Mercedes-Benz Transmission Problems
This article discusses the most common Mercedes-Benz transmission problems and how you can fix some of them. This troubleshooting guide will help you narrow down the transmission problem and teach you how to perform some repairs.
If your transmission is stuck in gear and you are far away from home, try this:
- Pull over and turn the car off. Remove the key.
- Do not press the gas pedal or any other buttons on the car.
- Wait 20 seconds.
- Start the car and drive to see if it is out of limp mode.
Your transmission may be back to normal, but there may be underlying issues that you need to address. If you ignore them, the car will go into limp mode more frequently. Don’t forget that first warning! It would be best to discover why your transmission is going into limp mode or not shifting.
You would be surprised to know that most transmission problems, such as hard shifting and no shifting issues, come from incorrect transmission fluid level or the infamous transmission 13-pin connector plug O-ring. These are easy fixes that you can even tackle yourself. Mercedes-Benz transmissions are very solid, and whole transmission replacement is rarely needed. The 5-speed transmission that was used between 1995 to 2008 is one of the best transmissions ever built. Now let’s see if we can help you troubleshoot your transmission.
You agree to use this information at your own risk by continuing to read this article.
- Low Transmission Fluid
- Transmission wire harness 13-pin (O-ring) connector oil leak/contamination
- The valve body or conductor plate defective
- Won’t come out of Park gear due to a defective brake light switch. (DIY How to replace the Brake Light Switch)
- Defective Shifter Module
Continue to read if you would like to learn how to troubleshoot your Mercedes-Benz transmission yourself and where you should start.
Transmission Troubleshooting Guide Step-by-Step
1. Check the Fluid Level
You want to start with the most common and least expensive problem. That is verifying that the fluid level is not low. On a 7 or 8-year-old Mercedes-Benz, the transmission fluid level is expected to be low due to seepage or other reasons. The car will go into limp mode when it detects a low level. The car has no dipstick to check the transmission level, but you can order one online for under $20. You will need Dipstick Tool for 722.6 722.9 Mercedes-Benz Transmission for measuring fluid level. Watch the video below to learn how to check the transmission fluid level on your Mercedes-Benz.
2. Read and Clear Transmission Control Unit Fault Codes
When the car goes into limp mode, it stores a fault code in your car’s computer. Specific codes related to the transmission are stored in the TCU (Transmission Control Unit), and generic codes such as P0705 are stored in ECU (Engine Control Unit). You don’t need to pay anyone to read the codes; you can retrieve them in a few minutes with a suitable OBD II scanner. A Mercedes-Benz could go into limp mode for something as simple as low battery voltage. If that is your case, replace your battery and use an OBD-2 scanner capable of clearing the fault codes from the transmission control unit. More on this later.
The video below shows you how to use a diagnostic scanner to troubleshoot your transmission and read and clear fault codes.
You may borrow one of these scanners from a friend or a local auto parts store. Your other option is to get one from Amazon, but you need to know what works on Mercedes-Benz cars. These scanners will cost about the same as a diagnostic at a repair shop, except you get to use them repeatedly. Here are two of our favorite scanners that will read and erase trouble codes from the transmission control units on Mercedes-Benz cars. Note that these work on 1996 and newer MB cars. They may be unable to read TCU codes on a few 1996-1999 models requiring you to use the 38-pin connector.
Full MB control unit diagnostics. Including Engine, Transmission, ABS, Airbag, SRS, SBC Brakes, ESP, EPS, Air Suspension, and more.
In-depth troubleshooting, but only works on MB cars. Offers bi-directional support.
Either one of these two scanners would get the job done. An advanced diagnostic scanner such as the YOUCANIC scanner is necessary for do-it-yourself Mercedes-Benz repair. They are affordable and can access most of the systems on your Mercedes-Benz, including Airbag, EPS, ETS, Transmission, Engine, front and rear SAM, SRS, brakes, and more.
