Aircon hacking - wider range

pprado · Unread post by **pprado** » 14 Jul 2007, 15:27

Hi,

I usually use my aircon on the hottest point before the "burn me" setting.
This is usually not as hot as I would want the car to be kept (I used to live on a beach city).

I thought in "hacking" the temperature sensor by adding maybe a resistance in series or in paralell, but I don´t really know what I would be doing

I know I would loose the precision of the air´s temperature settings, but does anyone know how I should proceed to get a wider range?

It´s ok to loose some of the range on the cold side, if it´s the only way...

wheeler · Unread post by **wheeler** » 14 Jul 2007, 16:16

the hot/cold airflow is controlled by mixer flaps in the heating ducting,when you have the heating at its minimum & maximum settings the flaps are either fully closed or fully open so tampering with the temp sensor wouldnt achive anything apart from messing up the climate control.

pprado · Unread post by **pprado** » 14 Jul 2007, 18:57

wheeler wrote:the hot/cold airflow is controlled by mixer flaps in the heating ducting,when you have the heating at its minimum & maximum settings the flaps are either fully closed or fully open so tampering with the temp sensor wouldnt achive anything apart from messing up the climate control.

Hi, yes - but that is for the maximum and minimum. I would like to have more degrees before I hit the maximum - say, able to regulate to 30 degrees for example.

Unread post by **Peter.N.** » 14 Jul 2007, 20:35

I think I know what you are saying, on the XM the highest regulatable temperature is 26C, beyond that it goes onto maximum heat. I would like to have a higher regulated setting on mine but I am afraid unless you have a circuit diagram of the control unit and are fairly well into electronics, I wouldn't attempt it. I am a TV engineer and I wouldn't even try without a circuit.

Unread post by **xantia_v6** » 14 Jul 2007, 21:31

I think that there is another way to acheive what you want.

The in car temperature sensor is (I beleive) a simple thermistor, the resistance curve is either published, or easily measured.

By adding resistors (series of parallel) to the thermistor, the control system will try to bring the in-car temperature to what it thinks is the correct temperature, but that will be offset by the resistance added to or subtracted from the thermistor.

Do you get the idea?

pprado · Unread post by **pprado** » 14 Jul 2007, 22:17

Thanks for the replies...

Peter, my idea is exactly what Xantia_V6 said. Only offsetting the readings.

For example - I would measure the element´s resistance at 22C and at 26C. Say that it´s 20 ohms at 22C and 10 ohms at 26C... Then, by adding a 10 ohms resistor in series, when at 26C the car would think it´s 22C and will heat; when at 30C, it will think it´s at 26C and will stabilize at that point.

If someone knows the resistance/temperature curve of the component... I don´t know even how to search for it hehehe

Unread post by **Peter.N.** » 14 Jul 2007, 23:07

Do you know, that had never occurred to me - must be the age!

Its not only simple to do but also to get at. If you connect a meter across it and heat or cool it you should be able to get rough guide. Hand heat should bring it up to about 30C but if you blow a hair drier on it, that should at least give you an idea of its lowest resistance. You could even put a small low value pot across it, then you could calibrate it at will.

pprado · Unread post by **pprado** » 15 Jul 2007, 04:03

Peter.N. wrote:Do you know, that had never occurred to me - must be the age!

I'm not that young so let's talk about other things...

Its not only simple to do but also to get at. If you connect a meter across it and heat or cool it you should be able to get rough guide. Hand heat should bring it up to about 30C but if you blow a hair drier on it, that should at least give you an idea of its lowest resistance. You could even put a small low value pot across it, then you could calibrate it at will.

Yes it seems I'll build the curve

That will be on my list then...

Thanks!

Unread post by **xantia_v6** » 15 Jul 2007, 07:58

In the spirit of hacking...
The document: einsa_Xantia-96 ESPA?OL [ 90.12 Mb ] mentioned in the post: http://www.frenchcarforum.co.uk/forum/v ... hp?t=22201 includes the resistance graphs of all of the sensors. I think that the one you are looking for is at the top of page 338.

MikeT · Unread post by **MikeT** » 15 Jul 2007, 19:09

Having successfully pulled my temperature sensor out today I thought I'd get some rough values for you pprado.

Using the 200K scale I got the following readings....

Touching ice = 29.1 - did see 30.0 at one point
Room temp = 11.5 - very warm day
V.Hot tea = 01.6 - just off boiling

At a guess, I'd say the freezing to boiling range is 0-30 (but not sure if that's x100Kohms?)

I haven't done the maths but perhaps you'll be wanting to add a resistor value of appx 1 to 5 (x100Kohms?) HTH

AndersDK · Unread post by **AndersDK** » 15 Jul 2007, 21:02

Mike, your readings are 30K to 1.6K ohms - on the 200K (max) value range.

That is : your meter will max show 199.9 at full scale when set to the 200K range. The leading "1" on the meter will only show once the value is beyond 99.9
I'd say its a fairly standard 10K NTC resistor. These resistors are designated by their room temp values.

Its worth noting that the ohmic value to temperature is NOT a linear function. But connecting - say - 4.7K ohms i series with the sensor would offset the value enough to make the controller think its colder than it really is.
Further connecting a toggle switch over the series resistor (to shortcircuit it), will make it very simple getting back to standard function : by the flip of a dash mounted switch.

weety · Unread post by **weety** » 15 Jul 2007, 23:35

if you want an easy bodge.....undo the flap mixture control (three screws) manually adjust the car to only run on hot air...

job done....

the flap controller is at the bottom of the air flow system behind the centre part of the dash (see the thread about air con bodge in air con section for my tale of woe)

pprado · Unread post by **pprado** » 16 Jul 2007, 01:17

Hey Mike much thanks for the values! I can now "plan my attack".

AnderSDK - I know what you mean... I am not pushing "n" degrees only, I am modifying the curve. This is really a dumb hack so I am expecting side effects - and will use a trimpot, and probably will also try to put a switch on the console. My Xantia does not have all the toys so I have space...

Weety, I want the system to regulate the temp and not to fix the air temp. I found myself many times trying to adjust the heater for the last possible temperature before the "hair dryer" function is enabled...

weety · Unread post by **weety** » 16 Jul 2007, 01:21

pprado wrote:Hey Mike much thanks for the values! I can now "plan my attack".

AnderSDK - I know what you mean... I am not pushing "n" degrees only, I am modifying the curve. This is really a dumb hack so I am expecting side effects - and will use a trimpot, and probably will also try to put a switch on the console. My Xantia does not have all the toys so I have space...

Weety, I want the system to regulate the temp and not to fix the air temp. I found myself many times trying to adjust the heater for the last possible temperature before the "hair dryer" function is enabled...

fair enough but if you want max temp my way definately gives it

a nice long rod from the flap control and it gives you full manual adjustment (you can tell i have been thinking about this

)

Mandrake · Unread post by **Mandrake** » 16 Jul 2007, 02:04

Have you considered the sensor may just be faulty ? 26 degrees should be plenty hot enough surely ?

Also have you checked the tiny fan in front of the sensor is working ? Without this fan the automatic temperature regulation system doesn't work properly as the sensor is not exposed to changes in cabin air temperature quickly enough.

Regards,
Simon