Quote:
Originally Posted by BMWRog
Immediately after I have switched off, I can hear what sounds like a softly buzzing relay, as if it won't quite latch. The BMW engineer didn't comment on this, but I gather he thought it might be the electric water pump.
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Yep...I was going to respond that the buzzing you hear is the electric water pump. The N63TU/S63 engines have three different cooling circuits:
- Engine cooling circuit
- Turbo-charger cooling circuit
- Charge Air/DME cooling circuit
Since the main engine cooling circuit uses a “belt” driven coolant pump that only operates while the engine is running & the electric coolant pump attached to this circuit is used to facilitate heating the interior...the Turbo-charger cooling circuit has its own 20 watt electric pump that can operate up to 30 minutes after shut down to cool the turbo chargers. There’s no need for the main cooling system’s electric pump operation after shut down since it is used to heat the interior. You don’t need to heat the interior with the vehicle shut down unless you initiate the REST (residual heat) feature like when stopped at a railroad crossing when temps are cool/cold...or when running errands in the winter and you want to keep the interior warm since you’re coming back within 15-16 minutes.
There’s a lot of stuff going on under the hood....see the illustrations below that explains the 3 different cooling circuits:
