Jaguar Forum banner

Turn off auto demist?

7K views 41 replies 18 participants last post by  OBC John 
#1 ·
I've looked so I presume it's not possible, but is it actually possible to turn off the auto demist function below certain tempretures. It was the same story in my old car and the number of times I get in the car and turn it straight of because it's not needed is getting silly. Surely it should have been a programmable option.
 
#2 ·
Phil, have you ever noticed it comes on even without the lights when the temperature is higher. There is a mist sensor at the top of the screen. I have seen the front and rear screen demise at 8 and 10° external temperatures and the button lights don't illuminate.
 
#7 ·
I'd like to turn it off because it doesn't need to be on? Heated screens use loads of power which means loads of extra fuel. I'll put it on if I need it on but don't see the point in needlessly wasting fuel if it's not required. Mine only ever seems to come on below 4 degrees, I've never known it come on by itself at any other time. Maybe that's something on facelift models?
 
#9 ·
The time the auto demist is on, is very short and certainly doesn't use a lot of fuel to run it and then for such a short time :)
 
#8 ·
Loads of extra fuel? How much are we actually talking about? 100ml over 100 miles?

If you worry about that sort of fuel consumption you should buy a bike and cycle everywhere. Cars aren't for you. :)


Sent from my iPhone using Tapatalk
 
#10 ·
Loads of extra fuel? How much are we actually talking about? 100ml over 100 miles?

If you worry about that sort of fuel consumption you should buy a bike and cycle everywhere. Cars aren't for you.



Sent from my iPhone using Tapatalk
Do you really think if I worry about fuel consumption I would have bought a 4.2 litre supercharged v8 engined car? It is however needlessly wasteful if it's not doing anything and seems a bit pointless having it come on automatically when I can just choose to turn it on if there is actually something to clear.
 
#11 ·
Surely there are better things to worry about?


Sent from my iPhone using Tapatalk
 
#13 ·
Do you also turn the AC off in winter? I would have thought this would have more of an impact on fuel consumption.


Sent from my iPhone using Tapatalk
 
#18 ·
True. And also, if you leave the aircon off and forget to give it a 30 minute exercise once a month you can dry out the seals in the system and cause refrigerant leaks.
 
#19 ·
I'm with you on this one Phil, it's really annoying when things come on when you don't want them on. Auto windscreen wipers, auto headlights, they're all a damn nuisance. They come on when you don't need them and stay off when you do.
In the case of the window heaters, they use a very large amount of power from the battery or from the alternator when most of the time they're not needed. The cabin heating and ventilation I leave on auto all the time.
 
#24 ·
I do leave it on full auto all the time yes as I know the ac does a good job of keeping the inside of the car dehumidified. I just can never see the point of the front and back screens coming on when most of the time they're perfectly clear anyway.
 
#25 ·
Then turn them off. But if the outside air temperature is under 4, then there is the possibility that the surface temperature of your screen is cold enough to freeze. So if you were to then try and clean the screen you might find yourself looking at frosting. Not fun if you've already driven off.
 
#26 ·
Amazing what you find when you read the manual....

