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Clutch-to-clutch coast downshifting in a motor vehicle automatic transmission

  • US 4,671,139 A
  • Filed: 01/27/1986
  • Issued: 06/09/1987
  • Est. Priority Date: 01/27/1986
  • Status: Expired due to Term
First Claim
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1. In a motor vehicle having an engine drivingly connected to a vehicle wheel through a fluid coupling and a multiple speed ratio transmission, where the fluid coupling includes an impeller connected to the engine and a turbine connected to the transmission, the transmission being shiftable from an upper speed ratio to a lower speed ratio by releasing a friction element associated with the upper speed ratio and applying a friction element associated with the lower speed ratio, and the motor vehicle being operable in a coast mode wherein the vehicle wheel and transmission drive the turbine faster than the engine drives the impeller until the engine speed falls below its no-load idle speed after which the engine drives the impeller faster than the vehicle wheel and transmission drive the turbine, a method of controlling the apply and release of the transmission friction elements to effect a shifting of the transmission from the upper speed ratio to the lower speed ratio as the vehicle speed is reduced in the course of a coast mode of operation, such method comprising the steps of:

  • maintaining the apply of the friction element associated with the upper speed ratio at least until the engine speed is driven below said no-load idle speed; and

    thereafterreleasing the transmission friction element associated with the upper speed ratio and subsequently when the impeller and turbine speeds substantially coincide applying the transmission friction element associated with the lower speed ratio such that (1) in the neutral interval between such release and such subsequent apply the engine speed substantially returns to said no-load idle speed, and (2) the engine speed immediately before and after the applying of the transmission friction element associated with the lower speed ratio is substantially equal, whereby torque fluctuations at the vehicle wheel are minimized.

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