The 4T80-E Transmission

The 4T80-E tranmission is the latest generation of the GM's Turbo-Hydramatic (THM) family of automatic transmissions. These transmissions are world renowned for ruggedness and reliability. THM transmissions are being used in cars and trucks from different makes, including Rolls Royce and Ferrari. Starting in 1966, GM has to make a drastic change to some of its THM transmissions, as they introduced front wheel drive Oldsmobile Toronado. Cadillac got the new tranny in 1967 in the front wheel drive Eldorado. Drivetrain layout on front wheel drive cars is such that the driven wheels are located under the powerplant and not behind it. Hence, the transfer of power must make some twists and turns before it can reach the front wheels. GM solved this matter ingeniously by actually folding the transmission. The tranmission parts behind the convertor were put in the folded part under the engine. A special chain linked the upper part (actually the convertor) with the lower part and thus accomplished the "power transfer twist". Initially GM was laughed at by other engineers who claimed the chain would never last long, especially with the high torque engines the transmission would be mated with. As experience has shown, the chain is extremely durable and one of the most reliable parts of the transmission. The 4T80-E still uses this kind of chain to transfer power. However, it is completely governed by electronic components which guarantee unnoticeable shifts and high performance operation. The Northstar system takes care of the interaction between the engine and the transmission.

For instance, just before an upshift takes place, the engine power output is reduced in order to avoid power transfer during the shifting phase. Furthermore, the transmission has a special adapt function. As transmissions are being used, their internal components (clutch plates, seals, springs) are subject to normal wear. As these components wear, the line pressure set at the factory is no longer valid for the optimum performance and comfort. That is why the 4T80-E constantly monitors its own shift harshness and slippage. Line pressure is adjusted accordingly.