Transmission Function, Fluid Level, & Condition

An automotive transmission converts an engine’s force into a controlled source of power to move the vehicle and provide usable power from a stopped position throughout all speeds. Without a transmission, the power from the engine could never reach the wheels. It acts as a mediator between the engine and the wheels and converts the high power produced by the engine into torque (rotational force), which is then transferred to the differential(s) and axles, which in turn rotates the wheels. 

The engine of your vehicle creates power and sends it to the crankshaft, but the power produced is too high and too variable to produce a usable speed for the driver. The engine operates at a high rotational speed (anywhere from 600 to 7000 RPMs), while the wheels need to rotate at a slower rotational speed (anywhere from 0 to 1800 RPMs). The transmission is able to keep the relationship of the engine’s RPM and the RPM of the wheels at optimal rates to maximum power, efficiency, and fuel economy.

There are two major types of transmissions used in most vehicles on the road today: automatic and manual. Several new varieties and hybrids of automatic and manual transmissions have been gaining popularity in recent years as technology increases. Today, over 95% of new vehicles sold in the United States are equipped with an automatic transmission.

Transmission performance and condition can largely be determined by two things; a test drive including various speeds from stopped to highway speeds and from cold to hot operating temperatures; and a fluid level and condition inspection (where possible). Transmissions are generally the second most expensive component to repair on a vehicle. If a transmission has been neglected, the fluid and overall performance will generally indicate this. If the transmission is a sealed system and has ‘lifetime’ fluid that cannot be easily inspected, the transmission performance during the test drive is even more important to determine.

High fluid temperatures are ultimately what kills transmissions. It is estimated around 90% of all transmission failures are due to excessive heat, but what exactly causes transmission overheating? It is often related to the transmission fluid itself such as low, dirty, burnt, or contaminated fluid. However, there are other factors that can come into play such as internal part failure, heavy towing, lots of stop and go driving, or a very hot operating climate.

It is important to note that no fluid, including so called ‘lifetime' fluids, can last forever. Realistically, they will not last for the lifetime of a vehicle unless lifetime is defined as when the component they are supposed to protect fails. Heat, friction, and normal wear and tear eventually break down all fluids. If the original owner keeps a vehicle beyond the factory warranty period, the reality of this will set in quickly. Worse still, once the vehicle is sold, the new owner will eventually have to pay the piper for the deferred service.

Oil is cheaper than metal. It is much less expensive to change fluids, even synthetic fluids, than it is to replace parts. If your vehicle has over 100,000 miles, invest in finding out what the transmission fluid condition is even in a sealed ‘lifetime' fluid transmission. This is beyond the scope of a normal used vehicle inspection on transmissions without dipsticks or inspection plugs.

A test drive is a critical component of the transmission inspection process. Hear a strange noise when changing gears? Experiencing grinding, shaking, shuddering, slipping, jerking, or an unusual shift? Is the transmission sluggish or delayed when shifting into drive or reverse? Any unusual signs you experience with your transmission are cause for concern. After all, normal transmissions do not display any of these behaviors. When they do, it means that something has gone awry.

If your vehicle is equipped with a transmission dipstick or fluid level inspection plug, follow the manufacturer’s instructions to inspect the fluid level to make sure it is full and carefully inspect the condition of the fluid by inspecting its color and smell. Transmission fluid generally starts out bright red or clear in color, looks clean, and is nearly odorless. If the color is brown to black and the fluid smells burnt and looks dirty, transmission service is a good idea.

If you are purchasing a used vehicle it is important to have the automatic or manual transmission inspected prior to making your purchase. Have an ASE certified technician test drive the vehicle, lift it on a hoist to inspect the transmission for any leaks or damage, and where possible inspect the transmission fluid level and condition.