on the w210 and other models there is a relay called the K40, its actually a big brick with multiple relays and fuses on it. It controls the power distribution to various items. When that goes a bit south the ECU does not switch on.

The ECU not being switched on is signalled by the electric engine fan running at full speed as soon as the key is put to run.

on the W210 the K40 is housed in the big box that holds the ECU, TCU and ABSCU. Its the most forward of the items.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)