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Downshift Delay


wwest
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With the advent of the move to (TOTAL) electronically controlled automatic transmissions Toyota/Lexus decided to take advantage of the additional capability to add several new "features".

1. Extend the fuel economy by significantly lowering vehicle "drag" from engine compression braking. Anytime the throttle is fully closed the transmission is automatically upshifted to lower engine compression braking.

2. Significantly reduce the danger of loss of control of FWD, or front torque biased AWD, on slippery roadbed surfaces by reducing engine compression braking.

3. Significantly extend the useful life of the transmission by reducing its "workload" from engine compression braking. Many of these vehicles were shipped from the factory with a transmission fluid life expectency of that of the vehicle itself.

Keep in mind that by the year 2000 the analog, fluid based, "computers" previously used to control automatic transmissions were virtually PERFECT in their overall control operation.

Toyota's electronically controlled transmissions represented a totally new, clean sheet, design.

So, the transmission ECU firmware, software, detects that the vehicle is in a "coastdown" mode, throttle fully closed, and accordingly begins an "upshift" gear change sequence (and/or an unlocking of the torque converter) to reduce engine compression braking.

But what happens if the driver suddenly decides that he/she wants to accelerate at just about the same time the transmission begins the upshift sequence?

The only "feedback" the transmission ECU has to "know" that the previously commanded gear shifting has completed (other than elapsed time)is via a comparison of the transmission input shaft rotational speed and the output shaft rotational speed.

Up until the 2004 model year these transmissions would react correctly to this sudden change of input, quick change from coastdown to acceleration, but by now Toyota had discovered an anomally in the design.

In some instances you couldn't just change the commanded gear selection right in the middle, or during, a previously commanded gear change without compromising the operation of the transmission. The transmission momentarily being in two gear ratios simultaneously, for instance.

So, the 2001 AWD RX300 required no transmission fluid replenishing or replacement for the life of the vehicle, according to the owners manual. But then a significant number of these transmissions began to fail prematurely. The dealers have now been told to advise owners of these vehicles that a 15,000 mile transmission flush and replenishment is recommended.

So, as of 2004 the firmware was rewritten and an e-throttle was added. With an e-throttle the engine can be prevented from developing torque until the transmission can complete the upshift it started at coastdown, and then be downshifted into the proper lower gear for acceleration once the e-throttle is given the GO signal.

A 1 to 2 second delay in acceleration wouldn't be all that unusual in this circumstance.

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