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Old 28th July 2008, 08:52   #46
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I think I neither need to measure NTC value nor to go for a better pot. After a week of it's use, my initial impressions continue to hold true, and I think it will be that way forever. I have set it to a setting which suits me best. Generally it is never required to be changed because it fairly adapts to changing ambient temp. It still gets "little" uncomfortable (as opposed to "quite" earlier) if the outside temp varies drastically. But then I have two short-cuts- switch it off when it starts to feel cold (it happens when A/C is not needed anyway!) and push the new button to put A/C into full blast mode when it happens to be parked under scorching sun. I feel like it's climate controlled A/C, well... almost!

Regarding pot value, I have used 5K. The NTC characteristic in manual showed about 6.5K when it switches the compressor and about 2K at 25degC. Beyond that it is not known, but I expect it to become less sensitive after that, because otherwise it would cross zero (superconductor!!), which is not possible I suppose. Right now, at about 90% setting, AC never cuts in, so the useful range is pretty wide which means fine resolution (easy to set). Next available pot value was 10K, I would ideally want 6.5-7K.

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Originally Posted by S.H.Shankar View Post
Anyway congratulations on a great job accomplished!
Ha ha... thanks for your compliment. But if you ask guys who replace engines and gearboxes, I think this would not even qualify as a mod I guess they would rather dream about improving quarter mile, 0-100 timing and the thrust in back alone, not such petty things!
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Old 29th July 2008, 15:56   #47
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Santosh, the question is not whether what you did involved rocket science but how many people it would benefit. And I believe it would be of extreme interest to far many more people than would be improving 0-100 timing or some such.

And thanks for sharing the whole experience. Some day when I find time I would like to borrow your idea for my Innova, with your permission.

Good luck and please post your next project here when you begin it.
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Old 31st July 2008, 10:12   #48
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Go ahead and use it Shankar, I haven't applied for any patent on that
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Old 21st March 2009, 23:26   #49
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Originally Posted by S.H.Shankar View Post
Thanks for the reply Santhosh. Yes I did reply inadvertently to a 2 year old thread. What happened was, I was directed here from some other thread and I forgot to check how old this thread was. However, no regrets, since this one opened my eyes regarding the functioning of a car HVAC system.

Well, regarding modifications to an existing a/c system, I believe adding a potentiometer to the thermistor (you have to find out if it is a PTC or NTC thermistor and then connect the pot in series or parallel to the thermistor as the case may be) would be one way of doing it, and it would be a simpler method definitely, but what I suggested was a method to convert the system to something very similar to a climate control system. That would necessitate the use of a second temperature measuring element, like a thermistor or PRT as I had posted.

Anyways please use the method that you find most suitable, and post the results please. And take care with the configuration when connecting the pot. Good luck.
Another reply, almost after an year now. Finally, i found what i was looking for. Great that i did a search before creating a new thread today which was to look like

-----
Hi members,

I have a question troubling me for some days now. In cars (with basic AC and heater) we have a temperature control knob. Idealy in an home air conditioner, decreasing the temperature (more cold) makes the thermostat to cut out later thus cooling more and vice-versa. It never controls the air temperature but only adjusts the time compressor runs.

But in cars, how is it done? To my observation in my U-VA, when i move the Temperature control from coldest to less cold, it looks like the temperature does reduce, however the AC compressor is still running (thus consuming the engine power also). Is it that the (hot)coolent, that is used when i turn the knob towards hot position begins to flow in its separate coil thus adjusting the temperature.
----

Now reading through the discussions, i am clear on thoughts. Santosh, How is the DIY climate control going on.
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Old 22nd March 2009, 11:15   #50
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Originally Posted by rapidsnail View Post
Santosh, How is the DIY climate control going on.
Very well. I use it all the time as it gives me a very precise/fine control of temperature as compared to the default "mixer". 95% of the time it is just fixed at a position which turned out to be the most comfortable setting for me. Sometimes when I have to park under Sun, I use the bypass switch for fast initial cooling, then release it to come back to my setting. In extreme hot whether I need too change the setting a bit.

