MPG gain - AC bypass

[RegularGuy]

Happened to realize with my 2007, whenever the blower fan is on, the AC compressor cycles.... There are some obvious efficiency improvements to be found here so perhaps someone can help me along here.

Searching the forums, it would appear most modifications involve adding a switch which turns off the compressor. I meanwhile am interested in the compressor cycling only when the system is set to AC or Max AC....

Does anyone know if this is possible? As I understand, the switch controlling the vents, has a single wire going to the computer signalling whether the system is on or off. The vent selection being controlled by vacuum. If my suspicion is correct, the switch lacking a separate signal wire for AC/Max AC would be unable to be used. Can anyone confirm my suspicion here?

My thought would be to add a normally open limit switch, in line with signal wire from the computer to the compressor such that when I turn the knob, it completes the connection allowing the compressor to cycle..... This would end up being wired exactly the same as the aforementioned modification, just triggered in a different way. If a more simplistic solution exists though, I'll take it!

 

[Dirtman]

The purple wire on the right side selector switch is the "command on" for the AC compressor. It just tells the computer it is allowed to use the compressor. It will run in every position except the foward vent position. The AC compressor cycles to dry the air so your windows don't fog even without the AC actually on. You can add a toggle switch to the purple wire to turn the "command on" off when you don't want the compressor to run.

It doesn't help to any noticable amount as far as MPGs... I did it to my 2009 a while ago and have all but given up ever actually turning the switch off.

 

[RegularGuy]

In that case, perhaps the electrical side of things isn't as I read... Are there separate wires back there? One going to the computer, and another completing the fan circuit? If that's the case it may be as simple as modifying the selector itself such that it only sends the "compressor available" signal with the ac on

 

[Dirtman]

Yes there are many wires behind the selector switch. When the selector switch activates the purple wire, that wire simply tells the computer it is "allowed" to use the compressor. When and how long the compressor is on is then up to the computer and the other settings on the switch. But disabling that single wire means the computer is no longer allowed to turn the compressor on in any setting at any time. When I get a chance ill post a pic of the wiring diagram for the selector switch.

You DO NOT want to disable the compressor to only run in AC mode. You still need it in defrost mode as well and sometimes with just the heat on. Like I said, it dries the air so on a cold or rainy morning your really gonna want the compressor running even though your not using AC or you'll have foggy windows.

 

[RegularGuy]

I wouldn't be cutting the wire, instead if I'm understanding correctly, I would be removing the "extra" contacts within the dial.

There may be something I'm missing here (I've probably oversimplified) if this approach doesn't work

 

[cbxer55]

My L is the same way. I heard it's done to keep the compressor lubed properly. It even comes on in the winter with heater use. I kind of hate it. I like driving with the windows down, and just want to use the vents some times to blow some air around, don't want the ac on adding heat to the radiator. On my L, whenever the ac is on, the temp needled moves noticeably higher on the gauge. And that's with a 10 degree cooler thermostat. So, I just avoid using it all together.

My 07 Mustang doesn't do this. It has a button you push if you want the ac on. And an orange light that illuminates when it is depressed.

 

[8thTon]

I modified the HVAC controls to prevent this, now the AC only comes on in the AC modes and defroster. I used a thin piece of Kapton tape on the rotary switch contacts - I had made a post about it at the time but I don't remember what forum that was on. I have the pictures around and I'll see if I can find them. It was sort of a PITA but everything looks completely stock, with no external switches or obvious mods.

 

[8thTon]

It was posted elsewhere, so I'll copy an abridged version here:

The wiring diagrams are likely correct in terms of point-to-point wiring, but what is shown withing the boxes representing switches and other devices is best considered an artistic rendering. It's not always useful for troubleshooting. Mine is a 2004, and I have diagrams for 2002, 2005 and 2008. The 2002 and 2005 don't show the floor/vent position at all and the 2008 shows the AC engaged for all modes but "off". So I pulled mine apart.

On my 2004 the AC clutch is energized for both AC and DEFROST modes, as well as FLOOR/VENT. It is off for FLOOR and VENT, as well as OFF. I thought about disabling the AC for the FLOOR/DEFROST mode but decided to leave that alone, and just disabled it on the FLOOR/VENT mode. Taking the HVAC unit out was pretty easy, but taking the mode control out is not for the faint of heart. Note the detent spring and ball behind it that can easily get lost. Also, there is a sliding vacuum control valve moved by the gear, and that must be phased properly on reassembly.

To disable the AC on FLOOR/VENT I cleaned off that one spar with brake clean and applied some thin Kapton tape over it. That is a tough resistive tape with a good adhesive used in electronics manufacturing, which was suitable for this, but there are probably other tapes or ways to insulate it. Now when the wiper hits that spot it will not make contact and the AC won't turn on. I reapplied some silicone grease and put it back together - had to replace a bulb too. Everything worked well. Since I use FLOOR/VENT and FLOOR most often I'm hoping I may see some mileage improvement.

 

[RonD]

IMO, if you are wanting better MPG then forget the AC modifications, the cycling in non-AC settings doesn't draw anywhere near the power that AC on does

Change engine from mechanical fan to Electric cooling fan, that gives both better MPG and more rear wheel power
Only modification that does BOTH

AC cycling also prevents musty smells in the cab as it dries the air, in areas with higher humidity you definitely want this
You live in a location that gets lots of rain, so your cloths are often a bit wet when you get in the cab, this moisture stays IN the cab when you get out, especially if you ran the heater
So if you disable this "drying" of the air it won't take long for you to get that musty smell after 1 winter, lol, and its really hard to get rid of once it starts

 

[8thTon]

It depends on your driving habits. I rarely use the AC and drive with the windows open, so it's just wasting power for nothing. And I still have the full factory clutch fan air capacity.

 

[Dirtman]

If you have the windows open why do you have anything on anyway? It doesn't waste anything when its turned off. And you can still use forward vents without the compressor coming on. It comes on in AC, MAX AC, DEFROST, FLOOR, AND FLOOR/FORWARD. It is always off on FORWARD AND OFF.

 

[RonD]

+1 ^^^

 

[8thTon]

If you have the windows open why do you have anything on anyway? It doesn't waste anything when its turned off. And you can still use forward vents without the compressor coming on. It comes on in AC, MAX AC, DEFROST, FLOOR, AND FLOOR/FORWARD. It is always off on FORWARD AND OFF.

I often have the windows open and the heat on. It's waste heat that blows out under the truck anyway. Have I mentioned that I like the windows open?

 

[Dirtman]

So use the heat on forward vent only...

 

[RonD]

Nah, you want to keep your toes warm and toasty if window is open