Delayed Reaction Auto Heating on Xantia S2

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Delayed Reaction Auto Heating on Xantia S2

Post by ecohouse1 »

Hi

I have noticed that there is often a "delayed reaction" when using the heater in the car when set to Auto.

I used compressed air to blow out the cave that the sensor lives in but it hasn't made any difference. If I turn the fan up to max this seems to "wake it up" and if I then switch back to Auto it is OK. I noticed for example the other day that the car was very warm and when I reduced the temp slider the fan "boosted" up but sleeps when I start the car for a fair few minutes.

Is there any reason for this to happen?

Cheers

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Re: Delayed Reaction Auto Heating on Xantia S2

Post by DickieG »

I've noticed a similar thing, the heater fan operation appears quite random at times.
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Re: Delayed Reaction Auto Heating on Xantia S2

Post by RichardW »

Indeed, mine does all sorts of odd things in Auto - I just leave it on the second fan position, increasing to 3 or 4 if demist required - climate works fine! Although, sometimes the fan speed is not constant..... :?
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Re: Delayed Reaction Auto Heating on Xantia S2

Post by Mandrake »

Mine does this too, it's behaviour is quite different to the Series 1 Xantias. I think some of it is on purpose though. For example if it's a 5 degree morning, the inside of the car is also cold, and you have the temperature set to 22, a series one car would blast you with full fan speed due to the large difference in temperature between selected temperature and current cabin temperature, completely ignoring the fact that it was sending in icy cold outside air because the heater hadn't warmed up yet...all it did to calculate the fan speed is measure the temperature difference between current and desired cabin temperature - the greater the difference the greater the fan speed applied.

Same problem in summer with the air con on - it would blast full fan speed with aircon on even before the aircon had cooled down so you would get a face full of very hot air from the hot air ducts before the aircon could get itself going.

The series 2 seems smarter, it also seems to measure the temperature of the air coming through the vents...and if the air temperature of the vents is not appropriate it will keep the fan speed very low.

So in the early morning scenario despite the cabin being cold and the slider set to 22 it's smart enough to know not to turn up the fan speed until the air coming through the vents actually heats up hotter than the ambient cabin air. I notice the fan starts very slow and a few minutes into the drive as the air starts to warm it speeds up dramatically, I'm pretty sure this is by design.

It doesn't work as well as it might though, I do find that the fan speed on auto is generally insufficient, while in the Series 1 it was sometimes a bit excessive. I also suspect that the random speed fluctuations are the result of an unwanted feedback loop formed by using air vent temperature as an input - for example if the air being warmer in the vent (when trying to increase the cabin temperature) causes the fan to speed up, the faster air flow will tend to cool the heater box and cause the air vent temperature to drop slightly, and slow the fan down again, and so on...

Another difference is that the auto fan speed on the Series 1 was a few fixed speeds that it switched between, similar to the manual speed settings. On the series 2 the auto setting seems to be continuously variable speed, so fluctuations of the sensor readings are likely to cause frequent small variations in the fan speed.
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Post by addo »

I'm of the contrary opinion; fan speed in the Series 2 cars with "climate control" is a series of steps based on sensor outputs vs a lookup table. While the Xantia was designed with option of a servomotor to control air intake choice (cabin vs fresh) it wasn't applied in any version - this would fit in with a more interpretive ECU.

Fan motor speed is lower at start in an aged fan - from my understanding - because of poorer electrical contact from the supply through to brushes/commutator. The AC bloke who is my "go to" when climatisation issues get incomprehensible, is rather down on the thermal cutout in fan speed controller (transistor) modules. He likes to plumb them directly so the thermal cutout can't affect current switching. I can sort of see his point; have yet to hear of a controller module that's cooked the whole deal good and proper.

The other point people overlook, is with demist - a popular choice for mornings in pays froids - it is valuable to have the airflow at full stick and starting close to ambient. The aircon will lightly demoisturise and as the heater core warms, airflow gently warms the glass.
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Re: Delayed Reaction Auto Heating on Xantia S2

Post by Mandrake »

Trust me, the fan speed in the auto mode on a Series 2 (at least mine) is continuously variable, I can tell the difference :wink: I spent some time debugging faults in the Series 1 climate control systems so I have a very good memory of their exact behaviour under different circumstances, and I noticed the different behaviour as soon as I got this Series 2.

