Diagnosing Common Pattern Failures on Mercedes-Benz Vehicles – UnderhoodService

Diagnosing Common Pattern Failures on Mercedes-Benz Vehicles

Mercedes-Benz cars have long been associated with fine design and manufacturing quality. The proof for me is its use as hired transportation (taxi and bus) in many Third World countries. But for the U.S. market, the addition of power accessories and electronics is much more prominent than in other parts of the world. A quick overview of the most common complaints and repair patterns points to electrical problems as a serious problem for many M-B models produced in the last 15 to 20 years.

 photo 1: gizmophoto 2: looks like trouble

According to Wiki, the term Gremlins was first used to describe aircraft mechanical problems during World War II that otherwise could not be explained. To this day, we continue to blame mythical creatures for a lot of things. Of course, there was the Gremlin car in the early ’70s, loved by some and questioned by many. In the 1984 movie, a small creature named Gizmo (see Photos 1 and 2) turned into a real problem for a small town.

Today, when a “gizmo” fails on a car and it can’t be easily found, we often just point to an unseen source of interference; a Gremlin.

Over the years, as many others have likely experienced, I’ve been haunted by problems on cars that just cannot be easily explained. Electrical faults, in particular, have been increasing in number on automobiles since more and more basic operations are turned over to modules, processors and linked controllers (CAN).

Mercedes-Benz cars have long been associated with fine design and manufacturing quality. The proof for me is its use as hired transportation (taxi and bus) in many Third World countries. But for the U.S. market, the addition of power accessories and electronics is much more prominent than in other parts of the world. A quick overview of the most common complaints and repair patterns points to electrical problems as a serious problem for many M-B models produced in the last 15 to 20 years.

In this article, I’m going to look at some common pattern failures on various models, and ways to approach and, in some cases, answer the questions that come with unexplained failures. This is not a hit on the quality or ­design of Mercedes-Benz ­vehicles in particular, as all manufacturers are experiencing more and more “Gremlins” of their own.

As always, this article should not be considered a substitute for a repair manual for a particular model or system, but rather a starting point for doing the diagnosis in a planned and ­focused way. You will still need to access specific information, and then proceed to test and diagnose with the proper tools, scanners and a lot of patience.

One other thing is possibly going to be needed, something that many technicians and more than a few other automotive professionals tend to be unable to access, and that is help.

I’ve come to the conclusion over four decades of automotive repair that whatever problem I might see on a particular car or truck, someone else has probably seen the same thing and was able to solve it. I just need to find that person. Today, that person is often just a few mouse clicks away or can be found with a phone call or a bit of research.

There have been so many cars over the years that just would not respond to any kind of intelligent thought process as to what was causing a particular problem. Resolution was not always complete or even particularly satisfactory. Some vehicles were just sent back to where they came from when an economically acceptable repair was not possible.

Throwing parts at a car today can often exceed the value of a 10-year-old model in a very short time, even on a Mercedes-Benz. This is where using time wisely to research a problem to see if it’s a pattern failure for a particular model can often save you some hair, and a customer some money.

The one story that keeps coming back to me when dealing with gremlins doesn’t involve a Mercedes-Benz model, but is a good illustration of how persistence and finding that one other person can be so important. It was nearly 20 years ago and the availability of on-line networking was just beginning to be used by the manufacturers to track and resolve issues that could potentially affect hundreds, if not thousands, of cars in an epidemic of simple mistakes that could ruin a car maker’s reputation.

That car was a semi-exotic, highly sought-after model that was produced by a foreign manufacturer but sold under another manufacturer’s badge. Treated by the producer as an orphan, these cars were electrical “monsters” with very little support from the factory when problems came up.

The problem was an intermittent loss of all electrical power that would cause a total shutdown of just about every electrical device on the car, but the car would always restart and then drive for days before it would happen again. Because of very low production and sales numbers, there just was not much in the way of pattern failure information to go on. This car was new, I was the responsible service manager and the car needed to be fixed.

Getting the problem to duplicate often enough was the first problem. We needed to find some “trigger” that started the failure. Our service team took turns driving the car, and eventually put more miles on the car than the owner had! It would never fail for us, at least not for long enough to trace and correct a problem.

After more than a month in the shop, countless hours of wasted time, and hundreds of dollars in swapping out every possible module and switch that might be causing the problem, I called a service manager buddy who worked for the car manufacturer’s dealer. Within five minutes, he was able to tell me what trim piece to remove, which screw to replace with a shorter one and why it caused the failure. This was a process the carmaker did to its model when the car arrived from the factory, but it was never mentioned in a service bulletin or other message, for fear that the word would get out that there was a serious problem, causing a recall! Finding information like that is much easier now.

Some of you won’t like to hear this, but, at some point, for your sanity, you just might need to ask someone or seek out help on a repair. There are some big names in auto repair information technology and I’ve used just about all of them at numerous times in my career. Help is available at a cost that will actually make your business some money if it’s used. Information is also readily available for professional technicians through networking organizations like ASA (Automotive Service Association) and iATN (International Automotive Technicians’ Network). Factory service bulletins are available from a number of sources, and factory repair information is easy (however, sometimes expensive) to get online. Just knowing that someone else has resolved a particular problem is a comfort when dealing with Gremlins.

