Odd Transmission Behavior At Highway Speeds

[LS 0181158]

While cruising at highway speeds--say 70-80 mph, for example--the transmission in my 1998 LS 400 (160K) seems to have become much more sensitive to throttle inputs. This relates only to the disengagement and re-engagement of the lockup torque converter. Before, I could always get somewhat firmly on the accelerator at highway speeds, and the car would accelerate briskly and smoothly without downshifting out of 5th gear OR disengaging the lockup torque converter; there was no activity in the drivetrain itself.

Now, even slight pressure on the accelerator--such as that needed to change lanes and accelerate smoothly past a car or to maintain highway speed up a slight incline--causes a slight nudge and an increase of 100-200 rpm as the lockup torque converter disengages, before re-engaging imperceptibly a few seconds later. This happens only in top [5th] gear, which I believe is the only circumstance where the converter operates anyway. My car feels kind of underpowered now, since the transmission now appears to have to "hunt" to accelerate gently or even to maintain a constant speed, where it didn't have to do so before.

Is this what I'm thinking it is? What can I do about it? Is this a known problem with the A650E transmission? 160K miles is quite a bit for most cars, but not a lot for the LS 400, and I really wasn't expecting transmission problems when I bought this car! I drive conservatively, and never experienced transmission problems with either of the AT BMWs I owned, both of which reached higher mileages than this. How much would it cost to rebuild/replace the transmission unit in my car, supposing it eventually came down to that?

I bought my car with 115K, and had transmission fluid changes performed at 125,046 and 130,429 miles (both times filled with Toyota Type IV fluid as specified). I wasn't planning to touch the AT fluid again until the upcoming timing belt/180K service planned for 174K miles, but might changing the fluid now help?

[amcdonal86]

I have heard rumors that a bad coolant temperature sensor can cause the problems you're describing without throwing a CEL or error code. Do a search on this forum and you might find out some more info.

[JeffreyV]

While cruising at highway speeds--say 70-80 mph, for example--the transmission in my 1998 LS 400 (160K) seems to have become much more sensitive to throttle inputs. This relates only to the disengagement and re-engagement of the lockup torque converter. Before, I could always get somewhat firmly on the accelerator at highway speeds, and the car would accelerate briskly and smoothly without downshifting out of 5th gear OR disengaging the lockup torque converter; there was no activity in the drivetrain itself.

Now, even slight pressure on the accelerator--such as that needed to change lanes and accelerate smoothly past a car or to maintain highway speed up a slight incline--causes a slight nudge and an increase of 100-200 rpm as the lockup torque converter disengages, before re-engaging imperceptibly a few seconds later. This happens only in top [5th] gear, which I believe is the only circumstance where the converter operates anyway. My car feels kind of underpowered now, since the transmission now appears to have to "hunt" to accelerate gently or even to maintain a constant speed, where it didn't have to do so before.

Is this what I'm thinking it is? What can I do about it? Is this a known problem with the A650E transmission? 160K miles is quite a bit for most cars, but not a lot for the LS 400, and I really wasn't expecting transmission problems when I bought this car! I drive conservatively, and never experienced transmission problems with either of the AT BMWs I owned, both of which reached higher mileages than this. How much would it cost to rebuild/replace the transmission unit in my car, supposing it eventually came down to that?

I bought my car with 115K, and had transmission fluid changes performed at 125,046 and 130,429 miles (both times filled with Toyota Type IV fluid as specified). I wasn't planning to touch the AT fluid again until the upcoming timing belt/180K service planned for 174K miles, but might changing the fluid now help?

I'm having problems with my 1992 LS400 torque converter lock-up. It can't make up it's mind to engage or not. Pleaes let me know if you resolved your problem and if it was the transmission's temperature sensor. I know that they keep the transmission from locking until the car is a operating temp. I was considering trying to disconnect the torque converter - but, maybe it's just a temperature sensor! Write me here and/or at byerjv@yahoo.com THANKS!