Each Mercedes vehicle comes equipped with a host of different cutting-edge safety features.
In addition to active safety features like active brake, lane keep and blind spot assist, your Mercedes is also equipped with a computerised system that detects electrical and mechanical problems.
When issues are detected, the drivetrain on your Mercedes enters a condition called limp mode to keep you safe, and avoid a serious (and potentially dangerous) mechanical problem.
Thankfully, regular Mercedes-Benz service helps you avoid the issues that can cause your vehicle to go into limp mode. Combine that with the brand’s reputation for reliability and quality engineering, and the majority of Mercedes-Benz owners never find themselves dealing with limp mode.
However, as with anything mechanical, there’s always a slim chance that something may go wrong, triggering limp mode.
Today, we will go into detail about how limp mode works, the various problems in your engine or transmission that can trigger it and where you should go for Mercedes service in Melbourne.
Limp mode, explained
Today, nearly every system in your Mercedes-Benz is operated by computers, which receive information from a range of different sensors.
One example is the mass airflow (MAF sensor), which helps the engine regulate its performance by providing information that helps the computer regulate the intake of air. Too much or too little air can affect your air-to-fuel mix, which can result in reduced performance (in the case of a lean mixture) or poor fuel efficiency and black smoke (in the case of a rich mixture).
These systems don’t just ensure fuel efficiency and performance, however – they can also double as a safety features, protecting you from electrical and mechanical problems.
As long as the information coming from these sensors falls within preprogrammed safe ranges specified by Mercedes-Benz, your transmission and engine will continue to function as normal.
When this information exceeds these safe ranges however, it may result in major problems with your engine or transmission.
For example, your turbo sensors may report that your turbocharger is supplying too much boost pressure, which can stress major engine components like the cylinder head, manifolds and vital engine sealing gaskets. This may even result in turbo or engine failure if the overboost condition is sustained.
To avoid such an outcome, your Mercedes-Benz computer will automatically enter limp mode when it receives sensor information that is out of safe operating parameters.
The purpose of limp mode is to protect you in the event that certain conditions have rendered it unsafe to operate your Mercedes-Benz at its normal level of performance. Its goal is to get you to your Mercedes-Benz mechanic safely, without further damaging any engine components on the way to the workshop.
It’s pretty hard to ignore when your Mercedes-Benz enters limp mode. Like the name suggests, your Mercedes-Benz will throttle throttle engine and transmission capacity while in limp mode. In particular, you may experience:
- Reduced engine power
- Limited or shut off turbo boost
- Restricted RPM
When limp mode triggers, your computer might shut off electronic shift solenoids, restricting your engine’s ability to switch gears and locking you into a lower gear that lessens the strain on your engine and transmission – usually second or third.
How transmission problems put your car in limp mode
Low fluid levels
As we’re sure you’re aware, hydraulic transmission fluid the lifeblood of your car’s transmission system.
Just like engine oil, transmission fluid plays a crucial lubricating role in your transmission. It ensures that all the different moving parts in your transmission system are moving seamlessly in harmony with each other, minimising wear and friction. Not only does the fluid lubricate, it is also required to actuate the hydraulic elements and clutches responsible for changing gears.
Low levels of transmission fluid can result gears not fully engaging, which in turn can cause your transmission to behave erratically. You may experience shuddering, slipping or thumping between gear changes.. Over a prolonged time, this will wear down your transmission to the point where you may need parts of it completely rebuilt or replaced.
In addition, the hydraulic oil in your transmission also regulates its temperature at a sustainable level.
While there are many causes of an overheated transmission, low levels of transmission fluid are one of the most common. This in turn can result in serious internal damage to your transmission:
- Seals can harden and leak internally
- Your transmission may start slipping
- Clutches prematurely wear away, depositing friction materials all through internal valving and hydraulic elements
To protect your transmission from further overheating (and keep you safe while on the road), your Mercedes-Benz will activate limp mode, throttling your performance, preventing gear changes and thereby reducing transmission temperature.
This will give you just enough power to get to a Mercedes-Benz specialist mechanic for an emergency repair or service. Once you turn up at the workshop, your Mercedes expert will determine why your fluid levels are low, top it up and if necessary, replace damaged or worn parts of your transmission.
