OK, here's my AC mod...
*** DISCLAIMER *** -- I did this for myself, with the understanding that issues could arise. I am not suggesting one way or the other that anyone else should attempt it. There was a long discussion about the pros/cons/ and issues that could arise in the Auto A/C forums, which you can read here:
http://www.autoacforum.com/messageview.cfm?catid=2&threadid=15144There was mention this could be dangerous... Read that thread carefully. Obviously, this mod can void a portion of your warranty.
If you choose to do something similar,
you are totally responsible for any issues that arise, or any kind of damages. I spent time to research this, I bought A set of manifold gauges to take readings, etc. and I feel confident that this will not be a problem for me, but again, I am showing this for reference only,
because I can in no way be responsible for anyone else..
<paranoia off>
It was 99F out today. had my car in the sun, but with the sunshield up in the windshield, and front windows down an inch. Got in at noon and turned the AC on 3. With the AC mod, I had 46F air blowing in about 2 min., and 40F in 4 min. cabin got comfortable (noo cold, but cool) in 10min.
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OK, basically, our system compressor cycling is controlled by a number of things involving special cases, like first starting the car, throttle opening >%55, pressure too high or low, etc,
but the normal operating cycle is controlled by a thermistor mounted on the evaporator face inside the car which measures the air temp blowing past it through the evap. The resistance changes with temperature. The CPU inside the climate control unit senses the resistance, and cycles the AC request signal to the PCM (which controlls the compressor's magnetic clutch) based on the measurement. It is supposed to cycle on at 41F, and the off at 39F per the service highlights manual. At this temperature, the thermistor has a resistance of approx 5.1K ohm.
My goal is to change this resistance curve slightly, so the climate control unit thinks it is not as cold as it really is, thus the compressor on-time is increased. Too much on-time will result in the evap freezing up, which is not desireable. This is the downside to the mod. I am shifting my curve down by ~5F, which I feel comfortable with in hot weather. The mod can be changed to do less than this. I do not feel comfortable doing more than this.
To shift the curve, I am placing a resistor in parallel with the thermistor, so that when the thermistor is saying 37F, the CPU thinks it is 41F, roughly the switchpoint of the unmodified design. This gets about 5F cooler air from the vents for me.
For fuel economy and safety purposes, my mod has a switch to disable it completely, like in moderately warm weather where my current system works pretty well.
In addition, it has a safety timer that automatically disables the mod after a time interval. I built my timer mod such that I can adjust the delay to fine tune it. I am currently using 7 minutes, but as hot as it was today (100F, full sun), I may go back and readjust it for 10 Min. I don't feel comfortable yet with anything longer.
I also am using a switch that has a built-in LED, and my mod has a control output to drive the LED to signal when the mod is on or off, whether from being switched off, or from timing out.
Got your interest yet?
Here's a block diagram of the mod:
The timer circuit is powered on by the switch which starts the timer running. It has a relay that closes to place the resistor in parallel to the thermistor. Once it times-out, the relay opens and things are back to stock operation. My timer has an extra control output that drives the LED in the circuit so I can see when the mod is off or on. I'm not going into a lot of detail about the timer, but it is just a straightforward one-shot with a long delay interval which I can adjust from 3-14 min or so. I currently have it set at 7 Min. I might build a couple more..
Here's a couple of pics of my install:
First, I had to decide where to get power, and where to place the switch. I chose the trim piece next to the lighter, so I had to remove it to tap into the power and to mount the switch. To do this you need to remove the cupholder and move the trim piece around the shifter out of the way to get to the two screws holding it in. Open the console, and then pull up on the back of the cupholder section, and it will come loose. It is held by snaps only. then lift the back of the trim piece around the shifter, lift it up to unsnap it from the console, and pull it back. Then these two screws need to be removed:
Once they are out, the trim piece for the lighter needs to come out. It is a little tricky. Held into place by a snap-in on each side. Partially open the ashtray cover, and pull outward on one side to unsnap it, then do the other side. It takes a little patience.
Here's the back view, and the connectors that need to be removed to free it for drilling for mounting the switch:
Rather than tap the lighter wires for power to the switch and gnd to the module, I decided to build something to insert between the lighter and the plug. To fit into the plug, I took a standard .25" spade terminal and cut it in half. Not real pretty, but it works. The female sides are unmodified 3/16" terminals that will attach onto the lighter body. I added a fuse link for safety with a 0.5A fuse.:
Now it gets inserted between the plug and the lighter body. The orange plug wire is +12, the black wire is ground :
In these two pics, I already have the switch mounted, and the wires from the module run inside the center console to the trim piece. You can see how it is tied down with the wire ties to keep the connections from coming loose from the plug. :
The switch I bought has three terminals. The top is the fused +12, the center is switched 12v to the module, and the bottom is for driving the switch's LED, which the module does with the green wire. You can also see the black ground wire to the module coming from the lighter plug tap.
Here's a pic of my timer module. The passenger side kick panel has been removed by pulling out the single plastic "rivet clip" holding it in. I also have the cover to the PJB (fuse box) removed. You can see the wires for power, ground, and LED control are routed behind the side of the console to the switch area. It is only a foot or so away, and is easy to see how to route them when you are looking into the space where th lighter trim piece came out. The twisted pair of orange and black wires goes up to the connector for the thermistor.
The two white dots on the module are for the delay and LED brightness adjustment. I fuond that I needed to adjust the switch's LED brightness to match the AC control's green indicator light brightness. The timer delay can be adjusted as well, but once I find the right setting, I'm going to leave it there.
The ends of the twisted pair have a set of molex terminals (spring clips) on them so I can insert them into the back of the connector for the thermistor. That way, no permanent tapping is required:
These ends slide into the connector slots for the red and white thermistor wires. I just push them in behind the connector wires to make contact:
to keep them from wiggling loose, I cable-tied them to the slot the kick panel goes into. Make sure the head of the tie is out of the way so the kick panel end can seat properly:
Now I just laid the extra wire behind the carpet, and placed the module behind it as well. There is kind of a padded opening it can sit in:
Done!
Anyway, with the switch off, after running the AC yesterday in 95F, my vent air temps stabilized at 43F. With the switch on, they stabilized at 38F!
Today at 100F outside it was 46F off, 40F on. You can tell the difference in less than 30sec when you turn it on...
From the design, you can see I chose a 36K resistor. If a larger resistor was chosen I might not have needed the timer, maybe something like 51 or 56K. It wouldn't get as cool, but it might not need timer protection. I would still keep the switch though, because once it cools down, I don't want the compressor running any longer than it needs to to remain cool. Or if it did freeze, it could be disabled.
I built a couple extra timer units.
P>S>, my car has standard tint sides and back. Not heavy tint, just light, but it also make a big difference..
Anyway, there it is...
Poll
