• There are several problems that can be resolved with an adjustment (A simple adjustment is one that can be made without removing the transmission from the vehicle.) or minor repair.

  If a late model transmission (computer-controlled transmissions started becoming popular in the early ’90s) is not shifting properly, it is often the result of a computer sending incorrect signals due to a faulty sensor, or the transmission is not reacting to the computer command because of a bad connection or defective solenoid pack.  These problems can be corrected while the transmission is in the car for considerably less money then a complete overhaul.

  If a non computer-controlled transmission is shifting too early or too late, it may require an adjustment to the throttle cable. Since throttle cables rarely go out of adjustment on their own or due to wear and tear, these mis-adjustments are usually due to other repair work or damage from an accident.  If the vehicle has a vacuum modulator instead of a throttle cable, there is an adjustment that can be made using an adjustment screw in some modulator designs.  In vehicles with modulators, however, it is very important that there are no vacuum leaks and the engine is running at peak efficiency. Engine vacuum is very sensitive to how well the engine is running.  In fact, many technicians use a vacuum gauge to diagnose performance problems and state-of-tune. Many problems that seem to be transmission problems disappear after a tune-up or engine performance related repair was completed.

  In some older transmissions, bands can be adjusted to resolve “slipping” conditions.  Slipping is when an engine races briefly when the transmission shifts from one gear to the next.  There are no adjustments for clutch packs however.

• Reseal job
  A transmission is resealed in order to repair external transmission fluid leaks.  If you see spots of red oil on the ground under the car, your transmission may be a candidate for a reseal job.  In order to check a transmission for leaks, a technician will put the car on a lift and examine the unit for signs of oil leaks.  If a leak is spotted at any of the external seals or gaskets and the transmission otherwise performs well, the technician will most likely recommend that the transmission be resealed.
  Most of the external seals can be replaced while the transmission is still in the car but, if the front seal must be replaced, the transmission must first be removed from the vehicle in order to gain access to it, making it a much costlier job.
• Replace accessible parts
  There are a number of parts that are accessible without requiring the removal of the complete transmission.  many of the control parts including most of the electrical parts are serviceable by simply removing the oil pan.  The parts that are accessible, however, vary from transmission to transmission and most transmission repair facilities would hesitate to provide meaningful warrantees on external repairs for the simple reason that they cannot see if there are any additional internal problems in the components that are only accessible by transmission removal.
• Complete Overhaul
  In a complete overhaul (also known as rebuilding a transmission), the transmission is removed from the vehicle and completely disassembled with the parts laid out on a workbench. Each part is inspected for wear and damage and then either cleaned in a special cleaning solution, or replaced with another part depending on its condition. Parts that have friction surfaces, such as bands and clutches are replaced as are all seals and gaskets. The torque converter is also replaced, usually with a remanufactured one.  Technical service bulletins are checked to see if the auto manufacturer recommends any modifications to correct design defects that were discovered after the transmission was built. Automobile manufacturers often make upgrade kits available to transmission shops to resolve these design defects.
• Replacement unit vs. overhaul existing unit
  When a transmission requires an overhaul, there are generally two options that you may have.  The first is to remove your existing transmission and overhaul it, then put the same, newly rebuilt unit back in your car.  The second option is to replace your existing unit with another unit that has already been rebuilt or remanufactured.
  The second option will get you out of the shop and on your way much faster but may cause you problems down the road.  The reason for this is that, in some but not all cases, a particular transmission model can have dozens of variations depending on which model car, which engine, which axle ratio, even which tire size.  The problems you could experience could be as simple as a speedometer that reads too high or too low (the speedometer is usually connected by cable to a gear in the transmission output shaft.) You may also experience incorrect shift points or even complete transmission failure because your engine may be more powerful then the one the replacement unit was originally designed for.  This is not the case with all transmission models so voice your concerns with your technician.   Most shops will rebuild your existing unit if you request it as long as they can afford to have a lift tied up with your car while the transmission is being rebuilt.  Of course this is only important if you are sure that the transmission you have is the original one and has never previously been replaced.Experienced engineers with ID lanyards can explain you better about the full process once you are at the sales office.

