Consistently rated as well engineered and well built, the Mercedes line fills almost every class of vehicle today, from small fuel-efficient people-movers like the Smart Car, to large over-the-road trucks and every niche in between.
Our shop performs routine maintenance and more involved repairs on hundreds of Mercedes vehicles every year. Most of the models we currently service are late-model (1995 and newer) cars that have been out of factory service for a few years. As a Washington State emissions repair facility, we also see a lot of “check engine light” problems. Older models still come in from time to time; mostly long-time customers with well-cared-for vintage models.
The focus of this article will be general maintenance, with an emphasis on common problems and how to diagnose and repair them. Special tools or technical assistance can sometimes be a reason to turn away certain jobs. For any given model of Mercedes, do a little information investigation before getting in over your head. What would seem to be a simple repair can go terribly wrong if the proper procedures aren’t followed.
Also, there is much to be gained by having scanner access to more than just the generic OBD information. Most Mercedes models since the early 1990s rely on interlinked control modules for the various systems, and each system can be influenced by another system. To attempt repairs beyond the basic maintenance category, scanner software that is Mercedes-specific is almost mandatory.
There will be a lot of generalities here. The transmissions used in M-Bs for the last 40 years have been basically the same and are generally bulletproof. Since we don’t have the resources to rebuild these transmissions, when there is a failure, we replace it with a used unit, or purchase and install a rebuilt unit from one of several M-B rebuild specialists. Note: Shopping for a rebuilt unit for these cars based on price alone is a mistake. The best warranty in the world doesn’t help customer confidence if the quality isn’t added in the process.
Service on M-B transmissions is pretty standard. Most models up to 1996 can be conventionally serviced (fluid flushed and filter changed) like any other car. And like many other Euro cars built after 1996, the transmissions weren’t designed to be serviced and even lack a dipstick for checking the fluid level. But many models do still have a fill tube with what looks like a dipstick covering the end. But just checking the fluid level requires that a “special” tool be purchased. That tool is, in reality, a dipstick (See Photo 1).
Though expensive to purchase, at least that one tool fits almost all models with the “dummy” dipstick plug.
Transmission codes can be pulled and diagnosed with a scanner. Depending on the year and model, the scanner connects either through the M-B round diagnostic plug that’s mounted under the hood, sometimes under a cover or in the fuse box or through the OBD II DLC. You probably won’t get transmission-related codes without M-B-specific software. As noted above, since the transmission and engine control computers are interlinked, there may be some crossover of information and codes.
When you do come up with a transmission fault, do your homework before replacing the transmission. External problems can cause shifting and operation faults that transmission replacement won’t solve.
One common failure we see on newer models is prompted by liquids that have spilled into the shifter area in the console, killing the electronics inside the shifter housing. Another problem with older models where shift sequence problems have suddenly started can be traced to an improperly adjusted kickdown linkage, or a failed kickdown switch under the gas pedal. On models with accelerator linkage (pre-drive-by-wire), the proper adjustment involves aligning marks or indicators on the linkage, and making sure the cable is still attached at the transmission. Almost all cars with drive-by-wire have the kickdown energized with the pedal switch and a solenoid at the transmission.
You will need to check your database for specifics on recommended fluids. There have been some changes to recommendations over the last 10 years.
The rest of the drivetrain is fairly conventional. Depending on the year, there may be flexible rubber joints at either or both ends of the driveshaft. Slight, small cracks in the rubber around the outside circumference are OK, but cracks that go from the mounting points to the edge of the flex disc are a reason to replace them. For some reason, we’ve seen a number of cars come in with a vibration in the drivetrain, only to find that the driveshaft has been out, and disassembled. The reason for the vibration? Incorrect phasing of the joints. This causes a vibration that will come and go depending on attitude and speed. If you separate the shaft at the center joint, mark it so it goes back together properly.
Over the past 25 years, Mercedes has become the leader in networking the functions of various vehicle control units. This provides a seamless operation in connected systems, but can also lead to failures in one system that affects the operation of others. There are several vehicle diagnostic systems available that will connect and interrogate some of these various systems, but not one that will cover them all.
