What is that whirring sound when you turn off a Prius 2005?

Engine coolant transfer pump moving hot fluid into the storage thermos. Also, sometimes, the brake accumulator pump.

 

And just a note that the 2010 no longer has that thermos, so I'm guessing that noise will now be gone.

 

Really? I thought that was more of the sucking 'whoosh' sound which was the hot fluid transfer.

 

It whirs. The description of sounding like an antenna is right on the money.

Tom

 

The North American model of the Gen II Prius has a thermos bottle which is used to save hot engine coolant. The hot coolant is used to heat the engine at the next startup, reducing emissions and improving mileage.

The brake accumulator pump is an electric pressure pump used to pressurize the power brakes. Most cars use engine vacuum for this, but with the Prius the engine is often not running, so an electric pump is used.

Tom

 

Spot on description on the coolant pump sounding like an electric antenna. Mine comes with a special note that when its really cold like below 10 degrees it sounds more like a refridgerator ice dispenser. Which I have been told is normal.

 

It goes a bit further than that. In normal running your foot pressure does not directly act on the brakes at all. Instead it moves a 'stroke simulator' and the pressure is measured by the Skid Control ECU. The ECU then decides how much pressure to apply to each wheel, factoring in how much regenerative braking effort the HV ECU says it can give, and modulates valves leading to the brake cylinders accordingly. The high pressure in the accumulator pressurises the brake cylinders, and the pump runs to maintain the accumulator pressure.

I'm not sure why the pump needs to run on shutdown. The pressure can be released simply by opening both the 'apply' and 'release' valves for one wheel cylinder at the same time. Speculating, the fluid is cycled a bit by pumping. Brake fluid is hygroscopic (absorbs water) and it's likely best that the water is evenly distributed rather than having quite dilute fluid in one part.

The diagnostic tester has a specific 'zero down' function, though this may simply be that you have to have the car on for tester communication to function, and turning the car on brings up the accumulator pressure. The next step in replacing an accumulator after running 'zero down' is to turn the car off, though.

For completeness, if the Skid Control ECU fails, several failsafe valves open, directing pedal pressure directly to the front wheel cylinders. There is no pressure assist in this situation and no rear braking.

The setup has changed a bit for the 2010 car and I haven't fully understood the changes yet. It's still brake-by-wire, though.