[santosh.s]

Here is some "official" information from MUL (tips in a feedback form) which confirms that there is absolutely nothing wrong in assumptions to start with... most relevant points are encircled

[rohitbagai]

While the AC relies on the compressor which is driven by the Engine the Heating element/coil derives its huge power requirements from the battery.

Hey Bro...Heter does not need HUGE POWER because it just uses engine coolant to send hot air throught the vent. It does not have any heating coil/element

[ritik123]

I wrote in to Maruti and this is the reply I got.

Dear Sir,

To get the maximum cooling you should keep the slider at blue position. It will not effect the working of the compressor. Movement of slider allows the heater to work and send the warm air inside the cabin and thus varying the temperature of the cabin.

With Regards
Ankur

Now my next question to them is "What affects cut off of the compressor". Will keep you guys posted.

[S.H.Shankar]

Hi guys, this is a very interesting thread. I knew next to nothing about car air-conditioning systems and I have learned one basic thing here. That is, to reduce the cooling in any way, car HVACs mix hot air with the cold air from the compressor! Well, this is a very inefficient way of doing things, but I am sure as someone posted above, there are reasons.

So now if the question is how to control the cooling of the car without tampering with complex plumbing, can I ask y'all a question? You see, right now, the mechanical thermostat which controls the compressor, cuts out electrical power to the clutch (inside the compressor) at 4 degrees and thus controls the cooling. So, what if you add an additional, electrical sensor based, relay in series with the line from the thermostat?

Set the existing HVAC control all the way to the blue end. Then rig up this system which will just be a PRT based sensor (PT100 is very common and very reliable at these temperatures, and it is cheap) connected to a comparator or schmitt trigger, driving a relay, which will be connected into the line that goes from the existing thermostat to the clutch. You will now have two relays in series on this line, the first one which is the original which is controlled by the thermostat and then after that another one controlled by our circuit. That is no problem at all.

I do not know if this is what you are looking for, but I believe that this will work, provided all the other facts mentioned above regarding the existing controls are correct.

I do not know how to post pictures here or else I could have given you a simplified wiring diagram.

[santosh.s]

Nice to see that someone has reopened almost 2 year old thread, and found it "very" interesting! By now, my car is more than 2.5 years old... and I am used to manually switch the A/C on and off every now and then! (Though it still irritates me)

I waited for response from Varun Roy regarding service manual for some time, but later almost gave up. Didn't want to be overconfident and take chance by trying to open complicated dash board and messing up with A/C. I knew that if something went wrong, I would not only face a big pinch in the pocket, but also a lot of headache and time wastage!

Anyways, it's never too late! The good news is that I did get a chance recently to download a soft copy of baleno's service manual (thanks to team-bhp, thanks to internet!). As expected, it has ample technical details, very useful for DIY enthusiasts. Now I think I really understand how baleno A/C works. Most of my initial assumptions and conclusions so far from this thread seem to be correct. One exception is regarding how hot and cold air is mixed- I used to think that after selecting re-circulated or fresh air, the flow might be bifurcating into hot and cold streams as per the "mixer" (temperature lever) setting, and then it might be recombined. In reality, however, all the air is cooled by A/C evaporator, and later only a fraction of this "cold" air is routed through heater core (depending on the setting). Probably that is the standard in car HVAC systems today (my guess based on some other readings on the net).

Regarding temperature sensor, let me correct Shankar- mechanical thermostat seems to be the standard practice for "engine" cooling system, while for A/C it is mostly electrical sensor only. As far as baleno is concerned, it is a -ve temperature coefficient type thermistor. The service manual even provides it's resistance versus temperature curve, which is really cool info! The compressor is cut off at about 2.5 degC and cut in at 4 degC. Additionally, the compressor control mechanism uses following other interesting criteria as well:
1. cuts it off at too low and too high engine RPM.
2. cuts it off under sudden acceleration (more than 80% of accelerator pedal open for more than a few seconds or something like that).
3. cuts it off if A/C coolant pressure is too low or too high.
4. cuts it off during engine starting.

Based on this info, I think it is far...far more easy to add desired temperature control than what Shankar has suggested. All that we need to do is connect a variable resistor pot of appropriate resistance range in series with the existing thermistor... that's all! I have already done some groundwork- on last Saturday, opened the center console, disconnected A/C and mode control switches, lighter etc., and located the thermistor. Confirmed that it is thermistor indeed, by disconnecting it's wire coupler and measuring the resistance value at normal temperature. As expected it was around 2Kohm. More importantly assembled everything back with absolutely no damage to anything whatsoever! (appreciate myself). Talked to a couple of DIY electronics friends regarding how to identify and where to get a good reliable pot, since I don't have prior experience in this field. This is what I need- a liner pot, 5-8Kohm max would be best suited. Linearity, power handling capacity and accuracy shouldn't be really important but it should be reliable enough in long run and it's temperature coefficient should be good, I think. I am also thinking of providing a switch to bypass the pot. This will enable me to quickly switch back and forth between coolest mode and "comfortable" setting without touching the pot at all. A/C is frequently required to be initially run in full blast, when you first enter the cabin on a hot sunny day. This bypass switch will thus reduce wear and tear of the pot to a great extent.

