Automatic Transmission Assemblies

Early automobiles were considered too difficult to drive by large segments of the populace. Pushing down on a stiff clutch pedal was arduous, and double clutching and rev-matching an unsynchronized manual gearbox was a skill that could be difficult to master, making these realities of vehicle operation a turn-off for many potential car buyers. The advent of gear synchronizers helped ease shifting, but automakers knew that a vehicle that was much easier to operate was what was needed to really open up the car market. Development work on an automatic transmission had been going on almost since the inception of the automobile, including proposed designs using centrifugal weights and compressed air to achieve gear changes, but it wasn’t until the 1930s that a transmission using hydraulic pressure was manufactured. There were initially a few “semi-automatic” transmissions that had some degree of hydraulic actuation, but in most cases the driver still had to manually disengage and engage a clutch. The General Motors Hydra-Matic that went into production in 1939 was the first transmission to feature fully hydraulic operation, a fluid coupling, and planetary gearsets, which are still mainstays of modern automatic transmission design.

Hydraulic fluid pressure created by the transmission oil pump enables an automatic transmission to function. Located in the front of the transmission case where it’s driven by the torque converter, the oil pump draws fluid from the transmission fluid pan. The pump pressurizes the fluid and supplies it to the torque converter, transmission cooler, and the transmission hydraulic system, and as it circulates throughout the transmission it lubricates internal components as well. The torque converter is a fluid coupling device that transmits engine power to the automatic transmission. It serves the same purpose as the clutch in a vehicle with a manual transmission, in that it allows the engine to continue running when the vehicle is brought to a stop while in gear, except the torque converter does this without requiring any action on the part of the driver. The torque converter, along with the transmission components that enable it to shift by itself, are what make the transmission automatic.

The torque converter is bolted to a flexplate, which is bolted to the engine crankshaft, and the converter is housed inside the forward part of the automatic transmission case. An impeller, located in the rear of the converter housing and turned by the crankshaft, is opposite the turbine in the front of the housing, which is attached to the transmission input shaft. Another component called a stator is located between the impeller and turbine. Both the impeller and turbine have many curved blades. When the engine is running the impeller blades direct fluid at the turbine blades, turning the transmission and moving the vehicle. Fluid enters the turbine at the outside and exits at the center. However, if it were allowed to flow unimpeded upon exit, it would contact the impeller in the opposite direction, hindering impeller rotation. Instead, the stator redirects the fluid, so it contacts the impeller in the same direction it’s rotating, in effect increasing the force being applied to the impeller by the engine, which is why the torque converter is so-named.

At engine idle speed the impeller isn’t turning very fast and so isn’t applying fluid with much force, so the turbine doesn’t turn, and the vehicle won’t move as long as the brake is applied. When the driver releases the brake pedal and presses on the gas pedal, the impeller turns much faster and there is a great difference between impeller and turbine rotational speeds. This is when torque multiplication is greatest and needed the most, to get the stationary vehicle moving. During acceleration torque multiplication continues but at a reduced amount as turbine speed catches up to impeller speed. At cruise, turbine speed is almost the same as impeller speed, however there is still some degree of slippage, which creates heat and inefficiency. To eliminate this inefficiency and increase fuel mileage at cruising speed, a torque converter clutch is used to lock the turbine to the impeller.

Fluid output from the oil pump also flows to the pressure regulator valve and from there to the manual shift valve in the valve body. The manual valve is controlled by the driver via the shift lever and directs fluid pressure to the appropriate valves and clutches and bands when the transmission is placed in Park, Drive, Neutral or Reverse. On older, non-electronically controlled transmissions, fluid is supplied to a governor and throttle valve, which in turn direct fluid to the shift valves in the valve body. The transmission output shaft drives the governor, which builds pressure directly in relation to vehicle speed. Under light acceleration, the throttle valve doesn’t have much effect on the shift valve and governor-controlled shifts are earlier and softer. Under hard acceleration, when engine load is high, the throttle valve will act against the shift valve to force governor pressure higher. This delays the shift and supplies higher pressure to the transmission bands and clutches, giving them the clamping power needed to control members of the transmission’s planetary gearset and enact firm, hard shifts.

On electronically controlled transmissions, shift timing and firmness are controlled by a computer. A network of engine and transmission sensors and switches provides input to the computer about operating conditions including engine load, temperature, and vehicle speed. The computer uses this input, along with internal programming, to manipulate fluid pressure and flow via electric solenoids in the valve body. There are shift solenoids that direct fluid pressure to the appropriate clutches or bands, and a solenoid that directs fluid pressure to engage the torque converter clutch. A solenoid contains a coil of wire with an iron plunger inside. When current is applied to the solenoid it becomes an electromagnet and the plunger changes position. When the plunger moves it opens or closes a valve, allowing or preventing fluid flow.

Automatic transmissions use planetary gearsets for all forward gears and reverse. Clutches and bands, applied by hydraulic pressure, are the components used to control the planetary members – the sun gear, outer ring gear, and the planetary gears in between – to shift gears. Clutch packs consist of multiple, alternating steel and friction material-lined plates housed in a drum or the transmission case. The friction plates are internally splined to fit into grooves on a hub, while the steel plates have external splines that engage grooves in the drum or case. A piston compresses the plates, locking the hub and drum together when hydraulic fluid pressure is applied. Return springs disengage the piston when pressure is removed. Bands are friction material-lined flexible metal rings that surround and are used to stop rotating clutch drums. One end of the band is attached to the transmission case, while the other is engaged by a hydraulic piston in a servo. The servo contains a spring to disengage the band when hydraulic pressure is removed.

Common automatic transmission problems include slipping, shift flare, delayed shifts or incorrect shift timing, noise, odors, and fluid leaks. Slipping can be caused by a failing torque converter, worn bands and clutches, and burnt fluid; shift flare, delayed shifts, hard shifts, and improper shift timing can be caused by sticking or worn valves, and a faulty vacuum modulator or throttle valve cable; noise can be caused by defective planetary gears; and the darkened or blackened fluid in a transmission that has been severely overheated will have a burnt smell. Automatic transmissions can leak at the fluid cooler line connections, line and governor pressure plugs, case-to-extension housing gasket, oil pump seal, oil pump-to-transmission case seal, dipstick tube seal, governor cover, servo cover, manual shaft seal, vacuum modulator O-ring, and throttle valve cable seal, but one of the most common leak areas is the transmission fluid pan.

Some transmission maladies can be repaired with the unit in the vehicle, but many jobs require transmission removal. Instead of paying to repair a high-mileage transmission or for an overhaul that could have your vehicle laid up for a considerable time period, installing one of our replacement transmissions backed by an extensive warranty is the faster, more economical, and smarter solution. No matter what you’re working on or what kind of transmission problem you’re having, we have the replacement automatic transmission assembly needed to restore proper shifting and transmission operation. We offer remanufactured automatic transmissions that are assembled according to original equipment specifications, so when repairs are completed you can count on the transmission performance your vehicle was designed to deliver. Our remanufactured transmission assemblies undergo numerous quality inspections throughout the remanufacturing process. All reusable parts are cleaned, inspected and measured to ensure they meet specifications and replaced with new parts if they don’t, and all seals, gaskets, bushings, and other common wear components are replaced. We also have performance components that will complement your vehicle's high-performance engine.