C180 Elegance, 1997 Auto, 97k mls. The ABS light came on and instantly the car juddered down to 1st and wouldn't shift up. Can't get in to test if transmission fluid still full however no warning light came on so presuming full. Why would ABS sensor be linked to transmission and what kind of major fault would result in car sticking in 1st?

The traction system is the source of vehicle speed input to the trans computer.

The trans computer needs that signal to know what gear to shift into.

Yes a differential sensor can cause the trans to go into limp.

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