Sometimes I feel like it is better than OEM climate control (except fancy interface and other frills) because I believe it works based on temp sensor somewhere in the cabin. When usually I run the blower at the lowest speed, the cooling process would be very inefficient and it would take a very long time for temp drop to reach to the sensor spot. In that case, OEM system would try to cool it as fast as it can by setting the thermostat to the coolest position. That is not what I want, in my case I am directly cotrolling the source temp, of the air being blown into my face! It would somewhat like having manual transmission versus automatic, isn't it?

Overall, "I am loving it!"

PS: I use the original temp knob (slider in my case) only when defrosting glasses, and very rarely when I need the heater.

Last edited by santosh.s : 22nd March 2009 at 11:18.
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Old 22nd March 2009, 23:23   #51
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Originally Posted by santosh.s View Post
You are right, if you go only by the title... but I hope you might have read the whole 1st post

Let me put it this way- I am not satisfied with the temperature control while using the AC. Because there is considerable fluctuation of temperature as the compressor automatically passes through ON and OFF stages. I am thinking that it is because of heater always being in ON state. If we are able to stop the heater whenever required, then IMHO, the temperature control will be easier, more accurate and most importantly the amount of temperature fluctuation will be considerably reduced.
INCORRECT!

The heater core is "off by default".

When in "max cool" - the A/C compressor is on and you get cold air.

When the knob is moved to the right - the A/C compressor cycles on and off based on a % and when the A/C is off you're getting air from outside or inside that is NOT temperature treated. The heater core is not functional at this point.

When the temperature knob is moved to ~ the 50% position, heat from the heater core is slowly mixed in. In most cars this is an infinitely variable adjustment.

In short - your heater core is sealed off until the temperature knob crosses the ~50% position. If set to the higher 50%, the A/C can still be turned "on" via the switch but I doubt it does anything - anyone know?
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Old 23rd March 2009, 08:19   #52
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Originally Posted by Atlblkz06 View Post
INCORRECT!

The heater core is "off by default".

When in "max cool" - the A/C compressor is on and you get cold air.

When the knob is moved to the right - the A/C compressor cycles on and off based on a % and when the A/C is off you're getting air from outside or inside that is NOT temperature treated. The heater core is not functional at this point.

When the temperature knob is moved to ~ the 50% position, heat from the heater core is slowly mixed in. In most cars this is an infinitely variable adjustment.

In short - your heater core is sealed off until the temperature knob crosses the ~50% position. If set to the higher 50%, the A/C can still be turned "on" via the switch but I doubt it does anything - anyone know?
I had created this thread as a newbie in order to get help. But I ended up facing more opposition and criticism like this one, though a few people really helped. I think it was because of may different (and valid) reasons-

1. When I created the thread, I had almost no idea as to how exactly heating part works and how the temperature control works. Moreover, I had probably described the problem in the most boring/irritating fashion.

2. Many other people who replied didn't have better understanding than mine.

3. There seems to be differences in terms of how heating is achieved in old versus new cars, Indian versus some other parts of the world. I have come across three kinds of heating mechanism being talked about- Electrical heating, Heating using warmed up engine coolant and Other fuel based heating without depending upon engine warm up. I know for sure that my car uses engine heating (Confirmed in service manual as well). I also believe that this applies to majority of other Indian cars. In this case, there is no question of heater being turned ON or OFF because it is ready for use all the time, once engine warms up. Only how much hot air gets used/mixed in airflow is something that is controlled.

4. Similarly there seems to be differences in the way temperature control is achieved. In my case, it is done by using air flow control to varying the amount of air being passed through heater core. It is like this- all the air passes through the cooling coil (A/C), there is no control on it. So, depending upon whether you have switched the A/C ON or OFF, all air gets cooled. A part of this cooled air is passed through heater core depending upon the position of temp control knob. When it is at coolest position, heater is bypassed completely, when it is at the hottest position, all the air passes through heater, and when it is in the middle, only 50% of cooled air passes through the heater while rest 50% bypasses it. A/C is not controlled at all, it has the thermostat only to cut it off when it reaches freezing temperature, irrespective of the temp control knob's setting. Hot air starts mixing as soon as you move away from coolest position, and I had posted another proof for this (a leaflet from Maruti). Again, I believe this is common for Indian cars.

With other heating mechanisms mentioned, there may be a different scheme to implement temperature control. Like what you said, they may be "directly" controlling A/C duty cycle (% time ON) and there may be heater control as well. Also it seems older cars didn't allow A/C to be ON while using heater, but that doesn't seem to be common these days. I am not so sure about methods other than in my car.

