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Climate Control Operation

MickhCz on Mon September 28, 2009 11:53 PM User is offline

Year: 2003
Make: Honda
Model: Accord
Engine Size: 3l
Refrigerant Type: R-143a
Country of Origin: Australia

Hi, due to the fact that Honda had incorporated the HVAC controller into the same board as the large screen display, I will be forced to try and either separate the HVAC controller part of the board from the display or find another way to control the HVAC.

Background – Industrial and process control hardware designer and micro controller programmer. Programming/building a new HVAC controller is easy for me and I intend to drive the old control board on a test bed and duplicate all sensor, timing and IO values.

In the mean time I am gathering as much information as possible on the operation of the HVAC system. My question is about the logic behind cycling the compressor from the point of fuel and cooling efficiency.
Keeping in mind that (in Honda) the Climate control board only requests that the compressor be activated. The ECU looks at various conditions and has final control of the fans and compressor clutch.

I have read that (in simplified terms) some units appear to run the compressor to just short of ice up and output air temp is controlled by air mix. I am sure that his is not the case?
1. Is there a limit to how often a given compressor can cycle per minute?
2. Is the compressor always run to near ice up or can the cycle be intelligently controlled to adapt to current weather conditions (within in limits).

Sorry for the long post. Hope it makes sense.

NickD on Tue September 29, 2009 5:35 AM User is offline

Is this what your climate control looks like?

Or is it like this one?

Strayed away from Honda, my local dealers are crooks, and got tired of driving 400 miles to a decent one just to buy parts. Feel they have a false resale value asking $16,000 for a vehicle with over 130,000 miles on it and they rust out just like any other car, but that is just me. They also wear out, brakes, exhaust systems, blower motors, engine controls, and over 400 bucks for a tiny fuel pump is way too much.

Top photo appears to be a manual system and Honda has used a blend door just like everyone else, coolant valves for regulating the heater temperature is history, heater core is always hot at engine temperature either a manually controlled door directs air from the evaporator directly or bypasses some or all of it through the heater core. In auto systems, a motor drives that door. Any automatic system I have seen gives you means to digitally select the desired interior temperature, not seeing this on the lower photo, but yet it has an auto button. You have to tell me what you have.

Honda's have typically used a TXV system that is self regulating, you are correct in that either climate control system, the controller applies 12 volts to the compressor system, but that circuit can be broken by either a dual function switch that will kill the compressor if the high side pressure falls outside of a roughly 40 to 430 psi range, lower is to shut it down when it gets cold outside, upper to prevent the system from blowing up. Another circuit breaker so to speak is an evaporator temperature controller, shuts down the compressor below about 33*F and switches it back on again at around 39*F mainly to prevent evaporator freeze up.

On older Honda's PCM would kill the compressor if the engine temperature exceeded around 220*F, also if you hit the gas pedal for passing to get more power. Engine idle control is also used, with the compressor on, engine idle rpm would increase by 50 rpm so it would run smoother TPS had to be closed, CAS would tell engine rpm, and it would signal the IAC to regulate engine speed.

My short cut for troubleshooting would be to activate the clutch coil with a remote variable DC power supply directly with the clutch coil vehicle connector removed, gauges installed to check on pressures while monitoring the evaporator temperature. Not sure if Honda is doing this anymore, though it was kind of dumb was the ground wire for the clutch coil was screwed to the compressor case requiring a rather long ground circuit through those corroded aluminum brackets then back to the block. With the remote power supply could quickly check for clutch pull in and drop out voltage, make sure the clutch current would decrease as it warmed up, and also monitor the pressures to be assured the AC system was operating properly with good cooling, proper evaporator temperatures and system pressures. With that under my belt, if the system would not work, then tracing out to find out why the clutch circuit is not getting it's voltage. PCM is first, then the climate control circuitry.

