[PLuKE]

My e91 330i auto (N52) has just started with a strange problem.

When the car is around 1500-1700rpm the revs fluctuate on the rev counter, very small amount 50rpm or less, you can just see it if you look hard enough, and you can feel the car doing it also, again very very small amount, nobody passenger has commented or said anything.

Its not hunting for gears IMO, it seems to be fine when on cruise control.

Thanks chaps.

[thakid22]

Originally Posted by PLuKE

My e91 330i auto (N52) has just started with a strange problem.

When the car is around 1500-1700rpm the revs fluctuate on the rev counter, very small amount 50rpm or less, you can just see it if you look hard enough, and you can feel the car doing it also, again very very small amount, nobody passenger has commented or said anything.

Its not hunting for gears IMO, it seems to be fine when on cruise control.

Thanks chaps.

My 330i sport with the ZF automatic did the same thing.
At about 1900 rpm, the tach would slightly jerk up and down a slight bit.
You'd have to look for it to see it. What made me look is I could also feel an ever so slight modulation of power during low speed cruising. I think it was at about 40mph.

In my case, the shift solenoids on the mechatronic (combined valvebody/TCU) had become lazy. I removed the mechatronic and installed new solenoids. There are 7 to 8 of them. They cost about $500 the mechatronic is located inside the transmission pan. You need to buy new transmission fluid to if you do this job.

Originally my car exhibited a jolt at stops when the car would shift from 2nd to 1st gear. Even sitting stationary and using the manual mode to shift from 1 to 2 and back produced a little bump to the car as the change was not smooth.

The new mechatronic solved the 1900rpm jerks. It cured the 2 to 1 downshift jolt. Additionally, all shifts became quicker, more precise, and the tranny felt like new. Even shifting from Park to reverse or reverse to drive surprised me with instant and immediate action. The change was so marked, that I instantly realized those engagements were previously slow, even though I didn't notice until it was fixed.

I do consider solenoids to be a wear item. At 100k + miles, there is definitely an improvement to be had by switching them out. Probably sooner. If the tranny is having issues, they are almost a certain bet, as the ZF6HP itself is relatively robust.

Solenoids in general, deliver diminished response and operation as they age. Honda/Acura trannies love a good solenoid refreshening too. Unfortunately for us ZF owners though, it is a slightly more involved procedure to swap ours out. But it is totally doable by the average Joe.

An indie can do it for a fee if you don't feel the DIY route.

[PLuKE]

Thank you for coming back to me.

I find the rpm fluctuations are very small, like 50rpm only. I thought it was a throttle sensor or something, not an autobox issue?

I scanned the car, nothing showed on the engine or autobox.

It's very slight, I don't always notice it. I can't find anybody else commenting on this problem.

Luke