All that said, I have done what I wanted to and it is working exactly as per expectations keeping me satisfied. BTW, your comments were based on which car?
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Old 23rd March 2009, 09:16   #53
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I did not mean to sound rude - it was just an objective response. My apologies! I've added my response in bold below.

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Originally Posted by santosh.s View Post
1. When I created the thread, I had almost no idea as to how exactly heating part works and how the temperature control works. Moreover, I had probably described the problem in the most boring/irritating fashion.

Ehh the point is we're all trying to figure things out - nothin boring or irritating here.


2. Many other people who replied didn't have better understanding than mine.



3. There seems to be differences in terms of how heating is achieved in old versus new cars, Indian versus some other parts of the world. I have come across three kinds of heating mechanism being talked about- Electrical heating, Heating using warmed up engine coolant and Other fuel based heating without depending upon engine warm up. I know for sure that my car uses engine heating (Confirmed in service manual as well). I also believe that this applies to majority of other Indian cars. In this case, there is no question of heater being turned ON or OFF because it is ready for use all the time, once engine warms up. Only how much hot air gets used/mixed in airflow is something that is controlled.

I have never seen electric heating in a car, only coolant based heating. I concur with the rest - the heater core itself is always hot.

4. Similarly there seems to be differences in the way temperature control is achieved. In my case, it is done by using air flow control to varying the amount of air being passed through heater core. It is like this- all the air passes through the cooling coil (A/C), there is no control on it. So, depending upon whether you have switched the A/C ON or OFF, all air gets cooled. A part of this cooled air is passed through heater core depending upon the position of temp control knob. When it is at coolest position, heater is bypassed completely, when it is at the hottest position, all the air passes through heater, and when it is in the middle, only 50% of cooled air passes through the heater while rest 50% bypasses it. A/C is not controlled at all, it has the thermostat only to cut it off when it reaches freezing temperature, irrespective of the temp control knob's setting. Hot air starts mixing as soon as you move away from coolest position, and I had posted another proof for this (a leaflet from Maruti). Again, I believe this is common for Indian cars.

From what I've seen, when you reduce the temperature on the "cool" side - the A/C cycles on and off but only mixes in with COOL/UNHEATED air. The heater core seems to be close until 50% from my experience. However this could very well be different with other cars.


With other heating mechanisms mentioned, there may be a different scheme to implement temperature control. Like what you said, they may be "directly" controlling A/C duty cycle (% time ON) and there may be heater control as well. Also it seems older cars didn't allow A/C to be ON while using heater, but that doesn't seem to be common these days. I am not so sure about methods other than in my car.

I see what you mean - instead of cycling the A/C, they just add hot air in various amounts to cold air to achieve the desired temperature. GM automobiles may use this method - but I haven't been able to tell. With a V8 its much harder to tell when the A/C kicks in compared to a 4 cylinder. I think I'll look under the hood and report back on this one.


All that said, I have done what I wanted to and it is working exactly as per expectations keeping me satisfied. BTW, your comments were based on which car?

old 1988 VW Jetta and a 95 Nissan. I have a late model GM car as well - I'll take a look at how the A/C cycles and let you know how it works.
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Old 23rd March 2009, 09:18   #54
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In short - your heater core is sealed off until the temperature knob crosses the ~50% position. If set to the higher 50%, the A/C can still be turned "on" via the switch but I doubt it does anything - anyone know?
Heater core sealed off. What does this mean?
Coolant is still flowing through it, as it does at ALL times. It's the airflow you mean.

Most cars have bi-level conditioning, wherein cold air is directed to the windscreen (for defogging) and heated air to the footwells.
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Old 23rd March 2009, 11:36   #55
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Heater core sealed off. What does this mean?
Coolant is still flowing through it, as it does at ALL times. It's the airflow you mean.

Most cars have bi-level conditioning, wherein cold air is directed to the windscreen (for defogging) and heated air to the footwells.
Yes I was talking about the airflow. The cars I have messed with have all been 10 years + old and did not have any fancy multi-zone control.
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Old 23rd March 2009, 12:29   #56
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I did not mean to sound rude - it was just an objective response. My apologies! I've added my response in bold below.
Haha... I need to repeat that line too.

