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Before the arrival of the ZF 4 HP-22 EH automatic transmission, the kick-down feature of automatic transmissions on the E31 tended to be implemented mechanically.
The EH transmission changed this. The position of the throttle was already being sensed by a throttle position sensor. On models with the EH transmission the throttle position sensor was improved to convey the position of the throttle precisely enough, so that the EH transmission electronics could identify when the driver was requesting a kick-down.
Before this, the throttle position sensor focused on identifying only the throttle-closed position and the wide-open-throttle position. Due to its simplicity, the throttle position sensor for models without the EH transmission has a rectangular 3-wire plug.
By contrast, the throttle position sensor for models with the EH transmission has a round 6-wire plug.