Torque Converter In and Out of Lock Up

[wrumble]

My 1997 528i while traveling at highway speed, the torque converter will go in and out of lock. While on the flat for example, the tach may read 2500, then when transitioning to a slight grade, the tach will actually go down by about 200 RPM. Or if at highway speed then accelerate, the RPM will first drop by 200 RPM, then continue accelerating. I believe the torque converter locks when going up a grade, and unlocks on the flat. Not sure why. Any ideas?

Thanks
Bill

[rf900rkw]

It shouldn't be doing that. My first step, after a code check, would be to verify that the computer is ordering such behavior. If it is, look to the sensors, engine and trans, for something abnormal. IF it's not, the command is staying steady, then it's an internal trans problem.

[wrumble]

Thanks, I will get a code check.

[wrumble]

Just had the codes read at Autozone. No codes were stored. My next step will be to monitor the lock-up solenoid voltage to see if it is being commanded to lock/unlock.

[rf900rkw]

Well, the good news is the computer isn't giving up and retrying due to a slightly slipping lockup. That woulds have flagged a P0740. And good on you for jumping in the the VOM.

[wrumble]

I connected a VOM to the lock up solenoid line and monitored the voltage while driving. The signal was stable during the periods when the torque converter goes in and out of lock up. So I guess there is something going on internal to the transmission causing the problem. Not sure where to go next.

[new200guy]

I have the same issue as you do on my 530D. had the transmission rebuilt and still have the same problem. i notice it from 85 to 90km/h. i have the GM 5L40E and i belive you have a GM 4LXXE transmission in yours. i have even swapped out the ECU and had it reprogrammed. I have no errors in mine either. Kind of just dealt with it over the last 60000km as the rebuilder says he can't find any faults either. I have heard that Vacuum lines that are cracked can cause an issue? don't quote me on that. Will probably be replacing mine when it gets warmer as i have over 284000km's on mine. i will subscribe, so maybe i can keep up with your post and troubleshoot from there.

[rf900rkw]

The voltage staying steady is not a good sign, no. But the next question is are you *sure* it's the lockup doing this? Could it be dropping back to 3rd? There have been cases where the line pressure in (low rpm) 4th isn't sufficient to hld the 3-4 valve stroked. The valve defaults back, the O/D drops off, the RPM goes up, line pressure goes up, solenoid can now overcome the valve and it shifts back to 4th.

The OP has a 4L30. Nothing in the operation, diagnosis, or repair applies to the 5L40.

[wrumble]

I cannot say with certainty that is not going into 3rd, but with only a 200 RPM variation I would tend to think it is not. Regarding engine vacuum lines, I am not sure how this would be related to my problem. Especially since I monitored the voltage to the lock up solenoid and it was not dropping out. With the voltage being steady on, the revs go up and down with slight change in throttle. The tach should act like a manual transmission when the torque converter is locked.

[wrumble]

For now, I disabled lockup. At the ECU, I disconnected the line going to the lock up solenoid. I then connected a 20 Ohm resistor in its place so the ECU thinks it is still connected and not go into limp mode. I hope to possibly avoid damage to the lock up clutch till I figure out what is going on. I have a feeling a rebuilt trans is in my future.

[djmhendrix]

I just noticed the exact same issue as OP described; Good to read that rebuilding and replacing ECU don't work. Still curious now... Anybody know what this is about???

[bimmer7410]

I use to have this issue. It got worse as time went by. I had the fluid replaced and it went away, but i do feel hints of it when driving.
I'll need a rebuild or torque converter replacement soon enough. 134k