There was a bit of sarcasm involved in my comment #2, as you have understood!

Regarding hot air not being mixed until you move to 50% position of the knob, if it is based on your "observation" then in all likelihood it might be actually getting mixed but you were not able to notice it easily. In my case, it appears almost like that, but careful observation reveals that mixing starts immediately. (Try switching the AC OFF, and it may be easier to see). When it comes near middle position, it becomes extremely sensitive- just a small notch more or less makes a big difference. When it goes to the other side, then it again looses it's sensitivity. It is like all-cool on left side, all-hot on the right side, and quite a narrow region in the middle where the transition from cold to hot takes place. Obviously, if you really want to control the temperature accurately, you end up in that sensitive area where the controlling ability is bad to say the list. It changes if you change blower speed, it changes if outside temp changes etc. That was exactly why I didn't like it.

What I have now is FAR better. It is very robust, precise and insensitive to outside temperature since the control is based on a fixed temp being measured by the thermostat. So, it automatically adapts to changing environment. Only thing is that the result varies to a small extent in extreme temperatures, and I think it is on the account of overall efficiency. What I mean is- AC produces air at a set temp., but in very hot conditions there would be more heat loss ("gain" to be precise!) before it reaches my body. In such cases, I just slightly change the setting, but as I have said many times by now- it is much better than OEM knob (slider).
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Old 23rd March 2009, 13:35   #57
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Santosh,

Interesting. In your over 2 year long post, you have well explained (and got others to explain) car HVAC working. What really impressed me is your custom job in your Baleno that you are also happy with!

In 2006, when I bought my first Santro, I was disappointed to see that the compressor was practically never cutting off as long as the A/C switch was on. I too initially thought that my thermostat was perhaps faulty since I was expecting it to behave like a A/C or fridge, cutting off the compressor as and when the desired temp is reached.

Did a lot of reading and understood the same things you and others have explained in this thread. Later, when I read the Santro manual again line by line to figure out how Hyundai wants me to operate the A/C, I found that they too meant the same thing, although they did not bother to explain so much in detail. The manual did say that you must switch on the A/C when you want cool air, or you want to defog the windshield. It also said that you can use the temp slider towards red if you want hotter air to suit your temp setting. It never mentioned about the two things together. The compressor runs as long as the switch is on, until it reaches the safety cut off of 4C or some such value.

Many have wondered in the initial posts in your thread as to why the inefficient hot and cold air mixing is done in cars. Here is one more reason why the 'inefficient' compressor on and hot air mixing is used. There are situations when and locations where you need warmth in the cabin, but also need the dehumidifying action of the compressor (in general and more specifically to defog the windshield). Then the compressor runs, and hot air is mixed as required.

In my Verna CRDI, which has automatic temperature control, I have observed its working over some time now. It of course does everything for me, and works wonderfully well. It not only senses inside temp and humidity, it also senses if there is direct sunlight. It auto adjusts the mixture of hot / cool air, decides whether to use recirculation / fresh air, decides on the blower speed, and also decides when to cut off the compressor. On cold Bangalore mornings, on many days when I switched on the ATC set at 22C, comp was off and hot air was blown in.

Warm regards,
Sandeep Menon
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Old 23rd March 2009, 13:59   #58
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There are situations when and locations where you need warmth in the cabin, but also need the dehumidifying action of the compressor (in general and more specifically to defog the windshield). Then the compressor runs, and hot air is mixed as required.
Correct, except for one little detail!
Hot air is not 'mixed' in this case; it is routed to the footwells and the cold air is sent to the windshield vents. BUT, you must set the knob correctly for this bi-level air conditioning.
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Old 23rd March 2009, 14:06   #59
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Correct, except for one little detail!
Hot air is not 'mixed' in this case; it is routed to the footwells and the cold air is sent to the windshield vents. BUT, you must set the knob correctly for this bi-level air conditioning.
True. But since most people do not change the selector to the bi - level setting (even a Maruti 800 has this setting), and the selector remains at the 'face level' vents, what eventually happens is a mixing.

In automatic temp control, the system automatically selects bi - level, and I can feel hot and cool air vented out separately from bottom and top vents respectively.
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