Mercedes CDI diesel and Sprinter SCR problems

Jason recently observed a factory Mercedes webinar for the complex problem of diagnosing and repairing Mercedes SCR systems for diesel cars and trucks and Sprinter vans. We put this knowledge to use today for repairing a 2010 Sprinter van with an SCR problem.

SCR stands for Selective Catalytic Reduction and the vehicles that have it are usually labeled or badged ” BlueTec ” It is the environmentally-friendly diesel technology from Mercedes-Benz used to reduce emission levels. BlueTec uses Urea or diesel emissions fluid injection into exhaust gas stream in specially designed chambers in the exhaust to reduce emissions, especially NOx (oxides of Nitrogen).

The factory scan tool can be misleading and it is common for technicians to misdiagnose the system and change expensive parts needlessly. This fairly new technology uses the Mercedes SDconnect to connect directly to the high-speed chassis CAN network of the vehicle. This allows monitoring of the raw sensor data and non-calculated information for precise diagnosis. The factory Xentry scanner cannot display and process this information the way this specially designed package can. On this vehicle we discovered a slight difference in the NOx sensor readings between the front and rear sensor and this led to diagnosing a faulty front NOx sensor. These sensors are around $600 at the time of this article but this is still way cheaper than if it was misdiagnosed as an SCR failure (currently $3700). 

Another Mercedes V12 wire harness repair job.

Autobahn Performance is doing another air mass meter wire harness repair. We are getting lots of these cars in for the repair, even being shipped in from across the country. We can repair the air mass meter portion of the body-mounted engine harness without removing the harness. Do not attempt to cut open the body harness and repair the wires; this customer did and ended up cutting and damaging non deteriorating wires.

You can see in the pictures the wires are molded into a pass-through loom. It’s impossible to separate and untangle the wires without damaging the entire harness. We have repair harnesses in stock to repair the body-mounted engine harness on V12 600’s. Usually, only the air mass sensor wires fail. $295 each, 2 needed per vehicle.

The harness can be run alongside the factory harness, sheathed in long lengths of heat shrink tubing and connected directly to the engine computers. The other harnesses that often fail are the lower engine harness (also called starter harness or positive cable harness). We can repair your damaged harness but it may be available at a reasonable price from the dealer. (Get me your VIN and I will get you a price.)

The throttle actuators have harnesses that come off of them and connect to the body harness. These harnesses fail internally. To check them you need to carefully make a 1 to 2 inch incision in the covering of each actuator harness and examine the wires inside. If they’re bad we can rebuild your actuators (done in house): $695 each includes internal motor, plastic gears and new cable harness connector end is  disassembled and re-used.

The fuel injection harness sits on the top of the motor and is usually available from the dealer at a reasonable price. These can be rebuilt by us but must be inspected to give an accurate price. The body harness of the car usually does not fail or deteriorate on 129 SL’s. If it has deteriorated it probably should be replaced with new from dealer. (We could rebuild this harness but it would be expensive and time consuming.)

Removing broken axle stub with welding rod.

It’s been a while since I needed to use this trick and it saved me (and the customer) about 8 hours of labor. This 2010 Mercedes C300 204 chassis vehicle broke the rear axle shaft (“half shaft”). It sheared clean off where it comes out of the side of the differential. Vehicle was in a minor accident and somehow the axle twisted and broke. We used a miller stick welder and the largest diameter welding rod we had around. We let the rod get a good molten puddle going in the center of the broken axle stub and then pushed the electrode straight into the weld puddle stopping the arc. We quickly unclamped the rod and shut the welder off. After allowing the rod to cool we used a slide hammer with a vice-grip attached to the pulling end (This is also a neat and usefull trick / tool I made). I tightly clamped the vice grip to the end of the welding rod and with one good pull of the slide hammer the broken stub popped out still attached to the electrode. The new axle went in fine and everything worked good without removing and disassembling the differential.