It's nothing to do with poor electrial contact of the brushes/commutator, otherwise the manual speeds wouldn't work properly either. What is probably happening with manually setting the speed higher for a short time as the original poster notes is that the increased air flow through the vents more quickly brings the air vent temperature sensor up to the temperature of the air flow. Leave it on auto (initially slow) and it takes a bit longer for that vent sensor to register the temperature of the vent air and increase the fan speed.

I've observed it pretty carefully on cold mornings (and warm ones) and it is definitely measuring the vent air temperature, as it's always at the same point as the air just starts to warm up that the fan speed increases automatically, regardless of how many minutes that took depending on driving style etc.

Regarding demisting - the demist setting has its own separate mode, you'll notice that when you move the slider to demist the fan speed automatically increases quite a bit (again, not as much as a Series 1) regardless of the temperature. This is as you say to help demisting, and because it's not coming out the face vents it doesn't have to worry about blasting you with icy air :lol:

What I was describing with a very slow initial speed in some circumstances doesn't apply in the demist setting.
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Post by addo »

It does come out the face level vents unless you shut them off!

What is the gain in constantly varying fan speeds? I would say it's nil compared to set speed thresholds for such an application, so why would they employ this approach? As someone of a musical bent I do listen to the fan motor "note" on Auto (among the variety of other noises clamouring for attention!) and find that it can always be replicated in one of the manual settings.

Image

Notice anything odd about the above? Not sure if the numbers are legible, but it carries two servomotor drivers. One DIL in the centre, one bottom right. That has me puzzled, unless it hearkens to the unused application.
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Re:

Post by Mandrake »

addo wrote:It does come out the face level vents unless you shut them off!
Yeah only a little bit though. Theres hardly any pressure through the face vents in demist mode compared to face only mode. :)
What is the gain in constantly varying fan speeds? I would say it's nil compared to set speed thresholds for such an application, so why would they employ this approach?
I'm not saying its a good idea, just observing that it happens. On my S1 (and other S1's I've dealt with) the Auto mode was always discrete speeds that match those manually selected, and as the conditions change to require a change in fan speed it would instantly jump from one speed to the next as if you had just switched the manual speeds yourself.

On the S2 it definitely ramps the speed from one to another. Now it's possible that the speed that it stops on matches one of the manually preset speeds, can't say that I have noticed that one way or the other, but there is definitely a smooth slow ramping from one speed to another that takes sometimes over 10 seconds to get there, and quite obvious from the pitch of the fan.

This is particularly noticeable in the cold start scenario (morning outside temperature near or below 0 degrees) that the fan comes on very very slowly for a few minutes and just as the vents start to warm up you hear the fan speed creeping up smoothly over a period of a minute or so as the the heater warms up.
As someone of a musical bent I do listen to the fan motor "note" on Auto (among the variety of other noises clamouring for attention!) and find that it can always be replicated in one of the manual settings.
One thing we haven't considered is the climate control could be different in different markets... My Series 1 was in New Zealand (although was a Japanese import, but my Dad's NZ new S1 Xantia's climate control behaved the same) and my Series 2 is in the UK, we don't know for sure if the Series 2's of all different markets have identical climate control, or whether there was more than one varient in the Series 2. I can only report on what mine does... :)

Also the slow fan for the first few minutes happens mostly when the outside temperature is close to or below zero, depending on where you are in Australia you might not experience those circumstances. :lol:
Image

Notice anything odd about the above? Not sure if the numbers are legible, but it carries two servomotor drivers. One DIL in the centre, one bottom right. That has me puzzled, unless it hearkens to the unused application.
You could be right about the internal/external air mixer flap. It does seem rather bizarre that it has a full climate control system but with a manual air mixer flap... that flap is my nemesis, as the bowden cable operating it stopped working on my previous Xantia soon before I sold it (stuck on fresh air fortunately) and the one on this Xantia is also broken, I suspect stuck part way between fresh and recirculate. :?
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Post by addo »

With the centre height vents on a Xantia, there is no internal shuttering. I think airflow favours the demist when that flap is opened fully due to it being a slightly less tortured route.

Not sure where my recent heater box assembly photo has gone - barely a month since it was in absolute pieces, being cleaned of sooty grime in all the passages. To its credit, the FIRST box I've had from PSA that hadn't warped to the point of binding on its air flaps. This is uaually what busts the servomotor gears and then burns out the op-amp. Overclocker approach was employed on this one:

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Re: Delayed Reaction Auto Heating on Xantia S2

Post by DickieG »

Re the operation of the fan motor on S1 and S2 Xantia's, the ECU design changed around late 96/early 97 which may explain some of the differences.
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