3. Replace 13 Pin Connector Adapter Plug O-ring
When you scan the car, you may get fault codes that point to communication problems with the valve body or incorrect gear ratios. Before you spend hundreds of dollars on a new valve body (I will talk more about the valve body in the next step), replace the Mercedes Transmission 13-Pin Connector Adapter Plug + O-rings. The O-ring plug leaks oil and disrupts the communication between the TCU and the valve body. The O-ring is very easy to replace. Mechanics unfamiliar with Mercedes-Benz transmission suggest replacing the whole transmission when the root of the problem is the O-ring plug. Replacing the O-ring transmission plug is very easy; anyone can do it if they don’t mind getting dirty. Here are the instructions on how to change the O-ring plug. Once you replace the transmission O-ring plug, you will need an OBD-II diagnostic scanner to erase the fault codes.
4. Replace the Valve Body
If you still have issues with the car going into limp mode and the scanner points you toward the valve body, you may consider replacing it. The valve body is inside the transmission. The transmission does not need to be removed. Replacing the valve body can perform with the transmission in place. A new valve body at the dealer can cost between $500 and $1000.
Luckily for you, the transmission does not need to be removed. This means you can do this repair yourself if you are willing to get under the car. A new valve body at the dealer can cost between $500 and $1000. But because valve body failures are so common, you will find them on Amazon for a lot less; see Mercedes-Benz valve body listings here.
If your Mercedes-Benz is equipped with the 722.9, you can’t install a used valve body because the transmission control unit is part of the valve body. You will either need to get a new valve body which gets expensive considering that it will also require programming. An alternative solution is to try a Mercedes-Benz 7G 722.9 conductor plate repair service. These independent shops will take your old 722.9 transmission control module and try to repair it.
5. Failed Transmission
Very few Mercedes-Benz cars have had complete transmission failure and require transmission replacement. These transmissions fail in rare cases when water enters the transmission via the oil cooling lines or the radiator on cars equipped with Valeo radiators. The top of the radiator on your Mercedes-Benz contains the engine coolant/antifreeze chamber, while the bottom contains the chamber for the transmission oil cooling. If cracks develop between the two chambers, coolant will mix with transmission oil, destroying the transmission. This has been a well-documented issue for Mercedes-Benz cars equipped with Valeo radiators.
If your transmission has been one of the few that fail, you should consider a remanufactured or used transmission with low miles. Before you rush into replacing the transmission, read the fault codes from the transmission control unit using a scanner such as the YOUCANIC Scanner and go from there.
What Scanner to use for diagnosing Transmission Problems on Mercedes-Benz?
The YOUCANIC full system diagnostic scanner is an essential tool for Mercedes-Benz owners and technicians looking to troubleshoot problems with their vehicles. This diagnostic scanner can help diagnose issues with the engine, transmission, brakes, airbag, ABS, steering systems, and more by reading and interpreting fault codes stored in the car’s computer system. With this information, you can make informed decisions about repairs and maintenance, saving time and money.
In addition to diagnosing problems, the YOUCANIC scanner can also reset maintenance reminders and service indicators, ensuring that your vehicle is adequately maintained and preventing potential issues from occurring in the future. This feature is especially useful for Mercedes-Benz owners who want to stay on their vehicle’s maintenance schedule without visiting a dealership or a mechanic. Overall, the YOUCANIC full system diagnostic scanner is a valuable tool that can help you keep your Mercedes-Benz running smoothly and reliably for years.
Frequently Asked Questions
What is Limp Mode?
The limp / fail safe / emergency mode is when your transmission is stuck in one gear and doesn’t shift. In this condition, you only get speeds up to 30 mph max.
Why does this happen?
When the sensor input values from the different sensors inside the transmission, in the valve body, MAF sensor, and other parameters are outside the normal operation range, the car could go into the limp mode, go home mode. It was designed this way to protect the transmission from further damage. Don’t panic! Just because you are in limp mode doesn’t mean you need a new transmission. You may also get a check engine light if your transmission goes limp mode. When a Mercedes-Benz goes into limp mode, it only operates in second gear and reverses. You may feel a bang as transmission engages when you put it in gear. The first thing you should try is to check the transmission fluid level. The transmission has a dipstick tube but no dipstick in it. You will need this special dipstick, though, to measure it correctly.
Top reasons causing Transmission To Go Into Limp Mode:
- Mass Air Flow Sensor could also cause your transmission to go limp mode.
- Defective shift module inside the car.
- Bad Speed Sensor
- Brake Light Switch
- Defective transmission control module.
- Old Battery
These are the leading causes for limp mode, but limp mode could happen for other reasons which may not be directly related to the transmission.
How to troubleshoot my car that is not shifting?
