BMW Slipping Automatic Transmission
This answer is applicable for many BMWs in addition to the one listed below.
I have a problem with the transmission on my 1995 BMW 540I. It runs very smooth on the highway, but from a start it slips and catches and sometimes goes to the transmission fault mode where it is stuck in a higher gear(limp home mode). If I turn off the engine and restart it, it runs good again unless I start off too strong and it will fault out again. I learned to drive carefully as to not set off the code. I replaced the transmission computer, but that didn’t help for long. I thought it was fixed for a while then it acted up again. It seems that as long as the engine is above 1500 rpms, the transmission works fine, below that it has problems. Is something clogged, dirty filter, a valve sticking or electrical connector loose?
In the case of your apparent transmission slipping, I would recommend a fluid and filter change as a first course of action. I would drop the pan, change the filter, refill, drive around the block and then just drain the fluid again (without removing the pan) and refill. This will get a higher percentage of fresh fluid in the transmission. Drive the car for a few hundred miles and see if things improve at all. If the slippage is due to poor fluid condition or a sticking valve in the valve body, the fresh fluid may show an improvement, over time. If you find that there is no improvement, you likely may have worn clutch packs and be in need of a rebuild.
The Bentley repair manual will cover the steps in performing the fluid and filter change. You can also check out the DIY that we did in the Spring 2006 issue of our Fast Times newsletter (copy is attached). This was on a 2000 540i, but the procedure will be similar for your model, your fill and drain plugs will be different (as outlined in the Bentley repair manual).
Let us know how the fluid/filter change goes.