The SL Market Letter got a letter from a subscriber recently with questions about the much talked about Mercedes wiring harness issues affecting some Mercedes Benz models from the early 1990s, we asked a couple of folks who know their way around these cars for the straight scoop…
Subscriber Question: Hi John, Saw the article in the last SLML re E 320 Convertibles. I have a 1993 CE 300 which I believe is the same as the E 320 except for the name on the trunk. I was recently told that my wiring harness should be replaced because of a defect? I’m not planning to sell it anytime soon. Should I be worried now, or wait for signs of trouble. I understand it is not an inexpensive repair. Robert S. (SL Market Letter subscriber since 1988)
We asked for comments from Chris Olsen in Minneapolis, MN, John Olson’s personal mechanic, and Pierre Hedary an MB mechanic based in Florida, that makes house calls.
Chris Olsen: True, 1993 300 CE is a 3.2 liter six despite being called “300”. More important, not all 1993 Mercedes were plagued by bad wiring harnesses. Check your VIN number with MB’s Classic Center.
Q: What is the actual problem with the harnesses?
Chris Olsen: Ironically, the change was a well-intended expansion to biodegradable and recyclable parts. Wire coverings degraded far quickly than intended from heat, cold and hum id weather. Short trips don’t dry out moisture. Abnormally vulnerable to engine heat, cracks formed allowing moisture fuel and oil residue seep in. This happened faster in extreme climates causing even further cracking during other repairs.
Pierre Hedary: Wiring harnesses on 1993-1995 Mercedes Benz M104 engines are notorious for disintegrating. If you are in the market for any of these wonderful cars, there are a few things you should know about these harnesses, and what signs to look for on one of these beauties.
Q: What models were afflicted?
Pierre: The 1993-95 SL 320, W124 E 320 in all body styles, the S 320 and the G 320. All W 124 and R 129 V8 models of this era were also affected.
Q: Which harness are we talking about? More than one?
Pierre: The fuel injection harness gets the most heat and stress, often the first to go but the starter/alternator harness had the same issue, and should be examined as a potential source of issues as well. The throttle actuator wiring was also flawed in the same way. Finally, on a car that has had the issue for years, the engine control unit can be affected.
Q: What are the symptoms?
Pierre: Electrical errors can be first. These can lead to rough idling, multiple diagnostic trouble codes, or even engine starting failure.
Q: Did Mercedes ever have a campaign to fix the problem? Was there a technical service bulletin to fix the problem?
Pierre: Ken Brown, parts department manager at Stahl Motor company, states that there was never a campaign to replace the harness, nor was there a Technical Bulletin. Mercedes dealers would sometimes help original owners with the cost after the harness failed, but this has not been the case for at least ten years. Failures during warranty period were fixed at no cost.
Q: How much do new harnesses cost, and are they any better?
Pierre: For 2014, the cost of a new fuel injection harness for a W124 E320 is either $1100 or $1220, depending on what year your car is. Replacement is a quick and painless process that requires only two hours, maybe three at most. The material quality has improved dramatically.
Q: How do I know if a harness is bad?
Pierre: Check the plug ends and look at the wiring insulation. If the plug ends are falling apart and you can see that the insulation going into them is bare, then the harness is falling apart. If the harness is still pliable and not stiff, the connectors are strong and the wiring is brightly colored, chances are it has been exchanged.
As far as symptoms go, rough idling, poor starting, poor fuel economy, random multiple diagnostic trouble codes, stalling with no apparent cause and other bizarre issues can often be traced back to harness issues.
Q: Should owners just plan on re-placing these harness? And what about the main body harness?
Pierre: If the harness is original and it is showing the beginnings of a failure, you should replace it. They are not getting any cheaper! Good news, body wiring isn’t failing.
Q: Can a harness be saved?
Pierre: You can make temporary repairs but you cannot save one. Don’t try.
Q: What about the throttle actuator and the engine control unit?
Pierre: These can be rebuilt with new wiring but they should always be checked for issues. New and used engine control units are available, but they have to be programmed to the car.
On a final note, while engine harness replacement is expensive, nothing can hold a candle to the smooth running, powerful and efficient M104 engine. This power plant is certainly worth the investment, and then some, for how strong and smooth it is. Happy motoring!
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