While manual transmissions generally require less maintenance and attention than automatic ones do, they're both still subject to the same issues and physical laws. Both require the proper type and grade of lubricants -- some manual transmissions now use automatic transmission fluid for lubrication -- in the proper amounts. Both are subject to the physical stresses of abuse, and both can suffer mechanical failure, too.
The primary differences between the two are in how the transmissions are shifted and how that shift is accomplished.
Automatic transmissions shift automatically, as the name implies, with the aid of a complex hydraulic circuit that uses fluid and pressure to apply and release clutches based on mechanical, hydraulic and electronic inputs. Manual transmissions accomplish the same goal, but the gears are changed manually using shift forks and synchros to smooth out the transitions. The clutch, which is engaged and disengaged to move through the gears, is also a manual input. Regardless of the type, maintenance plays a key role in transmission longevity.
Factors Affecting Transmission Longevity
Simply put, factors affecting transmission longevity are often related to maintenance.
Ken Chamberlin has seen his fair share of failed transmissions. In his 15 years as a Chrysler auto technician, and several more before that as an independent mechanic, he has overhauled a wide variety of automatic and manual transmissions and delved into why they failed in the first place.
But the elusive "why" has never had a consistent cause. "There are a lot of variables," Chamberlin said. "How is it being used? Is it being maintained? What type of driving and what kind of fluid?" In his experience, one of the baselines for failure was the transmission itself. "This is a mass-produced component," Chamberlain said. "They're not identical and they're not perfect. I've seen one transmission last 10,000 miles, and an identical transmission last for 200,000 miles. You just can't say."
While the factory and fate play a role, Chamberlin said there was always a cause-and-effect relationship leading to failure, which is usually dependent upon a few factors.
From a failure perspective, fluid is one of the key factors. Modern transmissions, despite their status as mass manufactured items, are built to close tolerances and engineered for very specific functions. Part of that engineering includes working with a specific type of fluid that acts in concert with the other components. "Not every fluid is the same," Chamberlin said, adding that some parts and service departments, as well as independent shops, believe it is.
Each type and grade of fluid -- and there are more than 50 on the market -- offers a different amount of slip. Within the automatic transmission world, clutches use the fluid slipperiness during clutch apply and release phases. A change in fluid means a change in shift feel and slip, and this often translates into more or less heat being generated, faster wear on parts or degradation of clutch material.
The reaction is much the same in manual transmissions, where the gears reside in an oil bath. The fluid transfers heat, allows for the smooth transition of gears and prevents wear. Change the fluid and the interaction between the fluid and the transmission components will change as well, leading to possible damage and failure.
Fluid also plays a critical role in heat and pressure. Chamberlin said there was a direct correlation between heat and transmission longevity. The hotter a transmission runs, the shorter its lifespan. For example, plow trucks used during New England winters are notorious for having short transmission lives. The transmission goes through thousands of hours of heavy use and generates significant heat during that time. As a result, the fluid breaks down, components fail and the car part longevity takes a nose dive.
Automatic transmissions use pressure to apply and release clutches, or essentially shift gears. This pressure is affected by a number of variables, but the baseline numbers are determined in large part by the fluid and its relative condition, which is affected by heat. Hot, older fluid can decrease (or increase) the pressures within the transmission beyond engineered tolerances. When this happens components begin to fail and soon the driver is left with an expensive repair bill.
The best way to keep a transmission alive is proper maintenance.