I have a really weird issue with my gearbox. I will be generally cruising at 70k/h (sometimes with the cruise control on) and every know and then it feels as though the gearbox is jumping. It's a very hard feeling to describe. To give an indication of what it feels like, I guess, is to

* Make a fist
* Rest is on top of your thigh (easier when sitting down)
* Push down firmly and let go (with about a 1/2 second gap between each push down - almost like you're jabbing yourself)

This only seems to be at this speed, and generally speaking all other speeds seem to be fine. I have found that if it starts doing this and I turn O/D off the car drops down a gear and doesn't do it anymore. So it looks as though it's overdrive that is doing this.

I have also checked the car when it is doing this jutting "thing" that there is no change in either the gear position (so i.e. not slipping to another gear), no change in current speed being held, no change in engine revs at all. No additional lights, no CEL, no knocking noise or anything.

I do know that I may have a small leak on the gearbox, and was contemplating that this may be related to a pressure loss in the gearbox doing this. If that was the case however, wouldn't the issue be there at different travel speeds?

 

Might it be the gearbox mount? Then again if it was then this would be apparent at other speeds.

 

i would say take care of the basics, according to the mileage.

clean your throttle body, maf, and check your wires. ive seen cases where just a coil is bad at a specefic rpm range. check your plugs. we basically need more information about the car. mileage?

 

Correct me if I am wrong, but as I understand it only happens in overdrive, at 70 kpm (app 60 mph). If you lightly press on the brakes, (enough to activate the brake lights, but not enough to slow the car down) can you still feel it? The torque converter clutch is designed to release as soon as the brake is pressed on most cars. Since you did not state what year your car is, I could not look up the schematics. What year is your car, and how many kilometers on it? Try the brake thing, that may narrow down what the problem is.

 

The car is a 97 Lexus ES300 (release year 1996 in Australia) so 3rd Gen. Also car has done around 211000km's (131000 miles). Recently serviced about 3 months ago. If I turn off over-drive when it does it, the feeling goes away. It's only when it's on O/D that it seems to do it. It's also a random thing and doesn't do it all the time.

 

Ok, I checked alldata, and according to their schematics, the lockup clutch is tied into the brake switch. So if you try lightly pressing the brake you will dis-engage the lockup clutch. This should help narrow down the possibilities.

 

Okay finally got roud to actually doing a couple of things. The problem has still not gone away entirely. This part Saturday I did a transmission oil change to see if it would resolve the issue. It has improved it, but it's still got that un-nervy feeling about it at around the 80km/h mark (again now more random rather than constant). I did notice tonight while driving it, I was doing a 100km/h down the freeway and when I disengaged the cruise control (and dropped to 80) it felt like it had a very slight shudder before settling. I have attempted to drive it while it has been doing it (and note it doesn't do it for very long), but when cruising (with the cruise control on) and then putting my foot back on the gas to keep the speed up and then tapping the brake it seems to stop it. I can't 100% say this or not, because when it does this annoying thing it's sometimes only very brief.

There are no CEL's or other indication lights on, so the car doesn't think it has a problem at this point. If it is the lockup clutch, would it be slipping? How expensive is it going to be? Might it be a dirty sensor having the issue at either 70 / 80 km/h? Again there's no visible signs on the Revs or the speed when it does this jerk thing.

 

I bought an ODB-II sensor (seeing as I always wanted one) and thankfully there are no codes on my engine. I recently took the car for a drive up north (about a 250-300km journey one way) travelling at speeds around 110km/h (and slightly higher when overtaking traffic). Lovely feel at that speed without any signs of problems or issues.

After I returned (so about a 600km trip + some driving at my destination) the car was fine at normal speeds. Getting to 70km/h on a pretty much straight road (so no big inclines to allow it to raise revs to keep engine speed; no down hills to allow it to coast) it did it for a second or 2.

If the car does do this jutting thing, I can pretty much drop it out of overdrive, hence forcing revs up and putting load on the engine and this will get rid of the issue straight away. I can also tap the brake to turn the cruise control off and it will go away straight away.

Could it be a sensor that is dirty? I was reading through the Full Service Manual (FSM) and one of the maintenance items is to remove the speed sensor, clean it and put on a new "o-ring". Would this help? If it is the lockup clutch, what sort of damage is this going to do to the wallet?