and $525 for the "Throttle Slider". Strange name, but I imagined it to be the Throttle Actuator. The rest - $1,800 - is supposedly labor. I agree the price was way too high, and expressed my... shall I say... displeasure with their greed, especially on the labor side of the equation.

nissanzx1 - HA! You and I think alike... my first stop tomorrow AM is an Indie in the area that could replace the cable at far less labor cost. However, if the harness is not that complicated, I may attempt it myself. It's just that as I am writing this, I am unsure of the correct routing. Fear of the unknown, I guess. Fortunately, the car is an 'extra' - I'm selling it anyway, so time to repair is on my side.

forp - Thanks. You have confirmed the pictures that were included in the web page Larry indicated. Yes, the insulation does appear pretty badly damaged. Personally, I've never seen wire insulation that bad. Jeez!

So gang, my plan is to seriously consider buying the parts and installing them myself. While I have the tools and expertise to replace wiring in a harness, I'm not thrilled about the possibility of soldering in a number of them. Thus, I probably have to buy the parts anyway. If, after looking at the harness and reviewing the harness run, I may just take the car to an Indie.

As I get moving through this repair, I'll post updates. Will also work on getting the codes, though the car is pre-OBD-II.

emerydc8 · #7 07-27-2007, 06:25 AM

It took me less than an hour to replace my harness. The hardest part was getting the connector through the hole at the firewall. I found that it was actually easier for me to run all the wires through the firewall hole from the back, rather than try to fit the 8-inch long, lever-type connector through the firewall hole.

You can build a test light from parts available through Radio Shack. Jim Forgione was nice enough to show how to build the "home brew" tool at the bottom of this link. K6JRF Auto Page. Just don't expect the dealer to know what you're talking about when you mention that you're getting a DTC 19. I recently got a blank look when I brought this up with an MB tech that I know has been working on the cars for several decades. They apparently don't know--or care--what the DTC flashes are because their expensive MB laptop converts it to English for them. Best of luck.

MB-Dude · #8 07-27-2007, 08:55 AM

Thanks emerydc8 for the information. I had that link already, but your message did jog my memory. I had suspected the harness replacement was fairly straightforward; most harnesses are. Not sure if my '94 SL500 is analog or digital, but will get the car back today, after the dealer buttons it up from all their 'diagnostic' work.

MB-Dude · #9 07-27-2007, 11:25 AM

Follow-on Question - When replacing the Throttle Actuator in a '94 SL500, is there a procedure that needs to be performed that calibrates or sets up the Actuator to engine management?

I'll probably change the Wiring Harness and Throttle Actuator. The harness would be straightforward, but the actuator (I would think) will need to be taught.

deanyel · #10 07-27-2007, 05:17 PM

There's something fishy here - at a price of $525 for the part whatever it is they are talking about it is not the throttle actuator. Dealer price on a throttle actuator for that car would be at least twice that and perhaps three times that.

MB-Dude · #11 07-27-2007, 07:06 PM

Talked to the MB Parts Manager, who filled in some of the questions. To recap, the recommendation by MB Service is to replace...
Engine Harness - MB p/n: 129.540.77.05
Throttle Slide - MB p/n: 000.141.52.25

The harness is 'the' harness. But the Throttle Slide is the (taking a term from my ol' carburetor days) 'butterfly' assembly.

The belief is the wire insulation in both the harness and the short cable that is part of the Throttle Slide is damaged, thus causing the CEL. Once the Parts Manager showed me what the Throttle Slide was, I decided to investigate it further myself... before ordering any parts.

Since the Throttle Slide is operating properly, except for a rather dubious possible interaction with the harness, it may be possible to rework the individual wires (if needed) in this assembly. Thus, saving a substantial amount of $$$.

Guess where I'll be tomorrow morning! As I dig into this further, I'll provide an update. Will try to get a couple pics while I'm doin' exploratory surgery.

MB-Dude · #12 07-28-2007, 12:29 PM

Yes, the engine harness is bad. Refer to the attached pics for details. As you can see, the copper wire is beginning to corrode, which increases the resistance per foot, which causes incorrect signals. So the harness definitely needs to be changed.

However, I found something that I think is odd, though it could be my inexperience with electronic engine management. My engine is not a drive-by-wire and the accelerator pedal is physically connected to the throttle body through the cable and linkage. At rest (idle) the butterfly is positioned correctly.

But when I simulate stepping on the accelerator, the butterfly does not physically open even though I can see the linkage moving where it goes into the throttle body. The butterfly does not begin to move until the pedal has travelled about halfway. If I continue to step on the accelerator pedal to its stop, the butterfly opens only about 50%. It's as if something is not connected properly inside the throttle body.

In addition, I cannot find any wires that attached to the throttle body, though arguably it is buried deep under the cross-over intake manifold and I may simply be missing them.

Question #1: Is it normal for the butterfly to open as described above?

Question #2: Is it possible that my Throttle Body is not electronically controlled or provides and electronic feedback?

Regardless, it looks like major surgery to R&R the Throttle Body/Slider Assembly. Thoughts anyone?

AMG CE 36 · #13 07-28-2007, 05:16 PM

Hi, not an expert but I guess your TA is fead by wire and the linkage is for "limp mode", which uses less throttle.

MB-Dude · #14 07-28-2007, 11:26 PM

Here is what I think is causing the CEL... The first attached pic (94 SL500 Harness-4.jpg) is of the ignition signal wire feeding the left bank of the 5.0 liter engine. You will note that there is *no* insulation between the two wires.

I believe the wires would just 'kiss' each other whenever the engine vibrated when starting, causing misfiring. The engine management system would capture this as a fault and trigger the CEL. Whenever I reset the ECU codes previously, the CEL would always come on during engine starting.

The second picture (94 SL500 Harness-5.jpg) is my temporary fix to prove if this is the cause of the Check Engine Light. Cheap and cheesy, I admit. Once this temporary patch (piece of electrical tape) was installed, I cleared the ECU codes by disconnecting the battery for about 3 minutes.

I have started the engine twice and the CE Light has *not* come on. In addition, the car is much more responsive to the throttle. Obviously, this little patch is doing something positive.

The wiring harness still needs to be changed. But this work today suggests the Throttle Slider/Actuator/Body does not need to be changed. I plan on driving the car around town tomorrow with various stops and restarts.

Larry Delor · #15 07-28-2007, 11:50 PM

Good Hunting!
Well...the wiring is bad, that is for sure. As far as the ETA goes (Electronic Throttle Actuator), it could be either way - don't know what the odds are - if you go back to the V12 uberalles page, you can see an ETA opened up. You will see that it is more electromechanical than just one or the other. Mine was good for a while...but then one day, the car just would not run, and if I got it to run, it was rich and stank like hell. A rebuilt ETA cured it. I opened up the old one before I ordered the part (they are NOT a dime a doz.) and found a few tiny bits of something, and mostly dust - the insulation was gone.

If you are going to remove the ETA, be sure and buy a gasket first, and also the black rubber tube that goes toward the firewall - it is hard and brittle by now, and will break. Be careful that you don't break any vacuum tubes - they are hard and brittle too.

Phil should be able to give you a good price on all this stuff.

Good luck!