As many of you know like myself the air conditioning is lack luster at best. The system has a built in evaporator sensor to control evap freezing and to shut the system off and cycle it to maintain a set evaporator core temp which increases mpg in theory.

Despite taking the truck to the dealer and even performing my own full recovery and charge of the system, the system was still horrible. Lowest vent temp was 61 degrees on a 97 degree day. I also noticed the vent temp would fluctuate every 30 seconds at freeway speed and climb to 68 degrees and rapidly fall back to 61. I drove around with a digital air conditioning probe in the vents monitoring for almost a week.

After looking at the HVAC parts schematic I found the cause of part of it. Ford used a thermistor to control the evaporator temp. This is nothing new but what did change was its use in 2011. The trucks use the signal to turn the compressor off to increase mpg at a preset 47 degree temperature. This I learned from a dealership technician (unknown if its fully creditable ) so I was on the hunt to bypass it and to save my air conditioning clutch some wear and tear.

After multiple trial and error runs and frustration I found a resistor that will work long term to bypass the thermistor. This mod is fully reversible and requires no cutting of any factory parts. The mod takes 3 mins total. Below are the steps required and the part.



Please take note as this is tricking the computer, the truck must be running with the air conditioning on max prior to beginning. Without it running the truck will not detect the resistance value and the ac will not turn on. I had that ussue, you may not.

1. Lower glove box by pressing the tabs inward.

2. Look at the plenum box and locate the black wires that look like speaker wire. Follow upward to the connector. Push the tab on the top of the connector and pull to release.

3. Take your 33k 1/2watt resistor and insert one leg of the resistor in each of the harness side connection pins.

4. Test the system to check function , once the resistor is installed the ac will immediately cycle on and cold air will follow.


5. Finish by using electrical tape to secure the resistor to the connector. (Not shown)

Final result is no more cycling and vent temps are down to 45 degrees on a 98 degree day.

Total cost is around $2.00

 

Very interesting. I have heard of a resistor mod but didn't know exactly what was involved. Up in the PNW it doesn't get too hot but this should be stickied.

 

Linked to in the stickies!

Great find and great write up!

https://www.ford-trucks.com/forums/1...l#post13550251

 

Pretty sweet trick ! Our AC can always use a little help. I don't want to sound ignorant, but I do have a question about this. I thought that the compressors need to cycle to some extent to prevent a total freeze over, so the question is, will this prevent compressor from cycling off at all or does it just prevent excessive cycling ?

 

What year of truck does this work on? 2011 and newer? Mine is a 2009

 

I have to use recirculate mode on mine to stay comfy on very hot days AND very cold days because the defrost SUCKS on these trucks also. I might have to try this. Thanks for the info.

 

I commented (to myself mostly ) this past summer how the A/C in my 2013 sux big time, a lot different from my 2010 I had. Thanks for figuring this out for us!

So, next question........since you bought a pack of 5 and only used 1.....if I paypal you 47 cents for a stamp how 'bout mailing me one ?

 

So, I went ahead and did this. The op's instructions were excellent but I do stuff like this for a living. I snapped a couple of pics for those folks a bit less experienced at things such as this.

Pic #1- the location. When you open the glove box and unlatch it to allow it to fall down this is what you'll see. Just like the OP said, eh? You'll have to scrunch down and look up a little, up under the dash. The yellow circle is the entire plug you'll disconnect. The red circle is the release tab- push it down (see pic #2). Mine was a little stubborn mostly b/c you have to go by feel.


Pic #2- detail of plug release tab. Pretty self-explanatory.


Pic #3- if you just slide the wires from the resistor into the plug it's a loose fit and I was concerned about not making good contact. So I bent each resistor wire in a "U" to create a spring-open type of effect. Worked great!


Pic #4- resistor in place. A couple pieces of electrical tape from one side, over the top to the other to hold it in and a couple wraps of tape so it won't rattle and done!




If you have any questions post them up! Plus I now have 4 extra resistors- send a PM if you want one and I'll drop it in an envelop for you. Free of charge, yours for the asking.

 

edit : Had to re-post. Somewhere along the line my subscription settings get wonky. Sorry about that

 

If I understand you are putting the resistor in to FAKE an actual temp. While this works very well, if the evap core ever freezes over, it may slug liquid refrigerant back to the compressor damaging it. If the system has a low pressure cutoff switch this mod would be safe. If the system does NOT have a low pressure cutoff switch, this mod should be avoided.

 

It might be better to wire a resistor in series or parallel in the circuit so that you are just changing what the computer thinks the temperature is.


that is to say, change it slightly so that it doesn't shut the compressor off so soon, but don't change it so much that it won't ever shut it off.


one would either have to know the resistance at the target value, or do some trial and error with a potentiometer wired inline with the sensor.


I agree that bypassing it completely shouldn't be the first and only approach.

 

Seems to me "the first and only approach" would be for Ford to leave the RTD out of the circuit in the first place, a practice done for years and years and years until some genius came up with the idea of cycling the compressor to increase fuel economy. Granted, some innovative techniques need to be applied to meet federal mandates but IMHO (down here in the sunny south) when my A/C system can't maintain less than 77*F in the cab on a summer day then something (anything!) needs to be done.

If you don't like the mod for whatever reason then don't do it but I'm tired of sweating like crazy in the summer and having my windshield suddenly fog over in the winter b/c the compressor has cycled off. You'll not convince me that it's good for my complexion to sweat outside working (or golfing ) for 4, 5 or 6 hrs and that it's also good to sweat all the way home. Sorry, I respectfully disagree.

And I don't subscribe to the "ifs" and "maybes". I'm willing to be the guinea pig to find out. It all comes down to choices: my truck, my choice.

 

Firstly, great writeup and thanks for posting.

So, does this mean the resistor needs to be removed/replaced every time the truck is started? If so, could an inline switch be added so you can cycle it on from the cab?

 

dude ....
settle down ....
its just a different opinion, geezus ...


not everyone has this problem. my 2010 will freeze you out no matter how hot it is.

 

Yeah, ok. Sorry about that. It's just that the worry-warts of the world have kept so many people from doing so many things by saying what could happen, what if the bad comes out, etc. I'm of the opinion that if someone is gonna post up "if you don't have a...." then that person ought to have to gumption to do the research to find definitively if I have one and state so rather than sling more "ifs", "maybes", etc. around. Go back and look- that one 4-sentence post contains nothing factual, 4 ifs, 1 may and 1 should. As a supervisor of men and a superintendent of supervisors I generally say,"when you know the facts come back and we'll discuss it. 'Tl then have a nice day."

Yeah, I know about the 2010 freezing me out. I had one before this 2013, until a lady blew through a stop sign and destroyed it. Even in October 2013 when I got this truck new I knew something wasn't right with the A/C but the dealership insisted it's right even when I could show recorded temps of 66-68*F from the vents.