Another significant source of information is found at your parts supplier. A pattern failure that affects hundreds or thousands of cars is usually addressed by the aftermarket in the form of a kit or rebuilt product that can be a significant upgrade from the OEM, and often at a lower cost. In a lot of cases, the dealer parts network has stepped up to also offer upgrade kits.

Like in the movie, Gremlins can become monsters when subjected to some seemingly minor conditions. Water, heat (sunlight in the movie) and food can be problems that can turn a relatively minor annoyance into a life-threatening condition.

You will need a compatible scanner for M-B vehicles (see Photo 3). There are many scanners and programs on the market, but some are limited in their ability to provide all of the functional tests needed to resolve electrical problems. Being able to activate and monitor various components helps with difficult ­diagnostic problems.photo 3: information such as component locations are needed to help with diagnosis. (courtesy of identifix)

If you don’t have a compatible scanner, you can still use a circuit tester to activate or test various circuits, but you will need accurate wiring diagrams to avoid causing additional problems. As mentioned earlier, there are many on-line sources for repair information, but having the repair information that exactly matches the car you’re working on is extremely important.

Training is also a particularly valuable tool. Knowing how to do voltage drop and draw tests is of primary importance. Having access to design and operation information is just as important. As I have indicated many times in the past, the owner’s manual can also be a great source of information.

Vehicle history, normal use and a description of the particular problem are paramount in attempting to repair any problem that’s intermittent or affecting more than one system.

Looking over the pattern failures for Mercedes-Benz cars built in the last 15 years, there is a common thread of electrical problems in just about all models. Trying to pick out some of the more common faults is difficult. The shear number of different chassis, series and classes of cars in the M-B line is daunting. And, of course, determining what model you’re working on is particularly important when dealing with obscure failures, or in the case of this article, Gremlins. Knowing when and where a particular vehicle was built can have a bearing on both tracing and repairing a fault, but it also has a lot to do with the source of both OE and replacement parts. 

I would say that basically, when more than one system is affected by an electrical problem, or there are failures in more than one module, there may be a Gremlin at work. For the record, I consider any problem that can’t be explained by a simple failure to be unexplainable. It’s just natural for Murphy’s Law to take over and cause problems, but a mythical cause is something else.

One common problem I’ve seen in a number of late-model M-Bs is a situation where the transmission lever won’t move, the key won’t crank the engine or the car won’t move, even though the shifter moves freely.

On automatic transmission models, there is no “shift linkage.” The transmission is controlled electronically on most models built in the last 15 years. On some models, there is a cable going to the ignition switch, but the connection to the transmission is by wire. If there are a number of codes related to the shift module (N15/3 or N15/5) or transmission range switch, look closely at the shifter/console and see if a Gremlin has been released in the form of spilled coffee, juice or maybe even the sunroof was open the last time it rained (happens all the time here in the Northwest).
photo 4: a visual inspection can sometimes turn up gremlin attacks, like this chewed abs wiring.
Another place to look on many M-B models is under the floor mat on the passenger’s side. Mercedes-Benz and many other manufacturers continue to hide modules, connectors and control units under the mat, toe board or in the right-side kick panel. Especially on cars that have had body repairs in the front end, water can make its way undetected into this area and cause serious problems. Obviously, cars that have been in high water areas (floods) can also develop problems many years after a Gremlin has been deposited.

On ML series cars (SUVs), a problem that occurs quite frequently is in the wiring near the windshield washer bottle. Corrosion in the connections at the ABS module can cause a number of codes relating to the ABS and the traction control system. Of course, there are also potential problems with any wiring that has been attacked either by living or environmental forces (see Photo 4). Visual inspections should be a first diagnostic process whenever an electrical problem is suspected.
photo 5: critters seem to go for certain colors of wiring, leaving others alone.
I don’t know if Mercedes-Benz uses environmentally friendly, soy-based wiring insulation, but I do know that critters of many types seem to have had a taste for its wiring and hoses for many years (see Photo 5).

I’ve long felt that some manufacturers must put flavorings in their wiring, as the most tasty ones seem to be the red (peppermint?) and black (licorice?) wires, with the brown (milk chocolate?) ones coming in a close number 3.

Breather hose (see Photo 6) also seems to be especially prone to snacking and there are any number of hidey-holes in which to store more palatable food (see Photo 7). This, of course, is also true of other makes.
photo 6: breather hoses have been popular snacks for many years.
Another big problem that Mercedes-Benz has failed to address for a number of years is the quality of its wiring harnesses. In particular, look at the wiring to the injectors and sensors (see Photos 8a and 8b). With multiple misfire codes, fuel trim or timing faults often occurring together, a close look at the condition of wiring is something that should be a part of any service.photo 7: rodents have their storage needs, too.
photo 8a: injector and sensor wiring is susceptible to heat damage and degradation of the insulation.Photo 8b: Camshaft sensor connector

Electrical Gremlins are going to be more and more common as the cars of the 21st Century age. With ­networked systems and environmental conditions evolving to make diagnosis harder and equipment more prone to attack, thorough inspection and attention to detail is going to become even more important.

My advice is to not try and fight Gremlins alone. It’s too big of a responsibility to attempt to prevent or find damage for you to do it alone. Learn to seek the wisdom of those who’ve been there before and follow their advice.

It worked against the Gremlins in the movie because once their weaknesses were revealed, they were eliminated. Happy hunting.

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