Transmission fluid should be changed periodically.  Your owner’s manual should give you the recommended intervals which could be anywhere from 15,000 miles to 100,000 miles.  Most transmission experts recommend changing the fluid and filter  every 25,000 miles.

Few transmissions have drain plugs to drain the old fluid.  In order to get the fluid out, the technician removes the transmission oil pan.  This is quite a messy job and generally not recommended for the casual do-it-yourselfer.  Even if the transmission has a drain plug, the only way to also change the transmission filter is to remove the pan. When the pan is down, the technician can check for metal shavings and other debris which are indicators of impending transmission problems.

In most cases during these transmission services, only about half the oil is able to be removed from the unit. This is because much of the oil is in the torque converter and cooler lines and cannot be drained without major disassembly.  The fluid change intervals are based on the fact that some old fluid remains in the system.

When the transmission is serviced, make sure that the correct fluid is used to re-fill it.  Each transmission manufacturer has their own recommendation for the proper fluid to use and the internal components are designed for that specific formula. GM usually uses Dexron, Fords prior to 1983 use Type F while later models use Mercon.  Late model Chrysler products use ATF +3 +4 (Not using the correct fluid for Chrysler transmissions is the most common reason for their transmission problems.)  Toyota sometimes uses Type T which is only available through Toyota and Lexus Parts departments.  Honda also specs out their own formula which is available from Honda or Acura parts departments.   A transmission will not work properly or may even slip or shudder with the incorrect fluid, so make sure that you double check.  Your owner’s manual will tell you which fluid is required.  Naturally, the owner’s manual will try to convince you to only use the manufacturer’s branded fluid, but they will also provide you with the specs for the oil.  If the aftermarket product indicates on its container that they meet or exceed the specs for a particular type of transmission fluid, it is generally ok to use that product.

An automatic transmission has many seals and gaskets to control the flow of hydraulic fluid and to keep it from leaking out.  There are two main external seals: the front seal and the rear seal. The front seal seals the point where the torque converter mounts to the transmission case. This seal allows fluid to freely move from the converter to the transmission but keeps the fluid from leaking out.  The rear seal keeps fluid from leaking past the output shaft.

A seal is usually made of rubber (similar to the rubber in a windshield wiper blade) and is used to keep oil from leaking past a moving part such as a spinning shaft. In some cases, the rubber is assisted by a spring that holds the rubber in close contact with the spinning shaft.

A gasket is a type of seal used to seal two stationary parts that are fastened together. Some common gasket materials are: paper, cork, rubber, silicone and soft metal.

Aside from the main seals, there are also a number of other seals and gaskets that vary from transmission to transmission. A common example is the rubber O-ring that seals the shaft for the shift control lever.  This is the shaft that you move when you manipulate the gear shifter.  Another example that is common to most transmissions is the oil pan gasket.  In fact, seals are required anywhere that a device needs to pass through the transmission case with each one being a potential source for leaks.

The Hydraulic system is a complex maze of passages and tubes that sends transmission fluid under pressure to all parts of the transmission and torque converter.  The diagram at left is a simple one from a 3-speed automatic from the ’60s.  The newer systems are much more complex and are combined with computerized electrical components.  Transmission fluid serves a number of purposes including: shift control, general lubrication and transmission cooling.  Unlike the engine, which uses oil primarily for lubrication, every aspect of a transmission’s functions are dependant on a constant supply of fluid under pressure.  This is not unlike the human circulatory system (the fluid is even red) where even a few minutes of operation when there is a lack of pressure can be harmful or even fatal to the life of the transmission.    In order to keep the transmission at normal operating temperature, a portion of the fluid is sent through one of two steel tubes to a special chamber that is  submerged in anti-freeze in the radiator. Fluid passing through this chamber is cooled and then returned to the transmission through the other steel tube.  A typical transmission has an average of ten quarts of fluid between the transmission, torque converter, and cooler tank.  In fact, most of the components of a transmission are constantly submerged in fluid including the clutch packs and bands.  The friction surfaces on these parts are designed to operate properly only when they are submerged in oil. Developing solid metal hitch covers is a good choice.