Rear Window Heater
Rear window heater operation is only enabled when the engine is running. The ATC module controls operation of the rear
window heater using a relay in the RJB. When rear window heater operation is required, the ATC module broadcasts a
message to the RJB on the medium speed CAN bus. On receipt of the message, the RJB energizes the relay by providing a
ground path for the relay coil. This allows a battery feed to flow across the relay to power the rear window heater element.
There are two modes of rear window heater operation; manual and automatic.
Manual operation is activated by pressing the rear window heater switch on the integrated control panel. When the switch
is pressed, the status LED (light emitting diode) in the switch illuminates and the rear window heater element is
energized. Manual operation is discontinued when the rear window heater switch is pressed a second time, 21 minutes
have elapsed (the heating phase), or the engine stops. If manual operation is discontinued by the engine stopping, the
previous heating phase is resumed if the engine is re-started within 30 seconds.
There are two variants of automatic operation; automatic operation at the start of a journey and automatic operation
during a journey.
Automatic operation at the start of a journey is initiated if the ambient air temperature is below 5 °C (41 °F). In this
instance, the switch LED is illuminated and the heater element is energized for 21 minutes. Automatic operation is
discontinued if the rear window heater switch is pressed or the engine stops.
Automatic operation during a journey is initiated when low ambient air temperatures are experienced and the vehicle has
been travelling for a set period of time above a threshold speed. In this instance, no feedback is given to the driver to
inform him the rear window heater is operational (the switch LED is not illuminated). The duration of heater operation is
variable depending on the ambient air temperature, vehicle speed and the amount of time the vehicle has been travelling.
Windshield Heater (Where Fitted)
Windshield heater operation is only enabled when the engine is running. The ATC module controls operation of the
windshield heater using two relays in the EJB (engine junction box). When windshield heater operation is required, the ATC
module broadcasts a message to the CJB on the medium speed CAN bus. On receipt of the message, the CJB energizes
the relays by providing a ground path for both relay coils. This allows a battery feed to flow across the relays to power the
windshield left and right heater elements.
There are two modes of windshield heater operation; manual and automatic.
Manual operation is activated by pressing the windshield heater switch on the integrated control panel. When the switch is
pressed, the status LED in the switch illuminates and the windshield heater elements are energized. Manual operation is
discontinued when the windshield heater switch is pressed a second time, 5 minutes have elapsed (the heating phase), or
the engine stops. If manual operation is discontinued by the engine stopping, the previous heating phase is resumed if
the engine is re-started within 30 seconds.
There are two variants of automatic operation; automatic operation at the start of a journey and automatic operation
during a journey.
Automatic operation at the start of a journey is initiated if the ambient air temperature is below 5 °C (41 °F). In this
instance, the switch LED is illuminated and the heater elements are energized for 6.5 minutes. Automatic operation is
discontinued if the windshield heater switch is pressed or the engine stops.
Automatic operation during a journey is initiated when low ambient air temperatures are experienced and the vehicle has
been travelling for a set period of time above a threshold speed. In this instance, no feedback is given to the driver to
inform him the windshield heater is operational (the switch LED is not illuminated) and the duration of operation is
variable depending upon the ambient air temperature, vehicle speed and the amount of time the vehicle has been
travelling.

Seems it does do what it wants anyway and it is able to switch itself on regardless of what it might be showing on the dash.
 
#27 ·
I note that this only applies to male drivers: "...In this instance, no feedback is given to the driver to inform him the rear window heater is operational ..."

In which case, let a woman drive and you'll have resolved the issue altogether...
 
#28 ·
The actual power drain isn't significant surely?

30 Amps (say) at 12V, 360W, is approx. 1/2 HP, not a great deal of fuel burn when the engine is putting out many times that anyway? And the drain is for a few minutes at a time?

You can waste far more fuel than that in one burst of lively acceleration.
 
#29 ·
The actual power drain isn't significant surely?

30 Amps (say) at 12V, 360W, is approx. 1/2 HP, not a great deal of fuel burn when the engine is putting out many times that anyway? And the drain is for a few minutes at a time?

You can waste far more fuel than that in one burst of lively acceleration.
 
#33 ·
I'm happy to let mine do what it want's. I leave the CC on auto all the time.

The only problem with the heated rear window is it ruins the DAB reception so unless it's really needed I turn it off.
 
#34 ·
The only problem with the heated rear window is it ruins the DAB reception so unless it's really needed I turn it off.
Really? I've never noticed that.
Do you have a rear-facing dash-cam though? I know when I tried one it affected the DAB signal.
 
#38 ·
Not quite the same symptoms, but I ended up with a new rear screen to get rid of DAB interference, would not pick up along several stretches of the M4. With or without screen heating on.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top