Auto and Truck Repair and Advice
Serving the Gainesville and Hawthorne Florida Area
Automatic Transmissions
The advantages of an automatic transmission are:
(1) Easier operation by the driver: no clutch pedal or gearshift to deal with. This frees the drivers attention, and lessens the training needed to operate the vehicle. Fatigue in stop and go traffic is lessened.
(2) Longevity and reliability: A manual transmission has a clutch which must be serviced eventually. An automatic will go much longer with only periodic fluid changes. In fact, an automatic, if never allowed to run out of fluid and burn the wet clutches, will go practically forever. (200,000+ miles!) It's harder to over-rev the engine with an automatic, so it can be easier on the motor.
Disadvantages:
(1) Slight horsepower loss: Although with lockup-type transmissions the "slip" from the torque converter has been eliminated, and many have variable displacement oil pumps, the oil pump which operates the transmission uses some power. Ditto for all those released wet clutch packs as they slide together. The automatics are a good deal heavier than the manuals.
(2) Slight loss of control of vehicle: It shifts when it wants to, not when YOU want to. Lots of folks prefer a manual where THEY shift the thing when THEY want to!
HOW THEY WORK:
The modern automatic transmission is based around planetary gears. A set of planetary gears consists of a sun gear in the center, 3 or more planet gears meshed with and encircling the sun gear, and a ring gear with teeth on the inside, encircling the planet gears.
The speeds are obtained by holding one of the three gears, applying power to another, and taking power off the third. For example: hold the ring gear, turn the sun gear, get reverse off the planetary gears. Hold the planetary gears, turn the sun gear, and get a forward gear ratio off the ring gear.
Automatic transmissions have one or more sets of planetary gears which are stopped by clutches and bands to get all the speeds and reverse.
A band circles a drum and looks almost like a western style leather belt , but made of sheet metal covered with composite material. A hydraulic piston tightens this around the drum to stop a planetary gear element.
Clutches have two types of elements: steel plates and composition on steel discs. The steel plates have splines ("teeth") on the outside. The composition clutch material circular plates have splines on the inside. Usually several (often 6 or so of each) steels and composition discs are stacked together, increasing the "area" of the clutch pack. A hydraulic piston presses these together to hold a planetary element.
The first auto transmissions had one set of planetaries and had 2 forward speeds. The later 3 speed transmissions had 2 sets of planetaries, and the 4 speed transmissions had 3 sets. I recently rebuilt a Chrysler front wheel drive transaxle, and they were getting 4 forward speeds using only 2 sets of planetaries! A clever design, but it had a VERY complex set of servos to accomplish this!
WHAT CONTROLS THEM?
For the newer cars, the answer is simple! (Imagine that!)
THE COMPUTER CONTROLS IT, taking in consideration your speed, throttle position, engine load, and other factors! Little solenoids supply fluid to those pistons and shift the tranny!
For older (and many new) cars the answer is more complex. They have a "hydraulic computer" which compares hydraulic pressures versus spring pressures and applys and releases pistons to control the planetaries.
The simplest ones shift using two inputs: throttle position, and governor pressure. A link goes from the gas pedal to the transmission. It moves a valve in the tranny. The more you press the pedal, the more it moves the valve, and the higher the THROTTLE VALVE PRESSURE is.
The governor has two weights which are opposed by two springs. As the transmission spins, centrifugal force moves these weights outward against the spring pressure.
The shift valves actually route fluid to the various hydraulic pistons (called servos) which operate the bands and clutches. Throttle valve pressure opposes governor pressure, delaying the shift when you're "floorboarded". When governor pressure gets high enough, the shift valve overcomes its spring pressure and shifts to the next higher gear.
HOW THEY FAIL
The actual clutches and bands usually last as long as the rubber seals in the servos. As soon as the pistons (servos) start to leak, the clutch elements fail rapidly. A "slipping" will be felt, or a "flare" where it slips when it first goes into gear then links up.
It's important to keep fluid in your transmission at all times. This is the main thing, especially newer vehicles. Transmission cooler lines are a big source of leaks. Check them and check your fluid regularly!
FLUID AND FILTER CHANGES
It used to be GM recommended no fluid change until 100,000 miles!!!
They don't say that anymore, although many people DO go that long and longer without a change. As with many things, READ YOUR OWNER'S MANUAL!!!
When in doubt, change the fluid at 50,000 miles.
ALSO: Look at your fluid! when you check it, it should be a pink color. If it's brown, change it. If it's nice and pink, you probably don't need to change it. They don't make the transmissions as tough as they used to, and an inexpensive fluid change can avoid or postpone a very expensive rebuild!!!
The Economechanix Cyber Mall
We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry!
Thank you for visiting the ECONOMECHANIX WEB SITE. Please feel free to comment. We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry! Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.
George G. Scott, Jr.