Oil Pump

The transmission oil pump (not to be confused with the pump element inside the torque converter) is responsible for producing all the oil pressure that is required in the transmission.  The oil pump is mounted to the front of the transmission case and is directly connected to a flange on the torque converter housing.  Since the torque converter housing is directly connected to the engine crankshaft, the pump will produce pressure whenever the engine is running as long as there is a sufficient amount of transmission fluid available. The oil enters the pump through a filter that is located at the bottom of the transmission oil pan and travels up a pickup tube directly to the oil pump. The oil is then sent, under pressure to the pressure regulator, the valve body and the rest of the components, as required.

On automatic transmissions, the  torque converter takes the place of the clutch found on standard shift vehicles.  It is there to allow the engine to continue running  when the vehicle comes to a stop.  The principle behind a torque converter is like taking a fan that is plugged into the wall and blowing air into another fan which is unplugged.  If you grab the blade on the unplugged fan, you are able to hold it from turning but as soon as you let go, it will begin to speed up until it comes close to the speed of the powered fan.  The difference with a torque converter is that instead of using air, it uses oil or transmission fluid, to be more precise.

A torque converter is a large doughnut shaped device (10″ to 15″ in diameter) that is mounted between the engine and the transmission.  It consists of three internal elements that work together to transmit power to the transmission.  The three elements of the torque converter are the Pump,  the Turbine, and the Stator.  The pump is mounted directly to the converter housing which in turn is bolted directly to the engine’s crankshaft and turns at engine speed.  The turbine is inside the housing and is connected directly to the input shaft of the transmission  providing power to move the vehicle.  The stator is mounted to a one-way clutch so that it can spin freely in one direction but not in the other. Each of the three elements have fins mounted in them to precisely direct the flow of oil through the converter

With the engine running, transmission fluid is pulled into the pump section and is pushed outward by centrifugal force until it reaches the turbine section which starts it turning.  The fluid continues in a circular motion back towards the center of the turbine where it enters the stator. If the turbine is moving considerably slower than the pump, the fluid will make contact with the front of the stator fins which push the stator into the one way clutch and prevent it from turning. With the stator stopped, the fluid is directed by the stator fins to re-enter the pump at a “helping” angle providing a torque increase.   As the speed of the turbine catches up with the pump, the fluid starts hitting the stator blades on the back-side causing the stator to turn in the same direction as the pump and turbine.  As the speed increases, all three elements begin to turn at approximately the same speed.

Since the ’80s, in order to improve fuel economy, torque converters have been equipped with a lockup clutch (not shown) which locks the turbine to the pump as the vehicle speed reaches approximately 45 – 50 MPH.  This lockup is controlled by computer and usually won’t engage unless the transmission is in 3rd or 4th gear.Use your business checks and upgrade your office automobiles for secure driving.

The modern automatic transmission consists of many components and systems that are designed to work together in a symphony of clever mechanical, hydraulic and electrical technology that has evolved over the years into what many mechanically inclined individuals consider to be an art form.  We try to use simple, generic explanations where possible to describe these systems but, due to the complexity of some of these components, you may have to use some mental gymnastics to visualize their operation.

The main components that make up an automatic transmission include:

• Planetary Gear Sets which are the mechanical systems that provide the various forward gear ratios as well as reverse.
• The Hydraulic System which uses a special transmission fluid sent under pressure by an Oil Pump through the Valve Body to control the Clutches and the Bands in order to control the planetary gear sets.
• Seals and Gaskets are used to keep the oil where it is supposed to be and prevent it from leaking out.
• The Torque Converter which acts like a clutch to allow the vehicle to come to a stop in gear while the engine is still running.
• The Governor and the Modulator or Throttle Cable that monitor speed and throttle position in order to determine when to shift.
• On newer vehicles, shift points are controlled by Computer which directs electrical solenoids to shift oil flow to the appropriate component at the right instant.