Residual at rest heat button

[acme]

REST button on a E9XX, I get the idea but does low battery power restrict the criteria? And should it always light up?

Thanks.

[Digitalize]

Yes, needs to have good battery power, and the car needs to be hot enough, I think they're the only two criteria. It has to be pressed fairly quickly after too I think.

Your additional waterpump needs to be working too of course!

[acme]

Thanks for the reply. Is it a known temperamental feature does anyone know?

Additional water pump?!

[Digitalize]

I've never got mine to work, but my additional water pump is broken.

It's an electric pump mounted to the inside of the passenger side wheel well, can see it in the engine bay. Assuming it's in the same place on the M's.

[acme]

Is the additional water pump just there for the REST button?

Cheers

[Digitalize]

As far as I know yes, but it's also possible they use it to keep the coolant moving for a little while after the engine is turned off, to prevent hot spots. No evidence but supposedly other manufacturers do.

[acme]

Really appreciate your reply. Now I'm a little concerned for the car I looked at, the pump isn't working as the button didn't work even 15 mins after switching it off when the engine was quite hot.....

Or are there specific criteria, key in/key in igntion on & engine off etc?

[Digitalize]

I can't remember the complete specifics of it, might be best to google it as I know it's an often asked question.

Without a diagnostics device there's no definite way to say what's wrong with a car, many small faults won't show up on the dash etc.

For example my E91 320D had both thermostats gone, 2 glow plugs, additional water pump, condensation sensor and air quality sensor all broken, but the car had no lights on the dash etc, you only find out when the codes are read, cleared, the car is driven and the codes are read again, bit of a pain but how all cars are now.

I think the ambient temperature has to be in a specific range for REST to work too from memory, but as said best to Google, maybe download a copy of the owners manual.