I just drove about 10 miles in 34 degrees C.  The hottest day ever for me and the car.

When I came to a stop with the car in gear and my foot on the brake, although the clutch disengaged, the revs stayed at 1000.

They usually drop to tickover at about 750-800.  Its the first time this has happened.  Is it something to worry about or was it just the very hot weather.  This was the same throughout the journey.

Water temp was normal, and the oil got up to 92 degrees C.

 

Thanks

Stewart

Was air con on? I’ve noticed if that’s going that revs hang a little higher (and not just on a dsg octy). 

3 hours ago, Benz3ne said:

Was air con on?

twas 34 degC, what do you think😂😂😂

 

But yeah, thats A/C running has caused that, if it ticks over at 1,200 you've accidentally knocked the DSG into sport 

Edited 11 August, 20205 yr by themanwithnoaim

24 minutes ago, themanwithnoaim said:

twas 34 degC, what do you think😂😂😂

 

But yeah, thats A/C running has caused that, if it ticks over at 1,200 you've accidentally knocked the DSG into sport 

They could’ve just had the window open? 🤣

Worth asking rather than assuming. 

Try pressing the brake pedal a bit harder and see what happens - mine does the same and couldn't figure it out for ages!

Guess it stays at slightly higher revs if you anticipate moving away more quickly.

Cheers,

 

Nick

Thanks guys

Yes, the air con was on from start up to journeys end, and all the windows were closed so I guess it was working overtime.  My first reaction was to press the brake a little harder than usual, but it made no difference.

I'll put it down to the A/C working harder than usual.

 

Regards

Stewart

I've noticed this too.  Seemed to raise the revs from about 27 degrees and up.

I had this issue with my VRS when I went to CZ last year and it was 38C driving around Prague.  The car will start injecting additional fuel into the mixture due to evaporation; you will also find that stop/start maybe disabled during this period too.

Its perfectly normal - the car is doing what is meant to do.

Thanks again guys.

I haven't been able to use the car since, but I just connected an OBD2 and no fault codes are stored.   So maybe its as varaderoguy  says, "Its perfectly normal - the car is doing what is meant to do".

 

Regards

Stewart

I'm a bit shocked reading this thread! I understand that AC compressor and PS pump loads raise idle RPM, but assumed that the Mk3 had an electric AC compressor to go along with the PS. Is that not the case?

No. An A/C compressor requires far more power and amps to run than a standard 12v system can output.

 

The power steering doesn't have a pump. It's an all electric system where a motor drives the steering rack directly.

 

Mercedes have done a system where everything is electrically powered (power steering, A/C, water pump and an electric super charger). But this uses a 48 volt system.

To put it into numbers. If you take the required HP to power an A/C compressor as 3hp minimum (can be more).

 

A 12v DC motor would require upwards of 150 amps!

 

I believe the power steering has a 50 amp fuse so uses under 50 amps.

Well bugger me, I was under the impression electric AC was commonplace on modern cars for the purposes of fuel economy...

 

Learn something every day, eh!

No not yet. But as more hybrids and light hybrids are coming out it's becoming more common as the higher voltages are available to power it.

 

But the A/C compressors either have a clutch to disengage when not needed or in the case of VAG they run permanently but then have a bypass valve inside so the refrigerant isn't being compressed and circulated the whole time.

On 16/08/2020 at 08:59, Phil-E said:

To put it into numbers. If you take the required HP to power an A/C compressor as 3hp minimum (can be more).

 

A 12v DC motor would require upwards of 150 amps!

 

I believe the power steering has a 50 amp fuse so uses under 50 amps.

That's also news to me.  I assumed it was hydraulic.  Does anyone have any technical info/diagrams of the electric power steering?

This is a motor from it. It's just a direct drive through a worm gear it seems:

 

 

And here is a complete rack. So you can see the motor is just mounted to it and there's no hydraulics at all:

 

Thank you Phil.

That's interesting, but also a little scary.  There must be a sensor somewhere that notices which way you are turning the steering, and how much effort you are putting into it.  I hope that sensor is ultra reliable, and that the effort the motor can apply to the rack is less than I can through the steering wheel!! 😟

 

Thanks for your research

Stewart