The part from the dealer is kind of expensive--around $160. If it is not working correctly, you should get a code that indicates that. I don't know what code it is on the 300-series, but it is a dtc 8 (switch not closing) or dtc 9 (switch not opening) on pin 17 (Ignition Control Module) for the 400-series.


I have read that the relay sends a signal to the ignition control module to retard the timing a very small amount before shifting, which has the effect of reducing the engine load on the transmission. This would make sense because the dtc is on the Ignition Control Module. I wish I could find that post.

It is hard to find much doing a search under "transmission overload protection." Maybe someone else can chime in.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles