Mercedes-Benz Transmission Problems
In this article, we discuss 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 of the 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 start going into the limp mode more frequently. Don’t forget that first warning! You need to find the reason your transmission is going into limp mode or not shifting.
You would be surprised to know that the majority of the transmissions 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 are very rarely needed. The 5-speed transmission that was used between 1995 up to 2008 is one of the best transmission ever built. Now let’s see if we can help you troubleshoot your transmission.
- Low Transmission Fluid
- Transmission wire harness 13-pin (O-ring) connector oil leak/contamination
- 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 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 it is common that due to seepage or other reasons the transmission fluid level can be low. The car will go into limp mode as soon as it detects low level. The car doesn’t come with a 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 come 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 yourself in a couple of minutes with the right 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 latter.
In the video below we show you how to use a diagnostic scanner to troubleshoot your transmission, read and clear fault codes.
You may be able to borrow one of these scanners from a friend or even your 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 you about the same as a diagnostic at a repair shop would, except you get to use it over and over again. Here are two of our favorite scanners that will read and erase trouble codes from transmission control unit on Mercedes-Benz cars. Note that these work on 1996 and newer MB cars. They may not be able to read TCU codes on a few 1996-1999 models that require 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.
|2. Launch Creader X431 VII works in European, Asian and Domestic Cars.
ECU, TCU, AirBag / SRS, Transmission only.
Only can read and clear codes of the four systems mentioned. Works on most cars and models including Honda, Toyota, BMW, Lexus etc.
Either one of these two scanners would get the job done. An advanced diagnostic scanner such as the iCarsoft or the Launch Creader listed here are a must 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 a lot more.
3. Replace 13 Pin Connector Adapter Plug O-ring
You scan the car, and you may get fault codes that point you to communication problems with the valve body or incorrect gear ratios and similar. Before you spend hundreds of dollars on a new valve body (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 who are not familiar with Mercedes-Benz transmission suggest replacing the whole transmissions when the root of the problem in 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 Valve Body
If you are still having issues with the car going into limp mode and the scanner pointed you in the direction of the valve body then you may want to consider replacing it. The valve body is inside the transmission. The transmission does not need to be removed. Replacing the valve body can per 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. Which 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 also find them on Amazon for a lot less, see Mercedes-Benz valve body listings here.
5. Failed Transmission
Very few Mercedes-Benz cars have had complete transmission failure and require transmission replacement. We see these transmission 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 part of the radiator on your Mercedes-Benz contains the chamber for the engine coolant/antifreeze while the bottom part contains the chamber for the transmission oil cooling. If cracks develop between the two chambers, coolant will mix with transmission oil and it will destroy the transmission. This has been a well-documented issue for Mercedes-Benz cars equipped with the Valeo radiators.
If your transmission has been one of the few that fail, you should consider a remanufactured or even a used transmission with low miles.
Before you rush into replacing the transmission always read the fault codes from the transmission control unit using a scanner such as the iCarsoft MB II and go from there.
Frequently Asked Questions
What is Limp Mode?
Known as 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. The reason it was designed this way is 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 into limp mode. When a Mercedes-Benz goes into limp mode, it only operates in second gear and reverse only. When you put it in gear, you may also feel a bang as transmission engages. The first thing you should try is to check the transmission fluid level. The transmission has a dipstick tube but with 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 into limp mode.
- Defective shift module, inside the car.
- Bad Speed Sensor
- Brake Light Switch
- Defective transmission control module.
- Old Battery
These are the main 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 if it is not moving at all. In limp mode, your car will drive in second gear.
– Get an OBD2/CAN scanner that also reads transmission codes and check the engine/transmission codes. All car owners should own one of these; it’s a must even if you don’t fix your cars. See our list of top recommended scanners here. You will get a fault code which will point 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 website 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 bad a torque converter, bad transmission oil pump, water contamination in transmission.
Understanding Mercedes-Benz Transmissions models.
– The 5-speed transmission was introduced around 1994 and was used up to 2007 models. This was a bullet proof transmission, and you should be glad if your car has 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 that were changed from the five-speed to the seven speed. Designated as 722.9
To find out which transmission your car is equipped with go to www.mercedesmedic.com/decode Use one 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 need to find and repair the problem that caused the car to go into limp mode first. Once you do that, in some cases the car will go out of limp mode right away. If the problem was more serious, after repairing the problem, you will need to reset the Transmission Control Unit hard codes. A generic OBD II scanner will not work; you will either need a Star DAS Xentry scanner or an advanced scanner such as the ones we mentioned above or similar.
Shifter stuck in park problem.
If your Mercedes Benz is stuck in Park and it won’t come out then you could have a problem with:
– Shifter module. 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 park. After 2000 this is controlled by an electrical wire.
TIP: If this is your problem, look up in the owners manual. There should be a section that shows you how to get the car out of 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 and the CLK-Class and maybe other models had defective radiators. The transmission fluid radiator is part of the coolant radiator in the 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 as well. You will need another transmission very soon otherwise.
Common transmission OBD-II Codes.
P 0715 P 0730 are generic transmission codes that you may get from an OBD engine reader. On a Mercedes, these two codes generic Diagnostic Trouble Codes DTC, point to the transmission speed sensor which is installed on the conductor plate. You can just replace the conductor plate. Can be done with the transmission still in the car. You can get a conductor plate here for less than $200.
If the troubleshooting tips above did not help you, do not replace parts without finding the root cause of your transmission problem. The best way to do that 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.