Fan speed when reversing.

This is the Forum for all your Citroen Technical Questions, Problems or Advice.

Moderator: RichardW

Post Reply
Jetjockey
Posts: 129
Joined: 02 Jun 2019, 22:15
Location: South Wales
My Cars: Citroen C5
x 3

Fan speed when reversing.

Post by Jetjockey »

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. :)
User avatar
Stickyfinger
(Donor 2016)
Posts: 10411
Joined: 28 Mar 2013, 21:05
Location: Somset my lovleee
My Cars: Xantia V6 ACTIVA 3ltr 24v Manual p1
Xm 2.1TD Ph2 Exclusive
AX, little Daffodil
SAXO White Mk1. Sally
x 1280
Contact:

Re: Fan speed when reversing.

Post by Stickyfinger »

Standard feature....to ensure you maintain visibility/demisted apparently.
Last edited by Stickyfinger on 20 Jun 2019, 07:14, edited 1 time in total.
Alasdair
Activa, the Moose Rider
3x C5x7 Steering racks and counting
Hell Razor5543
Donor 2023
Posts: 13727
Joined: 01 Apr 2012, 09:47
Location: Reading
My Cars: C5 Mk2 VTX+ estate.
x 2993

Re: Fan speed when reversing.

Post by Hell Razor5543 »

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.
James
ex BX 1.9
ex Xantia 2.0HDi SX
ex Xantia 2.0HDi LX
Ex C5 2.0HDi VTR
Ex C5 2.0HDi VTR

C5 2.2HDi VTX+
Yes, I am paranoid, but am I paranoid ENOUGH?
Out amongst the stars, looking for a world of my own!
Jetjockey
Posts: 129
Joined: 02 Jun 2019, 22:15
Location: South Wales
My Cars: Citroen C5
x 3

Re: Fan speed when reversing.

Post by Jetjockey »

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
Donor 2023
Posts: 13727
Joined: 01 Apr 2012, 09:47
Location: Reading
My Cars: C5 Mk2 VTX+ estate.
x 2993

Re: Fan speed when reversing.

Post by Hell Razor5543 »

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.
James
ex BX 1.9
ex Xantia 2.0HDi SX
ex Xantia 2.0HDi LX
Ex C5 2.0HDi VTR
Ex C5 2.0HDi VTR

C5 2.2HDi VTX+
Yes, I am paranoid, but am I paranoid ENOUGH?
Out amongst the stars, looking for a world of my own!
User avatar
GiveMeABreak
Forum Admin Team
Posts: 37005
Joined: 15 Sep 2015, 19:38
Location: West Wales
My Cars: C3 Aircross SUV HDi Flair Peperoncino Red (The Chili Hornet)
C5 X7 2.0 HDi Exclusive Mativoire Beige (The Golden Hornet)
C3 1.6 HDi Exclusive Aluminium Grey (The Silver Hornet)
C5 MK II 2.0 HDi Exclusive Obsidian Black
C5 MK I 2.0 HDi SX Wicked Red
Xantia S2 2.0 HDi SX Hermes Red
C15 Romahome White
XM 2.0 Turbo Prestige Emerald Green Pearlescent
XM 2.0 Turbo Prestige Polar White
XM 2.0 SX Polar White
CX 20 Polar White
GS 1220 Geranium Red
CX 2.4 Prestige C-Matic Nevada Beige
GS 1000 Cedreat Yellow
x 5646

Re: Fan speed when reversing.

Post by GiveMeABreak »

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
Please Don't PM Me For Technical Help

Marc
Richard_C
(Donor 2016)
Posts: 535
Joined: 15 Oct 2011, 17:31
Location: Cambridge, England
My Cars: C4 Picasso 1.6 Blue Hdi 120 Manual Excl+ 2016
C3 1.2 Puretech 110 Auto Flair 2017
x 99

Re: Fan speed when reversing.

Post by Richard_C »

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.
Richard_C

Current: , C4 Picasso 120 BlueHdi, C3 1.2 Auto
Past Citroens: Dyane (x2), 2CV, Visa, BX (x2), Xantia, Xsara Picasso, C3 (x2) C5 X7 Tourer, Synergie 1.9TD, C1
Others: Hillman Hunter, Cortina Mk 1, Maxi, VW Type 2, Granada, SAAB 900, SAAB 9-5, R5 Gordini
User avatar
GiveMeABreak
Forum Admin Team
Posts: 37005
Joined: 15 Sep 2015, 19:38
Location: West Wales
My Cars: C3 Aircross SUV HDi Flair Peperoncino Red (The Chili Hornet)
C5 X7 2.0 HDi Exclusive Mativoire Beige (The Golden Hornet)
C3 1.6 HDi Exclusive Aluminium Grey (The Silver Hornet)
C5 MK II 2.0 HDi Exclusive Obsidian Black
C5 MK I 2.0 HDi SX Wicked Red
Xantia S2 2.0 HDi SX Hermes Red
C15 Romahome White
XM 2.0 Turbo Prestige Emerald Green Pearlescent
XM 2.0 Turbo Prestige Polar White
XM 2.0 SX Polar White
CX 20 Polar White
GS 1220 Geranium Red
CX 2.4 Prestige C-Matic Nevada Beige
GS 1000 Cedreat Yellow
x 5646

Re: Fan speed when reversing.

Post by GiveMeABreak »

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.
Please Don't PM Me For Technical Help

Marc
Post Reply