The automatic transmission is one of the most complicated pieces of equipment in your car. Sure, the engine may be the heart of your car but it is the often forgotten automatic transmission that allows vehicles to achieve the type of fuel economy and performance that people expect in modern vehicles. From “3 on the tree” speed manual transmissions of the 1950s to the 10-speed automatic transmissions that are available in something as basic as the Honda Accord; for automotive enthusiasts and owners alike, there simply is no better time to be alive.
One of the most basic components of keeping your vehicle running is proper maintenance and maintaining the transmission is no different. Although most of us are not automotive mechanics, a great number of people take the task of adding in things like fluids or checking the oil to save money.
So what happens if you add too much transmission fluid? This article will cover these topics in further detail:
- History of automatic transmissions
- How automatic transmissions work
- What happens internally when you add too much fluid
- Symptoms of too much transmission fluid
- Cost to repair an automatic transmission
History Of Automatic Transmissions
There’s quite a story behind the humble automatic transmission and it starts way back in 1904. The Sturtevant brothers of Boston created a “horseless carriage” that utilized two forward gears that were driven by flyweights connected to the engine. As the engine speed increases, the transmission would shift into a higher gear to maintain or increase speed. When the vehicle came to a stop, the transmission would shift back into a lower gear. Although this was a genius design, the metalworking of the time was not up to snuff and the transmissions would spontaneously self-destruct when engaging gears.
Ford’s Model T utilized a planetary transmission (later used in automatics) that required the driver to initiate forward movement with the aid of clutches and bands but also low/high gear that was selected with a pedal on the floor. From that point on, more traditional manual transmissions became the norm. They utilized a clutch pedal to start off from a stop and a gear selector that would manually cycle through gears. Essentially, selecting a new gear meant that a spinning element would have to connect to a static element. This would create grinding, popping, or other loud noises as drivers shifted. They came to be known as crash boxes.
General Motors was the first to introduce a mass-produced, completely automatic transmission in 1940 via their Oldsmobile brand and later as an option on Cadillac vehicles. This genius (for the time) transmission utilized a fluid coupling and three hydraulically controlled planetary gearsets to produce 4 forward gears and reverse. Unlike other previously built transmissions of the era, the usage of a fluid coupling allowed the Hydra-Matic to take off from a stop without the aid of the clutch. The transmission would then shift based on engine load and throttle position. Although it was relatively smooth, the transmission had rough shifts but this was remedied in 1948 with the introduction of a transmission that utilized a torque converter. Torque converters basically multiply torque through amplified hydraulic pressure (completed inside the torque converter via turbines). These types of transmission have become the standard for today’s conventional automatic transmissions.
How Automatic Transmissions Work
For the purposes of this article, we will focus on the traditional torque converter automatic. For reference, there are several other types of transmission available on the market that retain the quality of being “automatic” that shifts gears automatically.
- Dual Clutch Transmissions
- Continuously Variable Transmissions
- Direct Drive Transmissions (Mostly utilized in electric cars and marine applications)
In modern, torque-converter automatic-equipped vehicles, the operation of individual vehicle brands may follow a slightly different approach but in general, the following steps will outline the basic operation of this complicated piece of machinery.
- During normal operation, the engine spins a crankshaft which exits the internal combustion engine into a torque-converter pump, which is essentially a fan blade meant to move transmission fluid.
- The driveshaft spins the pump which then causes fluid within the torque converter to move against a turbine (another fan blade) spinning in the opposite direction.
- As the two fan blades spin, centrifugal force moves the fluid to the outside and a third fan blade, known as a stator, then moves the fluid back to the pump blade. The process repeats and pressure is achieved.
- Above 40 (or so) MPH, the pressurized fluid activates a clutch piston that allows the torque converter to be bypassed. This is done because there is a loss of efficiency due to the kinetic transfer of energy via transmission fluid and the torque converter is not necessary once underway.
- This energy is then transmitted via an input shaft to the first of a series of planetary gearsets.
- A series of clutch packs and bands that are operated by the hydraulic pressure created by transmission fluid is utilized to activate and deactivate each of the transmissions forward gears. Clutch packs connect to move gears and bands hold them out of the way so they don’t turn.
In a nutshell, this is how this works but there are lots more intricacies here. If you want a fantastic visual example, check out this video here.
What happens when you add too much transmission fluid?
The purpose of the transmission fluid is twofold. First, it provides the lubrication necessary for the metal components within the transmission. Secondly, it serves as a hydraulic fluid to perform the tasks mentioned above. If you add too much transmission fluid, you will upset the balance of the system and take away its ability to adequately perform these tasks.
A primary concern for putting too much transmission fluid in is severe foaming, which is caused by increased pressure within the transmission due to air contamination via breached transmission seals. Your transmission is only meant to have a finite amount of fluid, so more fluid will put pressure on the transmission seals. If the seal busts, air can then enter the transmission, and fluid can leak out. Bubbles are pockets of air and when these bubbles make their way throughout the transmission, they are performing no practical function. Parts don’t get lubricated and hydraulic functions are affected because the air bubbles essentially provide a false pressure reading.
Without proper lubrication, the internal parts of the transmission could fail rather quickly or it could take some time for them to fade. Regardless, this will be a problem for your transmission and it will usually require a complete rebuild or replacement if air manages to get into the transmission. The other issue with putting in too much transmission fluid is that the breached seals may continue to leak. Without your knowledge, the transmission could be weeping fluid and continuing its pace towards a quick death.
Symptoms of too much transmission fluid
There are several telltale signs of too much transmission fluid in your vehicle. Some are diagnostic and some come from the experience of driving a vehicle that has a compromised transmission.
Here are the most common symptoms of too much transmission fluid:
- Rough Shifting. If a vehicle lurches, surges, shudders, or otherwise misbehaves; it could be due to too much fluid. Without the lubrication of the fluid (replaced by bubbles), critical parts come together abruptly and can jerk the vehicle around.
- Grinding. If you hear gears grinding, immediately stop and check your levels on your transmission. There is a good chance that the issue may have already had a terrible effect on your motor.
- Leaking Fluid. As noted above, too much fluid can bust a transmission seal due to operating pressures. If this happens, air can come in and fluid can leak out. If you notice fluid under your vehicle, it is always best to investigate immediately because that fluid SHOULD be in your car.
Cost To Repair An Automatic Transmission
If you do end up with damage from too much fluid, most reputable transmission shops will recommend that you either rebuild or replace the entire transmission. The average cost ranges from $1800 to $4000 and on up from there. A used transmission ranges from $800 to $1600, a rebuilt transmission from $1100 to $2700, and a remanufactured from $1400 to $3700. As you can see, this is one expensive repair and part of the reason why proper maintenance is so critical.
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