Failing transmission solenoids or sensors
When the transmission control unit sends out the message to upshift/downshift, it activates transmission solenoids, which then open up and allow pressurised hydraulic fluid into the clutch of the required gear..
This actuation is controlled by a valve body, which is a complex, maze-like structure that essentially serves as the switchboard for the transmission. It’s made up of countless passageways, channels and solenoids that redirect transmission fluid to where it’s needed for gear changes.
When pressurised hydraulic fluid is directed to its destination by various solenoids, it fills up different valves to activate the correct clutch pack or brake, shifting your car to the appropriate gear.
Whether it’s the solenoids responsible for directing fluid to the valve body to begin with or a transmission speed sensor, solenoids and sensors are essential parts of your transmission. Naturally, when one of these items fail, it can result in hydraulic fluid not being directed to the appropriate destination, or the transmission control unit receiving an incorrect signal.
And that in turn can result in transmission problems such as:
- Delays in shifting
- Shifting to the wrong gear
- Skipping gears altogether
- Shifting gears uncontrollably
These are just some of the consequences of faulty transmission electronics, all of which can present a major hazard while you’re on the road. To protect you, your Mercedes will switch to limp mode, giving the transmission a set of default values, just enough for the transmission to operate in its most basic functions, such as forward or reverse..
This protects the transmission’s internal components from further damage, while also allowing you to safely take your vehicle to a Mercedes-Benz specialist mechanic for an urgent transmission repair.
Missed transmission service
In other cases, a failed transmission could be the result of the transmission having never been serviced!
There is very little maintenance required on modern Mercedes-Benz transmissions, apart from routine service, were the oil and filter are replaced. If the transmission is not serviced when it is due, there is a good chance it can develop problems.
The oil filter in the transmission can become blocked with friction material, reducing the oil pressure and flow. This in turn increases the temperature, which causes friction materials on clutches to burn. The hydraulic oil degenerates quickly, and before almost every component inside the transmission is in some way damaged.
While a transmission service might seem like an extra cost, the benefits are clear. Service your transmission according to the schedule to avoid a total failure.
At Ammstar, our independent Mercedes-Benz mechanics use exclusively high-quality OEM and genuine parts, including Genuine Mercedes-Benz transmission oil specific to each vehicle. This ensures that your transmission is serviced correctly, and according to the manufacturer’s specifications.
Other causes of limp mode in your Mercedes-Benz
Problems with your EGR valve
Ambient air contains approximately 78% nitrogen, 21% oxygen and small amounts of other gases. When exposed to extreme temperatures (such as the heat generated by a powerful Mercedes-Benz engine), the air burns off and creates nitrogen oxides (NOx), which are pollutants that damage the ozone as well as human health.
Like the name suggests, the main purpose of your exhaust gas recirculation (EGR) valve is to recirculate a portion of the inert exhaust gases from your engine back into the combustion chamber. Instead of sending them to the exhaust, some (around 10%) is instead redirected to the EGR valve, which introduces already-burned gas into the engine.
This process replaces some of the air in the cylinder with inert exhaust, effectively reducing the potential energy that can be generated by combustion. As a portion of the air is inert, this will result in a slower explosion in the cylinder as well as a lower combustion temperature, which reduces the amount of NOx created by your engine.
A faulty EGR valve (for example, a clogged or stuck valve) will struggle to control the flow of exhaust, which can affect the air/fuel ratio. This in turn can reduce engine power, tank your fuel economy and lead to rough idling. You may also find yourself in violation of emissions standards.
In some cases, this will also trigger limp mode in your Mercedes-Benz.
Faulty wires and sensors
Sometimes, there’s no problem with your engine or transmission at all – in some cases, the issue is as simple as a faulty wire or sensor.
Wires in particular can be damaged by the heat generated by your engine, blunt force, abrasion, age, battery acid or an improper repair job carried out by an unqualified mechanic.
When these wires are damaged, they may not be able to send information to the computer, which will then interpret the lack of data as a failure of certain engine and transmission systems. As a precaution, your Mercedes will enter limp mode.
In other cases, the sensor itself could be malfunctioning and sending incorrect information to the computer.
