The Urbanite

Hello all,

Firstly apologies for dragging up an old topic. I have an interest in transmissions, especially those by ZF and stumbled upon this website when trying to find what causes the "steptronic slam".

Secondly I don't own a car with a ZF transmission, but a lot of the buses around here have a 4 speed (generally pre-2001), 5 speed (generally 2001 - 2006) or 6 speed (generally post 2006) ZF transmission. In fact thousands of them do, and they all suffer from these violent, jerky gear changes. There are exceptions - the buses that have had their kickdown rod removed. They do not suffer from any steptronic slams, and there is also no "nudge" as the bus comes to a halt. I appreciate that buses have very different ratios/mechanisms to cars, but they still suffer the same issues as listed within this topic.

There are two solutions to this problem from what I know. Either have the kickdown rod removed, or have the transmission programmed to disable the downshift element of the kickdown, but to still allow the engine to reach its full safe limit whilst accelerating. There are some buses with the latter setup, and they do not "lurch" or "slam" at all when stopping or accelerating from a low speed. However acceleration is hampered as it won't shift from "2nd" to 1st until a second or so after it stops.

Now I read a lot of claims that the manufactures of these transmissions deny that there's a problem. I have taken several videos in different buses where the steptronic slam is very evident indeed. These things must cost at least $300,000 and they still have the same problem.

Example of the purpose of this slam;


0:07 - Pullaway
0:11 - Shift into "2nd" gear.
Shortly after this, the bus almost slows to a stop, but begins accelerating again in the "2nd" gear.
0:24 - "Slam" back into 1st gear as the kickdown was activated. As you can hear, acceleration is greatly improved. In this particular bus it isn't violent, but it is noticable.

Example 2 (4 speed ZF transmission) -
<Fast forward to 0:24>

The bus pulls away with the kickdown in full use until the end of its 3rd gear. At 0:48, after a few seconds at crawling speed, there is a violent and loud "slam" as the driver hits the gas. <Fast forward to 7:18>. Again the bus is slowing down. At 7:23, a loud bang occurs as it drops from "2nd" to 1st whilst accelerating.

Example 3 (6 speed ZF transmission) -
<Fast forward to 1:20>

The bus is slowing down. You can hear that the shifting down is quite jerky. Steptronic slam at 1:29 as the driver accelerates again.

Example 4 (4 speed ZF transmission) -
<Forward to 3:20>

Driver pulls away. At 3:32, after slowing to take a tight corner, the transmission slams into 1st.

Example 5 (5 speed ZF transmission) -
The video starts with the driver pulling away. At 0:15, the transmission slams forcefully from "2nd" to 1st.