-First, determine if the car is in limp mode or not moving. In limp mode, your car will drive in second gear.
– Get an OBD2/CAN scanner that reads and checks the engine/transmission codes. All car owners should own one, even if they don’t fix their cars. See our list of top recommended scanners here. You will get a fault code pointing you in the right direction. Write that code down, then go to our Mercedes-Benz custom search to search all the Mercedes-Benz-related pages, forums, and websites in one place.
– If your fluid level is OK, then listen for a whine. Do you hear one? Make sure to check the fluid level first and add if necessary. If the level is correct and you hear a whine, you may have serious transmission problems. That means the fluid is not circulating in the transmission. This could be due to several issues, such as a bad torque converter, a bad transmission oil pump, and water contamination in the transmission.
Understanding Mercedes-Benz Transmissions models.
– The 5-speed transmission was introduced around 1994 and was used up to 2007 models. This was a bulletproof transmission; you should be glad if your car had this transmission installed. This is designated as 722.6
– The 7-speed transmission was introduced in early 2000. It was first installed in some 2004 models. The 4matics were the last models changed from the 722.6 five-speed to the 722.9 seven-speed. This transmission is known as 722.9 or 7G.
To determine which transmission your car is equipped with, go to www.mercedesmedic.com/decode. Use one of the decoding options in that link to see the transmission code. It will say if you have a 722.6 (5-speed) or a 722.9 (7-speed).
How to get the car out of Limp Mode?
You must find and repair the problem that caused the car to go into limp mode first. Once you do that, the car will sometimes go out of limp mode immediately. If the problem were more serious, after repairing the problem, you would need to reset the Transmission Control Unit complex codes. A generic OBD II scanner will not work; you will need an advanced scanner like the one mentioned above.
The shifter is stuck in the parking problem.
If your Mercedes Benz is stuck in Park and it won’t come out, then you could have a problem with the following:
– Shifter module. It could be defective.
– Brake Light Switch
– The connection between the ignition module and the shifter module. Some of the older models had cable used to unlock the car from the park. After 2000 this is controlled by an electrical wire.
TIP: If this is your problem, look it up in the owner’s manual. A section should show you how to get the car out of the park using a screwdriver or similar object.
If you don’t have a manual, download it here: Mercedesmedic.com/ownersmanual/
The car will not move at all when in Drive or Reverse.
Do you hear a whine? In that case, you may have serious transmission problems. Some of the E-Classes, the CLK-Class, and maybe others had defective radiators. The transmission fluid radiator is part of the coolant radiator in front of the engine. Even though the coolant and the transmission are running in the same radiator to be cooled, they have separate chambers. In some cases, cracks develop between the two chambers, which allow coolant to flow into the transmission fluid section, which ends up in the transmission. The glycol entering the transmission will destroy it. This is happening primarily in cars with Valeo radiators. If you had this problem and replaced the transmission, you MUST replace the radiator. You will need another transmission very soon otherwise.
Standard transmission OBD-II Codes.
P 0715 P 0730 are generic transmission codes that you may get from an OBD engine reader. On a Mercedes, generic Diagnostic Trouble Codes DTC codes point to the transmission speed sensor installed on the conductor plate. You can replace the conductor plate. It can be done with the transmission still in the car. You can get a conductor plate here for less than $200.
If you have a defective shifter, in most models years 2000 and newer (with a few exceptions), you need a shifter model which is “virgin” or new. The shifter module will need to be programmed to match the VIN of your car. The only exceptions when a used shifter may work are Mercedes model year 2000 and older plus:
- W210/W208 up to 2003
- R170 SLK up to 2004
- W163 up to 2004
You can’t install a used shifter on 2000-up S-Class (W220), 2003-up E-Class (W211), and newer models. It needs to be a virgin shifter module and needs programming.
Regardless of the information, you must verify if your shifter module needs programming.
If the troubleshooting tips above do not help you, do not replace parts without finding the root cause of your transmission problem. The best way is to connect your Mercedes-Benz to a diagnostic scanner. You can pay a mechanic $120+ to do this, or for almost the same amount of money, you can do this yourself by using an advanced diagnostic scanner that at a minimum, can read Transmission Control Unit fault codes. Check out our article on Top 10 Best Diagnostic Scanners for Mercedes-Benz troubleshooting.