Automatically temperature controlled systems normally use an ambient, solar, and in-car sensor, sure helps to have a vehicle schematic to find out what you have. Radiator fan controls that also serve the condenser are also apart of this circuitry. Find it easier to learn the problem while all this stuff is on the vehicle.

I am more familiar with GM auto climate control systems where the climate control is nothing more than like a remote for your TV set, sending commands to the BCM. I won't even buy a later than 1996 vehicle with our OBD II that stores valuable code in flashram, stupid, stupid, stupid. Have to be a dealer to reflash the ram, can do it now, but would cost me 5,600 bucks for a scanner, and several thousand bucks a year to get the firmware. With our temperature extremes, ambient temperature can corrupt the code and your problem may be you need a reflash. While I thought ATC was great, find it to be a PITA, if driving by a manure spread field or a paper mill, have no control over recir, with a manual system a quick flick of the wrist gets me in recir mode, and not that inconvenient to adjust the temperature, strapped in the seat anyway with those controls inches away. And when you have problems, can go crazy with an ATC system, that instantly wipes out any convenience.

Asked this question many times without an answer, is Toyota and Honda also using flashram? Nobody seems to know, can buy one and learn for myself. Already on my third upgrade in my Cavalier, programmers get awfully sloppy when it's easy for them to change the code.

Edited: Tue September 29, 2009 at 6:22 AM by NickD

MickhCz on Tue September 29, 2009 5:51 AM User is offline

Agree on dealers, I do all my own work. Cost here is about the same for most other cars at the moment (some good deals). That is almost the same but mine has dual climate control.

MickhCz on Tue September 29, 2009 7:05 AM User is offline

Something strange when I first read your reply, most of it was missing. Just had another look only to find the rest of your text?

So, yes I have the second unit which is dual. At this stage the system is working fine, the problem I have is with the integration (dumb cost cutting) with the Audio Head unit display as this has been a stumbling block with any after-marker upgrade or navigation modification.

As far as recir mode goes there is a button for that. I have the circuits and yes it includes solar, evap, inside and outside temp sensors.
The selected temperature is displayed on the LCD along with the current mode.

You have cleared up a few of the thing that were on my mind with the operation of this system.

I have been searching forums and the net trying to find a deeper explanation of the operation of the Climate control and compressor timing logic but have come up with just simple colour diagrams.

Flash-ram is in almost every micro. However whether the manufacture actually gives you access to it is another story. I doubt that the Climate control could be re-flashed in these vehicles as the control program would not be made available to the service centre. I think they would just replace the board if they screwed up the programming.

Edited: Tue September 29, 2009 at 7:08 AM by MickhCz

NickD on Tue September 29, 2009 7:10 AM User is offline

Dual climate control tends to ignore the people in the back seat, you do have a back seat, right? Do you have a digital readout for your desired preset temperature? What about your flashram, do you have that too?

Wife is from Venezuela, was surprised to see Toyota's and Honda's in our OBD II era with carburetors and zero PCM's. But I was looking for a cheap socket set that runs around ten bucks here, closest price was over a hundred for a made in China set, forget that. And Chavez is crazy, so not a good place to live. Can walk in a grocery store and find the shelves empty, can walk, but have to eat.

GFBrett on Mon October 19, 2009 11:28 PM User is offline

Hey MickhCz,
Interesting project you have going on there, I'm very interested in your plans regarding this.

Reason being, I have an MY07 Impreza STi, and the A/C's ability to remove heat from the cabin on a hot day is at best, rubbish. The decisions it makes by itself are often at odds with what I want it to do, and would like nothing more than to be able to modify it in some way.

For example, as seems to be common these days, the compressor cycles on and off at about 60-70% duty cycle - i.e. on for 25-30 seconds, off for 10. I understand there's most likely a few reasons, being icing, low pressure etc etc, but the end result is an average vent temp of about 8-10 degrees, which is hopelessly inadequate when the ambient is only in the high 20's C.

