Harsh downshift on 04 Sienna

[turbojimmy]

Just picked up my 04 Sienna on Friday the 18th. Drove it all weekend and noticed that it was "helping" me slow down. Like if I'm braking from 50 MPH, I'd notice that suddenly the car was slowing quicker despite steady brake pressure. I thought maybe it was some electronic nanny (like the "brake assist") but it turns out it's actually downshifting and slowing the van with the engine. I can see it in the tach. It will drop out of OD and the van will "engine brake'. It's not jarring or anything, but I've never had an auto do this. It will do it in other gears, too.

Is there something wrong with it?

Thanks,
Jim

[tidalcock]

Not as far as we can tell. Took ours to Grandfather Mountain this fall and it did the same thing. Strange feeling at first, but really cut down on downshifting and saved on break wear.

[04siennaowner]

I noticed the same thing. You'll get used to it.

[BMW_4.4i]

Not sure if the Sienna has this option but...
is there a "ECT Power" button somewhere in the car? This is used during accelleration to keep the engine reving higher before upshifting, and it may (???) be used during braking. If so, try turning it off.

[sixokahuna]

My 2004 sienna was doing the same thing...I took it back to the dealership for its 1000 mile free maintenance and mentioned the downshifting problem and a slight hesitation (or lag) on takeoff from stand still. The dealership told me they had to recalibrate the electronic control unit. It seems to run much better (the downshifting problem was fixed) although I think there is still a little lag from the time I press the gas pedal and when it actually moves.

[steph280]

There is a TSB out to reprogram the shift pattern on the 04 transmission which will correct this problem. If you go to any competent dealer and complain they will perform this TSB for you. If they deny of any problems, don't bother showing them the TSB because they probably don't even know how to do it.

http://www.siennaclub.org/forum/inde...showtopic=2190

[wwest]

When your Sienna is in OD the torque converter is locked and the engine is forced to turn at the same rate as the transmission input shaft. Unlocking the converter results in the engine turning slower than the transmission input shaft, thereby less compression braking.

So it isn't helping you slow down at all, exactly the opposite in fact!

The shop manual for my RX300 indicates that with even slight brake application torque converter lockup is instantly disabled. Additionally it indicates that with the throttle valve closed, coastdown circumstance, the transmission will actually upshift a gear of two depending on roadspeed and the gear it was in at the time of throttle closure.

I suspect this is to gain two, maybe three, benefits, improved fuel economy due to lowered engine compression braking, and safer operation of FWD and front torque biased AWD vehicles on adverse roadbed conditions, ice, packed snow, etc. Would overall transmission life be extended absent engine compression braking?

For those of you familiar with, or having actually experienced, the Toyota 3.3L V6, 5 speed auto, FWD hesitation problem, this "upshifting" circumstance is very likely the root cause of the 1 to 2 second engine lag or hesitation when you suddenly decide to accelerate from a just previously "entered" coastdown condition.

When you close the throttle and enter a coastdown situation the Toyota/Lexus transmissions are programmed to upshift almost immediately. It takes about 1.2 seconds to complete a shift from neutral to "D". So, you lifted the throttle and maybe even applied the brakes lightly but now the traffic light turns to green and you want to accelerate so you get back on the throttle.

But.....

The transmission had already started its "upshift" sequence and the electronic servo system has no intermediate feedback so it must fully complete the upshift before it can respond to the new acceleration "request" and then downshift into the most appropriate gear based on the roadspeed and just how far down you pushed the gas pedal.

So......

Within 1 or 2 seconds the transmission will have completed the downshifting and now the e-throttle will be given permission to GO!

[altenburgpj]

Did you know that the only reason automatic transmissions feel smooth, is that they are programmed to be in two gears at the same time during their upshifting and down shifting.

During this transition, the clutches are slipping and fighting each other. wearing down the clutch material.

If you want your Automatic to last 300K while towing a 3000lb trailer, just put a "shift Kit" into it which eliminates the transition, and makes the trany shift like a manual trany would.

Just out of curiosity, when did down shifting for engine braking become a bad thing?

Then again you might be the type who would drag their brakes while decending Pikes Peak.

[wwest]

Downshifting for engine braking became a VERY bad thing with the advent of the clear majority of vehicles on the roadbed being FWD or front biased AWD. Downshifting a FWD car will very often lead to loss of total traction, including directional control on a slippery or wet roadbed.

[altenburgpj]

Originally Posted by wwest

Downshifting for engine braking became a VERY bad thing with the advent of the clear majority of vehicles on the roadbed being FWD or front biased AWD. Downshifting a FWD car will very often lead to loss of total traction, including directional control on a slippery or wet roadbed.

I suppose that might be true on snow or ice, but my tires are just too good for that on wet pavement. I have never locked my front wheels or lost control on wet pavement no matter how hard I brake, or force my Sienna, or Buick Regal, down into a lowwer gear or two, or for that matter in my five speed MANUAL Ford Tempo. Now all those front whhel drive vehicles have been a little light in the rear, and I have spun them, I even spun a Dodge Maxi van 180 degrees on a two lane road without touching anything. But engine braking would never do it.

You might try it sometime. Find a large empty parking lot after a rain, and just try to lose control. I am sure it is much harder than you think.

[wwest]

Your automatic transmission does not know just what is "underfoot" at any given moment. So the safest thing for the FWD manufacturer to do is "upshift" the transmission anytime you are coasting with closed throttle.

Drive the BMW X3, AWD with rear torque bias, and experience its engine braking during coastdown and then any newer FWD with e-throttle for comparison.

[altenburgpj]

Sorry I don't see what coasting with closed throttle has to do with down harsh downshifting while braking. My 1972 Ford LTD would always upshift "anytime you are coasting with closed throttle". I think all automatic tranys have done this. Its what they are supposed to do.

I believe the thread started with Harsh down shifting while slowing down which if you read turbojimmy's first post wasn't even all that harsh. He just wanted to know if it was normal for his Sienna and the answer was and still is "Yes Jimmy it is normal"

Originally Posted by wwest

Your automatic transmission does not know just what is "underfoot" at any given moment. So the safest thing for the FWD manufacturer to do is "upshift" the transmission anytime you are coasting with closed throttle.

Drive the BMW X3, AWD with rear torque bias, and experience its engine braking during coastdown and then any newer FWD with e-throttle for comparison.