Huffing and puffing heater blower on Xantia

[charentejohn]

Any idea what causes this ?
Driving along for a while and suddenly the heater which was normally running quietly at the speed set bursts into full power for a second or two then back to normal for a while.

No big deal but sometimes comes as a shock.
Just the slider needs cleaning or is it programmed to do this ?

[MJM]

At a guess I'd say that the small fan that blows air over the cabin temperature sensor is playing up.

[charentejohn]

Thanks for replying, I asked a while ago and got no replies so assumed nobody had any ideas.
I did check this but only just noticed this so apologies...

Where is the fan that does this, somewhere in the dash I assume ?
Sounds reasonable as if air flow stopped the heater would think it had got cold ?
The main blower unit was replaced with a 2nd hand one before I bought the car, about a year ago, so maybe they made a bad connection somewhere.

Now I have a possible culprit I searched for 'cabin temperature sensor' and found other posts, seems a Lexia (which I have but haven't used yet) can test this.
That said my problem isn't hot or cold changes, always blows hot this time of year, but sudden 'flat out' blower fan.

[Mandrake]

If you have a Lexia you'll find a wealth of real-time data from the climate control ECU, I'd definitely suggest hooking it up and testing it. As well as reading all the temperature sensors it will read the control switches so you can confirm they're all working properly.

As for sudden changes in fan speed, I don't have any real suggestions as to the cause, but I have noticed a lot of quirks in the automatic fan speed control of both my current and previous Xantia's... the automatic fan speed control just doesn't seem that reliable or consistent.

Do you have any problems with the manual speed settings changing by themselves ? If they are steady and consistent that would at least rule out the fan itself. (Poor connection to the fan motor, worn brushes etc) I suspect you'll find its only the automatic speed mode that has a mind of its own.

[charentejohn]

Thanks, I hadn't tried the Auto setting as it is on normal at the moment, about 1/3 speed.
Perhaps Auto will produce different results.

It is not the end of the world just makes me jump a bit when it happens. Just floating along down the country lanes and WHOOSH the blower goes to full for 2 secs.
I have a moment of panic then settle down, for the rest of the journey it will do it a few times but it only catches me the first time. It was ok today now that I think.

I will check all connections and see if it helps.
I must try the Lexia just a matter of using it the first time, just good old techno-fear. Then again first time I rebuilt an engine I thought it would never work agin but it did.

[RichardW]

It might be the transistors on the control board going haywire - or it may just be an oddity of the climate system - mine is sometimes unsteady in about mid position (and I've just fitted a new control board) - I don't use Auto anymore as that results in all sorts of odd speeds!