Secondly, the air mixing has it in it's brain that I want my feet warmer than my head - always. Consequently, selecting head and foot mode automatically mixes in some hot air to the foot vents. Select foot only however and it goes cold. This even occurs if the temp is set as low as possible, and the compressor is on, meaning at all times the foot vents are about 3-4 degrees hotter than the head vents if both are selected.

Both of these situations combined mean that it takes a good 20 minutes of driving with the A/C on flat out to get the cabin temp close to comfortable on a warm to hot day.

The head/foot air mixing I can probably sort out one way or another, but I also would like to know if there are any other reasons for not running the compressor constantly other than ice up. I'm also very interested if you have a way of taking over control of the HVAC.


mk378 on Tue October 20, 2009 9:04 AM User is offline

I think they all work on reheating. If the evaporator is allowed to warm up it doesn't dehumidfy, and the air coming out has a damp mustiness to it. So whenever less than full cooling is required, the evaporator remains at 40F but the air is reheated by the heater.

Brett before tearing into things, simply block off the heater water lines so no reheating can occur. If this solves the problem you can just leave it like that during the hot season, or find the failed part that is allowing reheating. If the compressor still cycles off before the air is in the 4-5 C range, the evaporator temperature sensor may be bad, or it is cycling because of abnormal pressure.

Karl Hofmann on Tue October 20, 2009 2:50 PM User is offlineView users profile

Brett, If your Scooby is only giving your 8 to 10 at the vents then your car has issues...As MK378 has said start looking at the heating water valve of possibly a blend flap not closing off properly

Information for Nick, A Scooby is a nickname for a Subaru, most commonly used when directed at the Impreza ... I'll teach you English bud

Never knock on deaths door... Ring the doorbell and run away, death really hates that!

NickD on Tue October 20, 2009 4:43 PM User is offline

In my neck of the woods, backwoods that is, we call a Subaru a Subaru.

This what a Scooby looks like.

Karl Hofmann on Tue October 20, 2009 5:09 PM User is offlineView users profile

With all due to respect to Brett, but that is what the average Impreza driver looks like.... Perhaps with a Burbery cap on

Never knock on deaths door... Ring the doorbell and run away, death really hates that!

MickhCz on Tue October 20, 2009 7:00 PM User is offline

Originally posted by: GFBrett
Hey MickhCz,

Interesting project you have going on there, I'm very interested in your plans regarding this.

Both of these situations combined mean that it takes a good 20 minutes of driving with the A/C on flat out to get the cabin temp close to comfortable on a warm to hot day.
The head/foot air mixing I can probably sort out one way or another, but I also would like to know if there are any other reasons for not running the compressor constantly other than ice up. I'm also very interested if you have a way of taking over control of the HVAC.

Hi, It sounds like you might have a problem there (wonder if you have compared to other cars of the same model) If I turn mine down low it will freeze my whatever’s off, painful to say the least.
Full auto climate control programming is very complex and almost totally car specific. While it’s possible to program a tuneable/learning generic unit, the sensor interface trimming and amount of tunning would make it unwieldy and way to complex.

If the Aircon is only manual control than that simplifies it a lot, however with the amount of effort put in by most manufactures and considering how long they have been doing this, I would tend to think that you have fault somewhere in the system and the replies so far seem to suggest this.

GFBrett on Tue October 20, 2009 9:50 PM User is offline

Hi everyone, thanks for the feedback.

Of course, Subaru don't believe there is a fault (standard dealer line), I believe it is a poorly designed system. I will take mk378's suggestion and block off the heater core coolant to see how that affects the performance. It will certainly improve the foot vent performance, since I know that is higher because of reheating.

I should have been clearer in my original post - the vent temps go between about 3-4 degrees with the compressor on, then up to about 9-11 when it cuts out, so I don't think there's a problem getting the air temps low, but it seems to be the cycling in and out (and the re-heating) that results in poor performance.

Thanks for the input everyone, I'll play around and see what I can come up with. Give me manual A/C control any day!

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