Also talked to a Maruti dealer regarding getting an exactly the same wire coupler, with which I won't need to cut even a single existing wire to make these modifications. But he said that he can't provide the coupler alone, and that I would need to buy another thermistor itself! Instead, he suggested me to visit JC road and try my luck to find a matching coupler. Frankly, I can't wait to finish the planned modification... hopefully by the next weekend it may be done. Will post the outcome here, of course... and if all goes well and successful, then I should change the name of this thread to some thing like - "Poor man's climate control" or something like that

BTW, I had seen a reference to some cheapo temperature control on the very same forum by a member called "jaggu". Tried to find more details on that, but again gave up due to lack of enough information. For all we know, may be it is the same as what I have planned for!

[S.H.Shankar]

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.

[santosh.s]

Well, opening an old thread is not a crime, so it is not to be regretted anyway

As I mentioned earlier, the thermistor is NTC type, so I suppose it has to be a series pot connection. Visited SP/JC road shops on Saturday and bought a pot/knob/wires/push button (to bypass the pot, as explained earlier) etc. A friend recommended a wire-bound type of pot by a company called "Pankaj". I was told that it would one of the best in terms of reliability/accuracy/temp. coeff. etc. Both of us also figured out where to mount them. Though there is no space at all on the center console, there are many other dummy button's where new switches/knobs can be easily mounted. However, most of them are located near parking brake lever, which I don't find convenient for the purpose. Buttons near headlight leveling dial are too small to accommodate the pot (it is relatively big as compared to normal carbon pots). So, finally decided to mount them directly on the dashboard, on the left side near steering column where the dash surface kind of dips inwards (makes a concave shape). It would need a couple of drills in the dashboard, but I don't really mind.

The biggest problem now is that I had a hard time getting an exact match for the existing thermistor wire coupler (As could be expected!). I am confused as to whether to try once again for some more time and find it, or just to go ahead and cut existing wire and connect another different coupler where the additional components can be hooked. Really worried about damaging something while working on cutting and joining stuff, because existing coupler location is odd (deep inside dash) and therefore it is hard to do. I am also wondering whether I should buy a new thermistor itself just to make use of it's coupler alone (hope it has both male/female connectors!)... but forgot to inquire about the cost when I met a Maruti dealer last time.

Does anybody have an idea- how much could be the cost of baleno A/C thermistor?

Was occupied throughout Sunday, so could not finish the job. BTW, Shankar , I agree with you regarding the difference between your solution and mine. However, what I think is this- even though temp. measurement at cooling coil does not substitute measurement at cabin interior, it should still be indirectly linked to cabin temp. What I mean is, if the cabin is hot, cooling coil temp should fall at a relatively slower rate, and when compressor cuts off, it should rise faster. Exactly opposite if the temp of the cabin itself is low. So, control based on coil temperature should also adapt to outside changes, albeit little loosely. The culprit, as I understand , is that the target temperature is too low by default and I am trying to make it variable. If my gut feeling is right, it should be good enough to make the system comfortable enough. So, I will first try this simpler method, and if needed at all, can think about the second one. In the later case (which seems very unlikely!), instead of putting the new relay in the final compressor clutch relay circuit, I would prefer to hack the A/C on-off switch itself (it's just a close/open electrical switch). It won' t have any high current requirement, and more importantly it won't bypass all built in safety features of the existing HVAC system, in case something goes wrong with the new relay.

Oops... another lengthy post...!

[santosh.s]

Originally Posted by santosh.s

and more importantly it won't bypass all built in safety features of the existing HVAC system, in case something goes wrong with the new relay.

Correcting myself, actually none of the approaches would override existing controls as such, so both should be safe!

[santosh.s]

Some more updates on operation "climate control":
Things are moving at snail's pace, but are progressing nevertheless. No time, even on weekends! I had contacted the Maruti dealer again to inquire about thermistor. Nobody had any idea as to what exactly that is! They referred me to an authorized distributor of Maruti-Suzuki spares. Called the manager, but he didn't know about it too. He asked me for the part number, but I couldn't provide it because neither user's manual nor service manual mention it as such. So, personally visited their office on Saturday (wish my residence was close to SP/JC roads). By then I didn't expect people there to be too familiar with the word "thermistor". No issues though, I was glad to know that they have a very detailed catalog for every model which lists all the spare parts with not only numbers but neat diagrams showing smallest of parts like "o-ring" and "clamp". It wasn't difficult to locate thermistor there. But all in vain, they didn't have stock, would take 10 days if I order and the cost is Rs. 750/-. I felt like that was too much to pay just for a stupid wire connector. Moreover, as per my suspicion, it doesn't come with both male and female connectors! The other part would come with a circuit on the other side (manual calls it "AC amplifier", which is in turn hooked to ECM).

Therefore, I finally gave up on the idea of getting original thermistor, as well as wire cutting option. Instead thought of keeping original couplers as they are and connecting additional stuff non-intrusively. Never thought that I would go to this extreme, just for that otherwise trivial connector. Why not, I still have another year and a half of warranty period left (extended warranty). And in the unlikely event of me having to claim it, I didn't want to crate ready excuses! Managed to get all required things to do this. Today I even tested it- works fine, just as per the expectation! So, went ahead and drilled holes to mount the pot and the button. Looking forward for another few hours of spare time when I plan to do final installation.