That said, when there is a failure in any component on a Mercedes, there can be a symptom that leads you to what would seem to be a totally unrelated area. For this reason, it’s important to always learn how a system works, and what else is going to be affected. Though this information is sometimes difficult to locate in repair manuals, often the description and operation is clearly given in the owner’s manual.
One of the most common problems we see in later-model (1990 and newer) Mercedes is deterioration of the wiring harnesses. This can and does lead to multiple, seemingly unconnected, failures that are, in fact, interconnected but not by design.
One way to determine if there is a harness problem is to find a harness connection at an exposed sensor in the engine compartment and disconnect it. Pull back the insulation and bend the wires. If there are any cracks in the insulation on an exposed connection, you can expect that all wiring in that harness will be affected. There is a very good chance for shorts or opens to occur, especially in the main harnesses where many components and multiple circuits are run through conduits. If an OBD scan indicates more than three or four faults on a common harness, you can suspect wiring degradation. One common area for wiring degradation is in the harness to the injectors on inline 6-cylinder models with electronic injection.
One upside to the wiring in Mercedes vehicles is that many of the harnesses are readily available and aren’t too difficult to replace. The downside is that they are expensive. However, when compared to the replacement of numerous other components like sensors and switches, the harness cost can seem like a bargain. Replacing just the components without the harness repairs won’t solve anything for long.
Depending on the extent of rewiring needed, you could possibly get away with the repair of a harness but, as we have experienced, a break at one location often means there are also other breaks. Though there are several models that seem more prone to develop harness problems, it may be more of an age or mileage problem. Since we work on many high-mileage (200K plus) M-Bs, we may have seen a few more of these than some other shops.
Another place to look for electrical problems, as in any vehicle, is the ground side of any circuit. Like most other German-designed cars, the ground lugs will usually be populated by numerous wires. The combination of dissimilar metals, high current and temperature variation can lead to connection weakness. In DIN wiring harnesses, only solid brown wires should be attached to ground lugs. A tracer in a brown wire indicates that the circuit is switched and is not intended to be solidly grounded. In today’s aftermarket world, wiring can get moved when accessories are installed, sometimes affecting components in other systems.
While the older Mercedes models (before 1998) have some fixed maintenance recommendations, later cars have no set guidelines for general maintenance. Even routine oil changes have gone to a “flexible” service schedule based on the vehicle’s use and driver’s habits.
In the real world, preventive maintenance will add greatly to the vehicle’s reliability, longevity and resale value. There are no secrets to driveability issues on M-Bs, just the need to do proper, intelligent diagnosis based on the customer complaint, and an understanding of the events leading to the driveability or maintenance problem.
We routinely see M-Bs at 75K to 80K that still have the original air filters, spark plugs that are worn way past what would be acceptable on another car, and serious sludging problems from lack of maintenance. With a new M-B customer, the first thing you should try to sell is a regular maintenance program.
Probably the most common OBD II failures we see are misfire codes and MAF failures, very often related to the plugged air filter and worn spark plugs. We’ve also had to replace more than normal amounts of ignition coils, probably due to the worn plugs and the high current necessary to fire them. Often there will be no actual MAF code, but the symptoms of loss of power, lack of acceleration and lack of idle should lead you to at least test the MAF for proper values under operating conditions.
On the V6 and V8 models, plug replacement can be time consuming. Very often, the plugs are locked into the head and there are two for each cylinder. You may need a collection of tools just to remove the plug connectors. Access for plug replacement will test just about everyone. Unless you have the smallest technician’s hands in the industry, the space available will use every ounce of patience you possess (See Photo 2). For particularly difficult cars, I’ve removed the coils first just to have some working room.
Other repairs we make on a regular basis are to the cooling system and brakes. Depending on model and engine choice, there may be many possibilities to get the wrong parts for a given vehicle. You will need the VIN, production date, engine size and chassis series numbers. On models with a fan clutch, there are a number of different methods to remove it. Getting model-specific information will save big headaches when trying to remove fasteners in the wrong direction.
The braking systems are all conventional, and normal repair procedures apply. Use quality parts and there won’t be any surprises. Remember that with cars that are designed to exceed the speed limit many times over, the last thing you want to do is take a shortcut on the brakes. If machining a brake rotor will take it to the minimum spec, it’s probably safer to replace it. That will also prevent comebacks for warpage.