The transmission is a major component of any vehicle, it’s therefore extremely important that it functions properly and without fault. Most problems with transmissions are often related to fluid in some way shape or form. In a perverse way, overfilling the transmission fluid can actually be more dangerous and harmful than under filling. It’s really important to know what to look out for and how to rectify the problem if you have inadvertently overfilled your transmission, let’s take a look at the symptoms to look out for and explain the potential consequences;
What Are The Consequences of Overfilling A Transmission?
Overheating – The most common consequence of overfilling your transmission is causing an overheat within the transmission. One of the primary functions of the transmission fluid is to cool down the transmission when it gets hot. Working in tandem with the oil cooler, the fluid is cooled down to a level which allows for smooth operation of the transmission. If the fluid level is overfilled, it can result in fluid leaking out of the seals and other areas which then causes an overheat to occur. If the transmission overheats, you’ll likely get a warning message on the dashboard and the transmission will enter a safety mode often referred to as “limp home” mode. This a security mechanism built into the transmission to prevent damage occurring when a problem occurs.
Pressure Build Up – Another major consequence of overfilling the transmission fluid is increased pressure build up. When transmissions are developed, they are built within minuscule tolerances and calculate the exact amount of fluid that needs to be inside the system. When extra fluid is introduced, it increases pressure within the transmission due to the increase in air created. Naturally, this then introduces significant strain on the various rubber seals which haven’t been designed to withstand such pressures. The increased pressure build up will be obvious when driving the car as the transmission will behave in an unexpected way.
Fluid Foaming – Overfilling the transmission with too much fluid will cause the fluid to foam as it mixes with the air inside the transmission. When the fluid begins to foam, it’ll no longer lubricate the mechanical components as it should do which will then result in poor shifting and other unwanted issues. Eventually, over time, the transmission will cease to function entirely.
Erratic Shifting – When overfilling a transmission with too much fluid, you’ll almost certainly encounter harsh gear shifts due to the incorrect mixture of air and fluid within the transmission. Parts of the transmission which aren’t supposed to be submerged in fluid will become drowned with fluid, preventing them from doing their job properly.
Overfilled Transmission Symptoms
Now that we have summarised the most common consequences of overfilling your transmission, what are the symptoms to look out for so that you can rectify the problem because it causes serious harm?
Leaking Transmission Fluid – The first biggest sign and symptom of an overfilled transmission is a new leak of oil underneath the car. Transmission fluid is generally a red / cherry colour but over time becomes a darker colour. The colour of the fluid totally depends on how long it’s been in the vehicle for and when it was last changed. A good general rule of thumb is that it won’t be as dark as engine oil, it’ll pretty much always be a little big lighter in colour irrespective of age. If you do notice fluid on your drive or on the road where your vehicle was parked, take a look underneath the car and see if you can identify and confirm that the fluid is coming from the transmission itself. If it is, your next port of call is to have the level checked.
Fluid Level on Dipstick – It’s really important to carry out regular maintenance on your vehicle to prevent expensive repair bills further down the line. You should regularly check the fluid levels of your vehicle and if applicable, you should be checking the fluid level of the transmission. Now, the complication here is that not all cars have the option of measuring the fluid level via a dipstick. Some cars do have a dipstick for the automatic transmission, such as all Mercedes Cars which are fitted with their 722.6 5 speed transmission. To check the fluid level on those cars, you’ll need to buy a special dipstick which will accurately measure the fluid level.
It’s absolutely imperative that you follow the manufacturer’s instructions for measuring the oil via the dipstick. The vehicle must always be on a flat, level surface and in the case of the Mercedes 722.6 transmission, the vehicle should be running when taking the measurement. The dipstick will show two different temperature markings, one of hot and one for when the transmission is cold. Be sure to follow the correct markings when measuring. If you require fluid for the 722.6 transmission, our recommendation would be the excellent Febi Bilstein kit, Febi make some excellent OEM products and are our go to for servicing
Lots of other cars don’t have a dipstick for the transmission. If this is the case, you’ll need to take it to a local main dealer or specialist independent garage and have them confirm the fluid level. The procedure for measuring the fluid differs between all manufacturers.
Problems When Changing Gear – You may notice that your car is struggling to change gear as smoothly as it should be, if you do, this could be a good indicator that the transmission fluid is overfilled. Gear changes on an automatic transmission should always be smooth, crisp and relatively quick. If this isn’t the case and the vehicle is holding onto a gear for longer than it should, your first step should be to have the fluid level measured. The transmission may be suffering from foaming fluid as outlined above which can then result in the gears not operating correctly.
Strange Noises – It may sound a bit vague, but an overfilled transmission can sometimes cause strange noises to emanate from the transmission. If the fluid is foaming up and not lubricating the components sufficiently, you may hear a grinding noise. This really isn’t a pleasant noise to hear and will create a loud metal on metal type sound. If you do hear a grinding noise, our advice would be to stop the car immediately and inspect it when safe to do so. You may also hear a humming noise coming from the transmission area. This may sound similar to a faulty wheel bearing noise and can be quite difficult to distinguish between.
How To Correct an Overfilled Transmission
Prevention is always better than cure, remember this when carrying out any maintenance on your car and take extra precautions where necessary. It’s really important to not add too much fluid to your transmission when topping it up, always add small quantities and keep checking the level before finishing.
If your car is equipped with an automatic transmission which has a dedicated dipstick for measuring the fluid, you can follow these steps to measure and remove fluid from the transmission if necessary.
- Ensure that the car is parked on a flat, level surface and that the engine is running. When the engine is running, some of the fluid is pumped around the torque converter. If you measure the level with the engine off, the fluid from the torque converter will drain back down into the sump and give an incorrect measurement level.
- Pull out the dipstick, wipe it with a clean cloth and re insert back down into the tube ensuring that it goes completely all the way back in to ensure an accurate measurement.
- Pull the dipstick back out and visually check the fluid level against the marks on the dipstick.
If the fluid level is overfilled, you need to remove some fluid urgently. The easiest and safest way to do this is to siphon some out with the aid of an oil extractor pump system.
Extraction pumps are a fantastic tool and make oil removal an absolute breeze. Simply attach the extraction tubes together and insert the relevant tube down the dipstick as far it’ll go. Pump the handle a few times to generate suction and the oil will begin to drain into the reservoir via the tubes. At this stage you can choose to either drain the full oil and re fill with the correct amount or you can attempt to remove a small quantity to rectify the level and return it to its correct level. Removing the exact amount might be difficult so it may be a good idea to have some transmission oil ready to refill with once you’re finished.
- Now that you’ve corrected the fluid level, it’s important to measure it again and verify that the level is correct. Be sure to follow the manufacturers guidelines on exactly how much fluid should be in the system and how to fill correctly.
In summary, it’s absolutely essential that your transmission isn’t overfilled with fluid as it can cause major problems. Any damage to components within the transmission will absolutely certainly lead to an expensive repair bill, Automatic transmissions have a reputation for being extremely costly to repair. Be sure to treat your transmission fluid as a regular serviceable item and check it regularly. Keep an eye out for any of the symptoms mentioned above and if you do start to notice any of those particular symptoms, be sure to check it out quickly.
Just so you know, we may earn a small commission through the links on this page but we think that’s a fair trade off for the hours research and time that we put in.