[santosh.s]

Well, I have finally finished the job today. Attaching a few snaps to show how it looks, before and after installation. Initial impression is that it is working like a breeze! No more fiddling with AC button, and no more discomfort of shivering/sweating alternately in spite of that!!

[deepakvrao]

I'll be frank in saying that I did not go thru all the posts but I got the gist that the idea is for the compressor to switch on/off to maintain the cabin temp rather than having it on all the time and have the cabin temp controlled with mixing warm air at the vents?

If so that is how old ACs in M800s [afterfitted], Ambys etc would work and the problem would be non uniform maintenance of cabin temp.

Mixing warm air albeit not the most economical solution, maintains the in cabin temperature uniformly during the drive. Just my 2 bit of course.

[santosh.s]

Originally Posted by deepakvrao

I'll be frank in saying that I did not go thru all the posts but I got the gist that the idea is for the compressor to switch on/off to maintain the cabin temp rather than having it on all the time and have the cabin temp controlled with mixing warm air at the vents?

Absolutely. "warm" here is quite hot, actually!

If so that is how old ACs in M800s [afterfitted], Ambys etc would work and the problem would be non uniform maintenance of cabin temp.

Mixing warm air albeit not the most economical solution, maintains the in cabin temperature uniformly during the drive. Just my 2 bit of course.

I didn't get this part. I suppose by non-uniform you meant different temperature at different portions of the cabin(?). Can you explain why hot air mixing would do a better job at that? (I did this little mod because I thought it is exactly the opposite, and that it will not distract me while driving by eliminating the need to switch it on and off. Economy of fuel is a bonus).

Let me elaborate further- I wasn't happy with the use of "mixer" to set a temperature because:
- When I set it at the most comfortable level, it feels like alternate blows of hot and cold air.
- When the above is not noticeable, then it doesn't have much impact on the overall temperature either!
- Setting this lever at a desired position is always tough since it is very sensitive in the middle (where the comfortable setting usually belongs!) and on the other hand very less effective towards either sides.
- It needs frequent adjustments when outside whether/engine temperature varies. Not only that, even any change in blower speed also affects it's response.
Overall, I found it too crude.

I guess (I have to always say "guess" because I am not professionally associated with making or servicing cars) all cars have hot air mixing mechanism because it is essential for controlling heater in the fist place. So, manufacturers may not want to raise cost by providing additional (better) mechanism for AC alone. On the other hand, if you don't have a heater, then using temp. sensor would be the simplest method for AC alone systems. Do after market ACs come fitted with a heater? (guess not)

[deepakvrao]

I didn't get this part. I suppose by non-uniform you meant different temperature at different portions of the cabin(?). Can you explain why hot air mixing would do a better job at that? (I did this little mod because I thought it is exactly the opposite, and that it will not distract me while driving by eliminating the need to switch it on and off. Economy of fuel is a bonus).

No - I am not referring to different locations of the cabin.

What I mean is that if you set the temp to be maintained at say 21*C then the compressor goes off when the cabin reaches 21 but does not come on again till it reaches say 23. This is what I have seen practically in the cars which have a thermostat to switch on/off the compressor. So you dont get an even 21 but a temp that fluctuates between say 21-23/24.

Same happens if a window AC is installed too low in the room - compressor goes off, room gets quite a bit warmer and then only does the compressor switch back on again.

[santosh.s]

Temperature deviation during on-off control system would mainly depend upon two thing, IMHO. First is the accuracy/reliability of the sensor itself and the extent of hysteresis in the system. Some amount of hysteresis is a must to avoid switching too fast, and high amount of hysteresis would increase temp deviation as well. In my experience, with the mixer this deviation (of air being thrown out of outlets) is quite high. Remember, compressor would still switch on and off to avoid freezing. Now it is much better. If compressor happens to be *continuously* on, and never freezes, then what you say may be applicable (that is mixer may do a decent job).

As per the service manual of my car, by default compressor switches on at 4 degC and off at 2.5 degC. So, it is +/- 0.75 degC, which is quite good I think. Looking at expected response of their thermistor, this deviation should increase slowly until about 25 degC, beyond which it may rise faster (but that would be less of a concern. Why would I want to set the temp at 35 degC)

[S.H.Shankar]

Santosh, I am sorry I could not reply sooner to you since I was away on tour. I am glad that you got so much accomplished.

Yes, Pankaj indeed is a good make of wire wound pot (at least it was during the days when I used to dabble in electronics as a hobby) and it is a good choice as far as reliability is concerned. Carbon potentiometers wear out pretty fast and can be a real headache in this application. So no sweat about that. Cermet pots are also available nowadays in India (try RS, they should have them) in case you want a more expensive option. However I dont believe your application warrants their use, the Pankaj should be the best choice for you.

Now could you find out the value of the NTC? What value of potentiometer did you use?

Anyway congratulations on a great job accomplished!