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W140 Check Engine Light

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rexleung's picture
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I Have An Experience With My Check Engine Light That I Want To Share With The Friends In This Forum.

I Have A 92' 300se, W140, The Check Engine Light Of My Car Lighted Up. I Went To My Mechanic Who Told Me That It Was The Crank Shaft Position Sensor. I Replace The Switch For $278.00. About One Weeks Later, The Check Engine Light Came Again. I Took It To The Same Mechanic Who Told Me That It Is The Harness Of The Fuel Injectors' Problem. To Replace The Harness, He Charged Me More Than Seven Hundred Dollars. I Asked Him Whether He Could Guarantee That It Would Definately Fixed The Check Engine Light, He Told Me He Did Not Know For Sure. The Harness Is Surely Bad And I Need To Replace Them. As He Did Not Assure Me It Would Fix The Problem, I Did Not Agree To Replace The Harness.

I Went Home And Tried To Look For The Problem Myself. I Found Out That The Clamps Below The Mass Air Sensor Have Been Loosen To Such An Extent That Make Me Believe That Someone Has Loosen It Purposely. I Tighten Them Up And Reset My Check Engine Light. It Fixed The Problem Without Changing The Harness. It Has Been More Than Two Weeks That The Check Engine Light Has Not Yet Come Back.

The Loosen Of The Clamps Might Due To Two Possibilities; (1) Someone Purposely Loosen Them Or (2) Someone Forget To Tighten Them Up After He Fixed Other Parts. I Don't Know The Answer.

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jtripp's picture
Babylon, NY
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Joined: September 15th, 1976

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Vehicles
1996 S420 LWB
1995 S350

The wiring harnesses of all early '90's Mercedes were purposely made of an easily recyclable plastic. This resulted in wire insulation that broke down with heat from the engine. The insulation had radial and longitudinal cracks and also crumbled into powder with the wires shorted out. Check engine lights came on and good components were replaced unneccessarily. The lights remained on because the wires in the harness were shorted to each other or shorted to ground.

Now comes the bad news: The bad harnesses permanently damage the computer by burning out the traces in the printed circuit board. So, even a new harness will not make the light go out. A used salvage yard computer won't allow the engine to start. Mercedes won't install your used one. Mercedes would like to sell you a new computer. Info from your existing computer is downloaded at the dealer and installed in the new one. I believe that Mercedes is the only one with the equipment capable to do this job. Maybe George Murphy or Beckmann Technologies can repair your computer.

The reason the check engine light took so long to come back on is because when the DT Codes are cleared it takes four "drive cycles" for the light to come back on. A"drive cycle " is starting out with a cold engine,going through all the gears and then followed by a cooling down period.

The above information was supplied to me by club member joeym7 who is a factory trained Mercedes Tech with 25 years experience at a large Mercedes dealership outside New York City

I would examine the wires and replace them if the insulation is cracked and split. A new harness may make the engine run better but probably won't make the light go out. Do it yourself. No splicing of wires is involved. It plugs into a harness plug on the right fender under the black plastic trim. I became personally involved with the same problem on my cars and on a friends '95 S320. The harness had already been replaced on my friends car and Joe McCambridge said that the engine was operating perfectly even though the check engine light was on. I replaced the bad harnesses on my '93 300SD and '95S350 last summer. The '95 came from Arizona. All the wires plugged into the top of the injection pump had no insulation and were all shorted out. Surprising there was no check engine light. Records that came with the car showed that 2 components were changed in the past with a note that the same codes came back due to computer problems. I still get the 2 DTC even with the new harness but no check engine light. Let us know how you made out.

__________________

New York club member since 1976.

Continuous Mercedes ownership since 1964.

1993 300SD with '99 606 engine. 111,040 miles.
1994 S350   75,238 miles
1995 S350 148,124 miles.
1996 S420 with '99 606 engine  Sold
1999 S600  one of 14 imported  Sold 
1989 Bentley 8  Sold at 40,000 miles.

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Dean Yelenik 1's picture
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A 92 300SE does not have the problem wiring harness. It is a 104.990 KE engine which means it still has a distributor cap and rotor. The problem wiring harness began with the 3 coil HFM engine management system which did not come to the 140 body (104.994) until 1994. This is not to say there couldn't be a problem with your wiring harness - but you don't have the notoriously bad one. I too would not do anything unless and until a) the CE light comes, b) you have operating problems, and c) you physicaly see a problem with the current harness.

Anonymous
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Anonymous's picture
Wiring Harness

I was interested buying a 1999 S420 and currently have a 1995 S420 in which I have replaced the wiring harness. Does the insulation problem also exist in the 99 S420 that is discussed in the original post?

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Dean Yelenik 1's picture
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No, a 99 should be fine. Your 95 was pretty much the tail end of the problem.

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