Mercedes-Benz vehicles are unique in the number of sensors used to ensure optimal performance at all times. As a Mercedes-Benz diagnostic specialists, Ammstar is uniquely qualified to detect and fix issues with this intricate system, identifying the specific sensor or wire that’s faulty.
As more and more car manufacturers (including Mercedes) shift towards smaller, more efficient engines, many manufacturers are turning towards turbocharged engines in an effort to preserve engine performance and power.
This development allows even small displacement engines to punch above their weight, outputting a greater amount of power relative to their small size. Additionally, turbochargers are also commonly found in performance vehicles such as Mercedes’ AMG models to further increase engine output.
Turbochargers are designed to increase the amount of compressed air in an engine. Just like using a bellows to feed a fireplace, the extra oxygen provides more fuel for the combustion process, which directly translates to increased engine performance.
The problem is overboost. This is when the turbo runs at a higher pressure than the engine can safely handle on a sustained basis. While some cars have overboost programmed into their engine computers to provide extra power when needed, sustained overboost can be damaging to your engine.
Overboost can stress engine components like the cylinder head, gaskets, and intake manifold. Over time, it can even result in these components failing altogether, which can have disastrous effects on the rest of your engine.
If your intake system sensors detect sustained overboost pressure, the ECU in your Mercedes-Benz may trigger limp mode, shutting off the turbocharger altogether to minimise potential damage.
Getting out of limp mode
Sometimes, your Mercedes will (temporarily) operate normally after given a bit of time to “cool down”, returning to its baseline level of performance.
If you want to fix the problem for good, however, we strongly recommend taking your vehicle in for a Mercedes-Benz service.
Many times, it will be a transmission-related problem that will throw your vehicle into limp mode. As one of the most intricate and complicated parts of your car, DIY transmission repair is beyond the scope of most enthusiasts.
For example, say your Mercedes has gone into limp mode because of a valve-related issue such as a faulty valve solenoid. Since the transmission is located on the underside of the car, you’ll need a hoist to gain access. Oftentimes, it also needs the transmission to be removed altogether, which requires specialised tools and equipment.
Needless to say, very few home mechanics have this type of hardware in their garages!
Computerised transmissions such as those found in modern Mercedes-Benz vehicles make this even more complicated. For example, issues with a sensor or bugs in the computer that govern your transmission require skill and special computerised tools.
Even relatively straightforward transmission problems can be quite involved and complex. This is to say nothing about complete transmission replacement.
When it comes to your transmission, you want quality. Getting your transmission rebuilt or repaired by a skilled mechanic who’s familiar with Mercedes-Benz models can add many years to your car’s life compared to a generalist mechanic who offers you a cheap transmission repair.
At Ammstar, we are Mercedes-Benz diagnostic specialists. We are specially trained in Mercedes-Benz systems, and use specialised systems to identify problems with your transmission and other systems.
Is your Mercedes stuck in limp mode?
Fix it today – book a Mercedes-Benz service in Melbourne ASAP
If your Mercedes-Benz has entered limp mode, don’t ignore it. In many cases, limp mode is a symptom of a deeper problem in your engine or transmission.
For example, if your Mercedes has entered limp mode because transmission problems, you may experience sudden or unexpected gear shifts while on the road. This won’t just lead to a rough and inconsistent ride – this unpredictability can also make you a risk to other motorists.
While it’s possible to continue driving your Mercedes in limp mode, we strongly recommend making your next destination the nearest Mercedes-Benz specialist mechanic in Melbourne.
Ammstar is an independent Mercedes-Benz mechanic with a simple mission: to offer an affordable, independent alternative to dealership service.
Unlike other independent workshops, Ammstar has extensive experience working primarily with Mercedes-Benz vehicles. Thanks to years working on cars with the three-pointed star, we’re able to offer service equal to the dealership.
Using advanced diagnostic tools, we’ll perform a deep dive into your engine and transmission to determine the exact reason your Mercedes has gone into limp mode.
And once we’ve narrowed down the root cause, our team can resolve the issue, using high-quality genuine and OEM parts. Our 20 years of hands-on experience and brand-specific knowledge also allows us to perform specialist services if needed.
You won’t get that from the corner mechanic!
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