pressure attempts to open the valve. When speed and governor pressure become great enough to open the 2-3 shift valve, intermediate clutch oil passes through the shift valve and enters the direct clutch, thus shifting the transmission into third gear. Oil pressure to the direct clutch piston is applied only to the small inner area in third gear.
When the accelerator is released and the vehicle is allowed to decelerate to a stop, the transmission automatically downshifts 3-2 and 2-1. This results from the decrease in governor pressure as the vehicle slows and the springs closing the shift valves in sequence.
In this system, shifts would always take place at the same vehicle speeds when the governor pressure overcomes the force of the springs on the shift valves. When you accelerate under a heavy load or for maximum performance, it is desirable to have the shifts occur at higher vehicle speeds. To make the transmission shift at higher vehicle speeds with greater throttle opening, variable oil pressure, called modulation pressure is used.
Modulator pressure is regulated by engine vacuum which is an indicator of engine load and throttle setting. The engine vacuum signal is provided to the transmission by the vacuum modulator which consists of an evacuated metal bellow, a diaphram, and springs. These are so arranged that, when installed, the bellows and one spring apply a force that acts on the modulator valve to increase modulator pressure. Engine vacuum and the other spring act in the opposite direction to decrease modulator pressure which results in low-engine vacuum and gives a high-torque signal and high modulator pressure. High-engine vacuum gives a low-torque signal and low modulator pressure. Modulator pressure is directed to the 1-2 regulator valve which regulates modulator pressure to a lesser pressure that is proportional to modulator pressure. his tends to keep the 1-2 shift valve in the closed or downshift position. Modulator pressure is also directed to the 2-3 modulator valve to apply a variable force proportional to modulator pressure. This tends to hold the 2-3 shift valve in a the closed or downshift position. The shifts can now be delayed to take place at higher vehicle speeds with heavy throttle operation.
Line pressure is controlled in D (drive) range so that it will vary with torque input to the transmission. Since torque input is a product of engine torque and converter ratio, modulator pressure is directed to a pressure regulator boost valve to adjust the line pressure for changes in either engine torque or converter ratio.
To regulate modulator pressure, and, in turn, line pressure with the torque converter torque ratio that decreases as vehicle speed increases, governor pressure is directed to the modulator valve to reduce modulator pressure with increases in vehicle speed. In this way, line pressure is regulated to vary with torque input to the transmission for smooth shifts with sufficient capacity for both heavy and light acceleration.
The 1-2 shift feel and the durability of the intermediate clutch are dependent on the apply pressure that locks the clutch pack. At minimum or light throttle operation, the engine develops a small amount of torque and as a result, the clutch requires less apply pressure to engage or lock. At heavy throttle, the engine develops a great amount of torque which requires a higher apply pressure to lock the clutch pack. If the clutch locks too quickly, the shift will be too agressive. If it locks too slowly, it will slip excessively and eventually burn and ruin the clutches due to the heat created by the slippage.
Automatic transmission service can be easily divided into the following three parts: preventive maintenance, troubleshooting, and major overhaul. Before you perform any maintenance or repairs on an automatic transmission, consult the maintenance manual for instructions and proper specifications.
PREVENTIVE MAINTENANCE. - Normal preventive maintenance includes:
1. Checking the transmission fluid daily
2. Adjusting the shifting and kickdown linkages
3. Adjusting lockup bands
4. Changing the transmission fluid and filter at recommended service intervals (Example: 15,000 miles or yearly for heavy or severe service)
Checking the Fluid. - The operator is responsible for first echelon's (operator's) maintenance. They should not only be trained to know how to look for the proper fluid level but also to know how to look for discoloration of the fluid and debris on the dip stick. Fluid levels in automatic transmissions are almost always checked at operating temperature. This is important to know since the level of the fluid may vary as much as three-fourths of an inch between hot and cold. The fluid color should be pink and clear. The color varies due to the type of fluid. (Example: construction equipment using OE-10 will not have color to it but stillContinue Reading