Fan speed when reversing.

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Jetjockey
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Fan speed when reversing.

Post by Jetjockey » 20 Jun 2019, 01:34

Hello.

I’ve noticed a feature on my recently acquired 2008 C5 exclusive tourer. When selecting reverse gear (automatic), the climate control fan speed increases. :?

Can anybody shed any light on the reasoning behind this?

Thank you. :)

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Stickyfinger
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Re: Fan speed when reversing.

Post by Stickyfinger » 20 Jun 2019, 06:21

Standard feature....to ensure you maintain visibility/demisted apparently.
Last edited by Stickyfinger on 20 Jun 2019, 07:14, edited 1 time in total.

Hell Razor5543
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Re: Fan speed when reversing.

Post by Hell Razor5543 » 20 Jun 2019, 06:53

It is also likely that, while you are reversing, the a/c system switches to recycling the air, so exhaust fumes are not drawn in.

Jetjockey
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Re: Fan speed when reversing.

Post by Jetjockey » 20 Jun 2019, 08:10

Strewth, very impressive, even my last Merc didn’t do that! What other wonders does the car have hidden?
I found the automatic door bin lighting last night.


Thanks.

Hell Razor5543
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Re: Fan speed when reversing.

Post by Hell Razor5543 » 20 Jun 2019, 08:16

Citroen, while they can be quirky, can also come up with so VERY good ideas. For example, the Citroen DS was, AFAIK, the first car with an angled engine bay bulkhead. This meant that, in the case of a heavy front end impact, the engine was deflected under the car. Prior to that cars had a vertical bulkhead, which often resulted in the engine coming into the passenger space in a similar impact.

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GiveMeABreak
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Re: Fan speed when reversing.

Post by GiveMeABreak » 20 Jun 2019, 09:50

Yes it is to prevent Exhaust fumes from entering the cabin as you reverse. The system engages recycled air mode, and the A/C is switched on as the air is recycled - which also stops the demisting as Alasdair notes, and the increase in fan speed is done to prevent misting until forward gear is selected again.
viewtopic.php?f=12&t=58626&hilit=exhaust+fumes#p551114

Richard_C
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Re: Fan speed when reversing.

Post by Richard_C » 20 Jun 2019, 18:11

I've had a car, maybe even the C5 X7 tourer or was it the SAAB, where it goes into recycle mode when you squirt the screenwashers. Seemed a bit pointless really.

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GiveMeABreak
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Re: Fan speed when reversing.

Post by GiveMeABreak » 20 Jun 2019, 19:38

It's all down to the Air Quality Sensor on the Dual Zone Climate system.

When the system is put into automatic comfort mode, the air quality sensor analyses the air. When it detects pollutants, it isolates the exterior airflow to the passenger compartment. So it activates air recirculation mode. Of course it can't detect bad smells - only pollutants - so if you have a passenger snacking on a pickled egg (God forbid), you're out of luck - it's a clothes peg jobby I'm afraid.

And for those that are interested in such things, here is the AQS:
Air Sensor X7 B.PNG
The air quality sensor informs the air conditioning control system about the air quality exterior to the vehicle passenger compartment.
The air quality sensor detects oxidising and reducing gases (NOx and CO) which are the main air pollution agents.

Below is the data graph showing the signal type when it detects CO2 and NOx

The sensory layer of the air quality sensor must be set to a temperature to enable its’ responses.
A heating resistor, integrated into the air quality sensor, sets the sensory layer of the sensor at a temperature to enable the following responses:
  • Reduction of the air quality sensor resistance by reducing gases which set the display loads
  • Increasing the resistance of the sensory layer of the sensor by oxidising gases which "consume" the display loads
Air Sensor X7 A.PNG
Key
  • "T" Time.
  • "V" Battery voltage .
  • (2) High status time.
  • (3) Period .
  • (4) Example of the signal during a sensor failure.
  • (5) Example of the signal during the detection of NO2.
  • (6) Example of the signal during the detection of CO.

    The exit signal of the air quality sensor is a signal type PWM.