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Old 09-24-2013, 11:32 PM   #1
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Installation of electronic a/c temp control

Dear friends.
I own Ford Focus, ca.2006 or so.
I dislike the dumb a/c system since it has no REAL temperature control. As much as I understand, the only way to control the temperature is by mixing hot air, which is an unacceptable strategy in greening world. This means that the a/c always works if engaged, which eventually freezes us, but more disturbing, it is a terrible waste of energy.
I have a little, electronic temperature controller, with more than necessary features (LAE, MTW11 pt100). I would have liked to make use of it and control the compressor on-time just as commonly dome with many a/c systems.
My intention is to wire the output controller relay to compressor clutch relay.
My questions:
1. Am I right and there is no hidden thermostat somewhere that I can just adjust the temperature to a bit higher?
2. Am I going to do a stupid mistake? Will it damage my a/c system?
3. If, in general, it is not a bad idea and might work, does my idea of hooking it to the clutch relay the best way to achieve my goal, or is there a better way?
4. Where would be the best position to locate the temperature probe?
Thank you answerers in advance, for any hint.
Yossef Gofer


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Old 09-25-2013, 06:27 AM   #2
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Since you wish to do R/D engineering.

You will have to understand 100% your system and obtain ALL Ford manuals
You will also have to study automotive refrigeration completely

On a home/building refrigeration system one cannot start/ stop the compressor whenever , there must be a delay interval after shutting it off.

If it breaks, you get to fix no options for that
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Old 09-25-2013, 09:48 AM   #3
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The simplest solution is to keep the fan on high and set for Recirculate. Run the AC until you're cool, then turn it off but leave the fan running. When you get too warm, turn it back on.
That's essentially what your home AC does. Doesn't make it smarter, IMHO.

Like mikeeshaq says, you have to have an interval (usually 3 minutes or so) between shut-down and restart or your compressor will have a very short life on a home system. Not sure but I'm guessing that's why there's no automatic cycling of the compressor in a car--the intervals would be too short for the system and too long for comfort.
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Old 09-25-2013, 03:08 PM   #4
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Since I think you are determined to build your own climate control system, let me see if I can help.

Cycling your compressor clutch is the most common way to control the cabin temperature other than using heat to compensate for the A/C being too cold.

Two options for locating the temperature sensor would be (1) the evaporator coil, or (2) the return air going into the evaporator coil.

You would need to have enough hysteresis built into your design to allow the compressor's high side pressure to come down before re-engaging the compressor clutch. The hysteresis would be a combination of several things: the mass of the sensor, the location (air vs. coil), and the settings of the controller.

Take some pressure readings to see how your compressor is operating normally, and then how it is operating with your controller. As long as you are not short cycling the compressor with high pressures, it is unlikely you will damage it.

One more thing, put a diode (i.e. 1N4004) across the clutch...it has quite an inductive kick when it disengages. Anode goes to ground.
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Old 09-25-2013, 03:19 PM   #5
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Quote:
Originally Posted by FocusKnot View Post
Two options for locating the temperature sensor would be (1) the evaporator coil, or (2) the return air going into the evaporator coil.
If he were to control compressor cycling using a sensor for the return air, how would that work with recirculation turned off?
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Old 09-25-2013, 03:28 PM   #6
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His emphasis seems to be on conserving energy, so recirc mode would be the best way to do that. Otherwise, he could put a small hole located before the blower and sample the cabin air temp even when not using recirc mode.
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Old 09-25-2013, 04:57 PM   #7
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I toyed with this idea previously on a Buick and also on my Focus.

I was just going to add a thermostatic controlled switch to do the equivalent of pushing the AC button on/off. It would be in series with the AC button.

That's low voltage, low current; why go to the power side of the system instead of the control side?
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Old 09-25-2013, 08:33 PM   #8
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Going 'green' on a system that dumps its hydrocarbon achieved load every time the car door is opened.

Or, LOL there....................

If you take over clutch switching with a controller, you will be switching a/c on/off without PCM control, I can see idle bumpup issues there when sitting at a light. The idle could get jerky since the IAC not brought to the party.
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Old 09-26-2013, 08:53 AM   #9
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Why not just adjust the fan speed rating while leaving it on max cool or heat? The A/C system uses High and low pressure sensors to determine clutch engagement. If you lower the fan speed, then the condenser will not drop in temp meaning the high pressure side will stay at a higher pressure and the clutch for the AC compressor will not turn on till needed.

That is why when you have the A/C on with a low fan speed in a mild climate it cycles a lot less and is on for less time. where in the middle the heat when you have MAX A/C its on basically for 90% duty cycle or higher.

Just lower the fan speed appropriately. I would work with controlling temp that way before touching the control of the A/C system.

If you tie directly to the clutch then you are ignoring the high and low pressure switches and can cause major damage to the compressor and create a possible dangerous environment.
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Old 09-26-2013, 09:19 AM   #10
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Quote:
Originally Posted by Magus2727 View Post
If you tie directly to the clutch then you are ignoring the high and low pressure switches and can cause major damage to the compressor and create a possible dangerous environment.
If Yoseff installs his controller in the existing clutch wiring, all the other controls remain in place and the system is fully protected.

Yoseff, you are receiving many suggestions here. What ends up working well for you may be a matter of trial and error. Designing climate control systems for cars is not easy...I did it professionally for many years. But, in those days we often controlled the clutch, heater, and blower to achieve the desired results. Conserving fuel was not something the manufacturers had us consider.
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