In normal operation automatic transmissions generate heat and some debris. Most heat is generated in the torque converter. The torque converter supplies the mechanical link between the engine and the transmission. It is also designed to slip, allowing the vehicle to sit still while the engine continues to run.
In order to lessen heat buildup and help prevent energy loss, torque converters today, have a lock-up feature. This means once the vehicle is in motion, the computer energizes a solenoid which "locks" the converter and stops the slipping. When the brakes are applied the converter is released.
Heat causes transmission fluid to darken in color and loose its protective qualities over time. The more heat, the more quickly this occurs. This is why most vehicles have transmission coolers built in. The coolers help eliminate heat and helps the fluid to last longer.
Another problem that develops over time is debris. Most debris generated in a transmission is from the gears meshing together and "normal" wear from bushings and thrust washers. With today's technology, clutches rarely wear out or generate much debris, unless caused by another problem.
Beginning in the late 1980's, vehicle manufacturers began using computers and [electromagnetic] solenoids to control when and how the transmission shifts. The computer was mounted outside of the transmission. The solenoids, mounted inside of the transmission were vulnerable to the metallic particles being generated.
Early transmissions used as little as one solenoid for torque converter lock up and two for gear changes. Today transmissions may have twelve or more solenoids. Worse, the computer itself may be inside of the transmission. Since some of these solenoids and computers come only as an assembly the are VERY expensive. To protect these expensive components, one or more filters are used.
Transmission filters are designed somewhat like a sock. That is, fluid enters the neck of the "sock" and flows out through the fabric. The pores in the fabric are small enough to trap debris yet large enough for fluid to pass. Once inside the filter the debris is trapped.
As more debris accumulates, more filter pores are clogged. In time the filter will become restricted and lessen the flow of fluid. This is critical as transmission fluid acts as the lubricant for the transmission. It also acts as a hydraulic fluid to activate clutches as well as a coolant, a cleaner and a liquid coupler in the torque converter.
When fluid flow becomes restricted, the transmission can block fluid from lubricating the gears. It does this to insure enough fluid is available to activate the clutches. This keeps the vehicle moving but leaves the gears without lubrication. The problem gets much worse very quickly. This is why a transmission service can help extend the life of a transmission.
Proper Service
Before any service is performed the vehicle should be driven by a trained technician. The test drive indicates problems or adjustments that may be taken care of during service. With a complete transmission service, the unit is then inspected for leaks or any external signs of problems.
The old fluid is then drained and inspected. Excessive metal content, a very dark color or burnt smell may indicate a problem. On transmissions with removable pans, the pan is removed. This allows inspection of internal transmission components. A careful inspection can reveal early warning signs of trouble. Many times tightening of a bolt, or adjustment at this point will prevent a major failure in the future.
On transmissions with replaceable filters, the filter is also removed and replaced. The best technicians often cut open the old filter and examine its contents. By the nature of the debris in the filter much can be learned about the condition of the transmission.
Transmission fluid types
Because of the huge number of designs in common use, transmissions often have a specific fluid that they require. For instance, Honda requires a fluid, with different characteristics than the fluid used in Toyota. In fact, even different model Toyota vehicles require different fluids. The same is true of Ford, Chrysler, Mitsubishi, late model GM and most others.
Often after design, a transmission may exhibit a characteristic that is not desirable. When this occurs, design engineers may formulate a fluid to alleviate the undesired effect. The same is true for problem that occur in service. Sometimes fluid is revised and upgraded. With so many fluids on the market it is imperative the correct fluid is installed in your vehicle.
Transmission flushing
In recent years, the practice of flushing a transmission has become very popular. It is quick and easy, and very profitable to those that perform it. With this method the cooler lines are disconnected. With the engine running, fluid is pumped out of the transmission while new fluid is added. In our opinion, this method is NOT a substitute for a proper service.
The pan is not removed and the filter is not replaced. It is not possible to properly inspect the inside of the transmission. This method may also stir up debris and further restrict the filter.
The filter in the above image is from a transmission that had been flushed about one week before.
Another problem with flushing is, often one type of fluid is used to flush all transmissions. It is not practical to have a machine for each type of fluid in use. Generic fluids are also considerably less expensive than the specialized fluid the engineer may specify.
Other possible service problems
The most common problem with automatic transmissions is fluid leaks. Leaks can occur when the transmission pan is not properly sealed or the bolts improperly tightened. Leaks can also occur when cooler lines are removed for flushing. Bent lines sometimes leak or come disconnected when driving.
Leaks result in the fluid level being lowered in the transmission. As the level drops the pump can not pick it up as it should. If the pump sucks air, pressure may drop and the transmission can shift to neutral or quit pulling. Extensive transmission damage can be quickly occur from low fluid.
Other problems that can occur are improperly installing the filter or using an incorrect filter. Some transmissions have more than one filter design. Each filter may appear the same and may fit, but not function properly. An expert technician will always make sure that the proper, quality filter is use and installed correctly.
A proper service can never harm a transmission. Properly done, service can prevent many problems and extend the transmission’s useful life. Improper service can be worse than no service at all. I feel it is very important to have only a trusted specialist service your transmission.
