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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 E28 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 simple wiring: just three wires.
By contrast, the throttle position sensor for models with the EH transmission has more complex wiring: a round 3-wire plug into the rheostat unit, plus two individual wires that go to an additional switch that, I’m concluding, performs some of the simple on/off sensing function that is traditionally performed by the simple-design throttle position sensor.