Servicing Your Transmission
One of the most critical, unobtrusive and, therefore, often neglected parts of a vehicle is the transmission. While it isn't as maintenance-intensive as the brake system, for example, both manual and automatic transmissions require at least periodic inspections if not regular service to prolong their life. But before we explain how to best service transmissions, a brief explanation of how one works is required.
The transmission multiplies torque from the engine through a gear reduction and/or torque conversion. A typical manual transmission has anywhere from four to six speeds with the final or highest gear being either a direct 1:1 drive ratio or an "overdrive" ratio, less than 1:1. For the most part, they are trouble-free (except for the clutch, which can be problematic if adjusted incorrectly or abused). An automatic transmission, on the other hand, multiplies engine torque as it passes through the fluid coupling known as the "torque converter" and then through three or more separate gear ratios. The transmission oil is critical for lubricating the transmission and reducing friction. When the fluid breaks down and loses its viscosity, it no longer effectively lubricates the transmission. This causes premature and excessive wear and results in transmission failure.
This logically begs the question, how often do I need to have my transmission serviced? And like many things in life, there is no straightforward answer. Service intervals will vary depending on how the vehicle is used and the operating temperature of the transmission. The leading problem with transmission failure is fluid breakdown due to overheating. And fluid life expectancy is directly related to the operating temperature of the transmission. Consequently, many transmissions fail long before they realize their potential design life because of heat. In an automatic transmission, for example, a normal operating temperature is about 215 degrees Fahrenheit. At this temperature, the oil should easily last for 30,000 miles (unless you have a new vehicle that is filled with Dexron III ATF which is supposed to be good for 100,000 miles). But for each 20 degree increase in operating temperature, the life of the fluid is cut in half.
Cars that are driven at track days should be inspected before every event. When you periodically check the fluid level, pay particular attention to how it smells. You will notice a burnt smell long before the color changes. And, when you notice a strong burnt smell, have the transmission fluid flushed immediately.
Therefore, and as a general rule for automatic transmissions - that are not tracked or autocrossed - John recommends replacing the fluid every 30,000 miles. There are three ways to service automatic transmissions. The first is commonly referred to as a transmission service and consists removing the transmission pan, draining the fluid (about half of the total), changing the filter and pan gasket, and adjusting the bands if necessary. The second technique for changing the fluid is called suck and fill. With this method, the fluid is sucked out the filler tube then new fluid is used to refill the transmission. The last service method is a total transmission flush. In this case, a special machine is attached to the cooler lines of a transmission and the new fluid replaces virtually all of the old fluid.
The biggest disadvantage to a conventional transmission service is that it does not completely drain all the old fluid. The remaining fluid, therefore, can form deposits on transmission components that can eventually lead to erratic shifting and even transmission failure. Using a transmission power flush and fluid exchange system extends fluid life and helps prevent leaks and expensive repairs. If the transmission is shifting and operating as designed, regular transmission flushes are the recommended procedure for prolonging the life of your transmission since it flushes almost all of the old fluid from the transmission pan, cooler and lines, and not just from the pan as with the first two methods. Flushes are also particularly important when converting from conventional transmission fluid to synthetic fluid (which is used under severe conditions such as commercial use, heavy loads, or towing).
Unlike the fluid in an automatic transmission that is being constantly churned (which generates heat) and contaminated by particles worn off the clutch plates, the fluid in a manual transmission or transaxle has life pretty easy. Most manual transaxles use automatic transmission fluid (ATF) to keep the gears lubed. ATF works well because it stays much more fluid at low temperatures and this oil stays relatively clean and runs fairly cool. However, it should still be changed at 30,000 miles (60,000 miles if synthetic oil is used).
The reasons you would change the fluid in a manual transmission more often are if you regularly track the car, or are experiencing hard shifting problems. And the only time you should have to add oil to a manual transmission is if the transmission is leaking oil. If you see any grease or wetness around the tailshaft or driveshaft seals, the oil level in the transmission or transaxle should be checked because it may be low. Needless to say allowing the fluid in the transmission or transaxle to run too low on lubricant can ruin it.
We has not seen a correlation between mileage and transmission failures. If the transmission is not abused and is properly maintained, it will last for the life of the vehicle. As related innumerable times above, heat is the number one culprit of transmission failures. We are firm believers that a periodic service/inspection is the best way to ensure that your transmission has a long life - and in this regard, fluid inspections and/or flushes/changes in both manual and automatics are critical to avoid premature transmission failure.