C5 Heater fan - full on and will not go off.

Gastropod · Post by **Gastropod** » 17 Jan 2006, 22:46

Hi, I have a problem with my 2002 2.2HDi Exclusive SE Estate.

This car is far too clever for its own good! When you start the engine, the heater fan now comes on at full power. Everyting worked properly until a few days ago.

For a few seconds, the aircon lcd display seems to react in the normal way to the fan control rocker switch, then indicates zero power whilst all the time, the fan remains at full blast.

Few other clues as well. I cannot select 'economy' aircon setting but the controls which direct the air to my feet or the screen all seem to work okay. As the controls appear to work normally for a few seconds after starting the engine, I would guess that one of the nineteen brains in the car must have a dose of amnesia! I wonder if this is one of the sensors gone faulty.

I posted a question about this on Yahoo and somebody kindly came back and told me that their Citroen dealer had taken five days to discover it was a wiring short behind the aircon control panel. I have taken the radio out to look and as far as I could see, there was no obvious problem with the wiring. I now need to get the aircon control panel out of the dash but do not want to risk messing up the high tech plastic dash on my luvverly new French's revenge for Agincourt!

My questions are:

1. How do you get the aircon control panel out of the dash? The radio comes out easily but the aircon panel seems to much more tricky to remove.

2. A few other people have posted about this problem, so (like the rear brakes) it is probably a 'stock fault'. Does anybody have a fix for this?

Any info welcomed. Driving to work is deafening but at least I do not get any problems with the windscreen steaming up!

Post by **Peter.N.** » 17 Jan 2006, 23:56

If it has the usual citroen blower motor, the most likely problem is that the speed control transistors have gone short circuit. I did put a list of usable transistors on here a while back and 'clogzz' resurected it a few days ago, so have a look for similar strings further down the list. It also tells you how to change them, if in fact that is the problem.

Gastropod · Post by **Gastropod** » 18 Jan 2006, 09:49

Thanks for this.

There is no Haynes manual as yet, which is a pain! Looking at the little information which I do have (this is the C5 pdf training files which another member kindly posted on Yahoo), it would seem that the speed control of the fan is by means of three relays with resistors associated. By switching different combinations of relays/resistors, it is possible to get eight different power settings for the fan. Is this whay you call progress?

This is certainly how it did function before this problem arose. there is no 'analogue' control of fan speed. It goes in steps.

Thanks for taking the trouble to respond.

dnsey · Post by **dnsey** » 18 Jan 2006, 09:57

By switching different combinations of relays/resistors, it is possible to get eight different power settings

In which case there must be a simple binary encoder of some sort. If I had to guess, I'd start by looking there, as it seems to be stuck at '111'.

Gastropod · Post by **Gastropod** » 18 Jan 2006, 18:04

Yep, thanks. Already sussed the binary implication.

The problem is in the driver for the relays. These are three outputs of one of the nineteen brains in the car. I presume that it has switched all three relays to on.

There is something, somewhere which is asking the brain to switch all three relays on.

I understand that a similar fault was caused by a wiring fault behind the aircon panel.

I hope to goodness that I do not have to replace the brain itself.

Anybody got any experience in using the OBD socket?