Mercedes SL600 with wiring problems

We are working on a very clean and original 1995 SL600 (129 chassis) that has less than 30,000 miles on it. Unfortunately this vehicle was made by Mercedes with faulty wiring insulation that has deteriorated and caused short circuits in many components.  The main engine harness (just includes the fuel injector wires and a few others) is in the worst condition. The main body mounted engine harness has some wiring circuits that are bad, Mostly the air mass meter harness. This harness is over$5000 for the part and approximately 60 hours labor to replace. It runs from the headlights to the trunk of the car, behind the dash and under the interior. This job at the Mercedes dealer could legitimately be $14,500. Just for the body mounted harness. Here at Autobahn Performance Inc. in Ft. Lauderdale Florida we have developed a repair kit and procedure to repair this harness without removal for less than $1800. It will work on V12 engine in the 129 (SL convertible) chassis and the 140 chassis (4-door sedan or 2- door coupe). In rare cases more than the normal wire circuits are affected by the insulation deterioration and the harness is unrepairable. This is usually only in European-delivered vehicles and the sedan 140 chassis. If a wire suffers from insulation deterioration the entire length of wire must be replaced. Trying to splice onto a section of the wire will not work properly and will result in short circuits and possibly a fire. The insulation will start to crumble near where you are trying to repair it and you will probably make your problems worse rather than better.

The throttle actuators (electronic controlled throttle body, TBA Throttle Body actuators): There is one for each side of the engine. (note: the one on the right side of the engine feeds the air into the left side of the engine and vise versa) The actuators have a short wire harness coming out of them, it’s about 14 inches long, that connects to the body-mounted engine harness. These harnesses also have the insulation failure problem. The harnesses are part of the actuators and are not available as a separate piece from Mercedes. The Actuators can be rebuilt and new harnesses soldered into place. Again, do not try to repair these as you will not be able to properly connect new wires to the electrical connector end of the harness. Short circuits in the wiring of the actuators commonly cause damage to the E-GAS actuators (the control units that control the throttle actuators). These also can be rebuilt.

Mercedes-Benz will not sell valve body for Mercedes 722.9 transmission

Update 9/6/2013
We are able to supply the 722.9 conductor plates and valve bodies to our customers. They take a few days to obtain and we have to provide the correct documentation, but we are able to get the parts for most local customers. (The dealers do not even stock these, so it takes a few days if you take your vehicle to the dealer.) We’ve had issues with private dealer owned vehicles that have open titles. There is currently no way to get a TRP part if the vehicle is not owned and registered by an individual.

Mercedes now will not sell 722.9 valve bodies or conductor plate control units, even with the TRP paperwork completed. The newer Mercedes 7-speed transmissions (also called G-tronic) have the electronic control unit mounted inside the transmission.These control units are failing frequently with speed sensor codes. (The usual codes are related to speed sensor faults.) The speed sensors are not replaceable as they are part of the electrical conductor plate that bolts to the valve body inside the bottom of the transmission. The transmission does not need to be removed to replace the valve body or the conductor plate. A valve body comes complete with the conductor plate / control unit. Early valve bodies must be replaced as a complete unit, but later second or third generation units can be repaired by just replacing the conductor plate / control unit assembly. We have replaced many of these and have the capability of performing the SCN coding to complete the installation of this repair.


These units are failing so often that the dealer has restricted selling them even in their own shop. To order a part the dealer must submit a request with computer scans and control unit log reports of the suspected failed vehicles electronics. The factory then sends the parts they deem necessary for the repair (conductor plate assembly or complete valvebody).

It’s rumored that Siemens VDO (the manufacturer of the electrical control units) is in a battle with Mercedes over the validity of failing units. Why should an internal problem with Mercedes and its suppliers affect our ability to acquire parts and repair our customers’ vehicles? Mercedes is requiring the vehicles be brought into one of their dealers for any valve body or conductor plate repair even if the vehicle is not covered under warranty and the customer is paying for it. In our customers’ view this would be perceived as an inability for us to repair their vehicles and give the dealers an unfair advantage at gaining the customers’ business. One customer lost for life could be worth hundreds of thousands of dollars.


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