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Discussion Starter #1
OK. Right off the bat... I do NOT take any credit for this mod in any way, the work, research and effort was all done by members over at F150Forum. I also take no responsibility if you have issues with your AC... This is a "Your mileage will vary, use at your own risk" mod. One of our own forum members JJM gets credit for finding the thread over there. I decided to do a guide here simply so no one needed to look it up. Original thread is found Compressor cycling fix - Ford F150 Forum - Community of Ford Truck Fans here.

OK. So... my 2013 with auto AC system in AZ. I thought it was ok... blew 47.9 degrees after 25 minutes or so of driving, never got any colder than that. But AZ is a 115 degree state.... I figured it was pretty darn good.

In comes JJM with mention of a mod to make it colder.... I asked for the link, wanted to read up on this (I troubleshoot and repair multi-million dollar semiconductor machines for a living.. where mistakes cost potentially millions of dollars... I'm very careful).

A deep dive through the thread (and other threads linked within) led me to believe that the mechanics, shade tree testers and other general users had done their homework. This mod goes after the PCM's cycling of the compressor based on the temperature of the evap thermistor, who's job is to ensure that the evaporator does not freeze up and to allow for better gas mileage. There is either a wide variety of incorrectly set thermistors out there, or Ford has built in a very healthy margin of error based on the multitude of climates your truck may be in (my personal opinion).

  1. Measure your temps:
    1. Ensure accuracy of temp probe. I used a meat thermometer that I "calibrated" by sticking in a glass of ice water... 32.6 degree Fahrenheit was accurate enough for my testing.
    2. Measure temps at vent with fan on 100% as well as 50%. Minimum 30 minutes driving to ensure your results are accurate.
    3. If temp is greater than 45 degrees Fahrenheit you may be a great candidate for this mod
    4. If temp is cooler than 42 degrees I'd be pretty darn happy and move on!
  2. If you decide to proceed, the cost and materials are quite cheap. Based on the data collection made on the other site as well as my own, between 330 to 400 K-Ohm resistance is the sweet spot. Your mileage may vary on this, so several members have tried a rheostat so they can adjust in the resistance needed. I went with a 360K-Ohm resistor, D-Sub crimp male pins and some heat shrink… total investment was $8.52….. That’s because I bought 20k-ohm resistors as well in case I wanted to increase the total number (didn’t use)
  3. For my first test I literally just wedged a resistor into the plug (pics are below), this worked fine but I knew it would come out…..
    1. Crimped on pins, cut 2 pieces heat shrink. Heated. Done…
  4. Glove box gets “dropped” for this… see pics, super easy. Just empty it before you do it LOL.
  5. Plug is located right above evap box, it’s clipped in. You will be able to see a little white plug to squeeze, then line comes down.
  6. Insert resistor assembly into the plug as shown in pics…. If you went with the d-sub male pins it should be a tight fit, no movement.
  7. Replace wire harness back up in it’s cradle and re-install glove box.
Start her up and monitor temps. Within 15-20 minutes of driving you should have passed your original temp and headed 7-10 degrees cooler. Pay attention!!! Too cold and you will ice your evap up and potentially sludge your Freon, so ideally you should be between 36-40 degrees at this point. If it is colder, add resistance. If you are warmer, remove resistance…. Each truck will respond differently to this and it is on YOU to ensure it’s right….

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Hey Duc, thanks for taking the time to open our eyes to this mod. Thanks to the other people that experimented to try this too.
 

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Awesome! I'm gonna do this!

-58's, Wagner, Solo, aFe, Livewire, LET Tuned!
 

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Good job on thinking of using D-Sub pins as it makes for an inexpensive noninvasive piggy back connection that can be removed without a trace.
 

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There's a reason they limit the temp coming out of the vents. You're eventually damage the ac unit and or just going to freeze up the coil and not going to get any air out of it if you run it that cold for extended run times. In household AC units we want a temp difference 18-20 degrees from in coming to out going air and any more it's not getting the heat out or you're going to freeze up the coil. I know automotive is a little different but if a coil is below freezing it will turn into a big block of ice.
 

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Discussion Starter #9
There's a reason they limit the temp coming out of the vents. You're eventually damage the ac unit and or just going to freeze up the coil and not going to get any air out of it if you run it that cold for extended run times. In household AC units we want a temp difference 18-20 degrees from in coming to out going air and any more it's not getting the heat out or you're going to freeze up the coil. I know automotive is a little different but if a coil is below freezing it will turn into a big block of ice.
That's why you have to measure the vent temps.... testing with the Ford diagnostic software showed the pcm cycling the compressor almost the same amount as stock if you target high 30's / low 40's... a few guy went to low 30's and froze their cores...
 

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That's why you have to measure the vent temps.... testing with the Ford diagnostic software showed the pcm cycling the compressor almost the same amount as stock if you target high 30's / low 40's... a few guy went to low 30's and froze their cores...
And one ran his at 20 for a while and did not freeze up.

But yes, you gotta tweak it to get it just right.
 

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That's why you have to measure the vent temps.... testing with the Ford diagnostic software showed the pcm cycling the compressor almost the same amount as stock if you target high 30's / low 40's... a few guy went to low 30's and froze their cores...
And one ran his at 20 for a while and did not freeze up.

But yes, you gotta tweak it to get it just right.
I'm not sure if the South West makes much of a difference with the air moisture but if it can freeze up it will. I'm just putting it out there for someone that does it and then wonders why it doesn't work after awhile running it that cold, then tries to take it to the dealer to fix it.
 

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I'm not sure if the South West makes much of a difference with the air moisture but if it can freeze up it will. I'm just putting it out there for someone that does it and then wonders why it doesn't work after awhile running it that cold, then tries to take it to the dealer to fix it.
Don't worry, I'll keep an eye on it :)
 

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I'm looking into this mod. My AC is lackluster at best. I've never been impressed with it 'cept for when I turn it on and it's 78° outside.
It does indeed seem like the compressor shuts off too quickly.
 

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Thanks Ducatti

I've glanced over those threads once or twice but never got the time to go through it in detail.

Cant wait to try this along with insulating the few exposed AC lines under the hood.
 

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I too was not satisfied with the A/C in my truck. Installed a 330K resistor. Vent temp before mod was 43F after 36.5F Outside temp was 84F(parked in driveway). Fan was on #2, recirc was off and max air was off. Will try for a couple of days (supposed to get hotter). Thanks OP for the write up. Used link to find out how to remove the plug from the top of the air box(white tab on bottom, who knew!).
 

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I live in Houston so its 95+ outside right now and very high humidity. I added a 1M resistor to prevent the box from freezing and to ensure the compressor would cycle. I made a 750k resistor and the temps got too low, around 40*, this is too low if it is humid in your area... I went from 51* to 42-44 with the 1M and haven't encountered any issues...
 

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That's why you have to measure the vent temps.... testing with the Ford diagnostic software showed the pcm cycling the compressor almost the same amount as stock if you target high 30's / low 40's... a few guy went to low 30's and froze their cores...
Does the on board computer control the final temp output? When it was 104 the other day vent temp was 41. This morning it was 68 so tried to see how low ac would be on a cool day, guess what? Still 41. Non mod a/c system.
 

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Discussion Starter #19
Does the on board computer control the final temp output? When it was 104 the other day vent temp was 41. This morning it was 68 so tried to see how low ac would be on a cool day, guess what? Still 41. Non mod a/c system.
You sir are lucky.... I wouldn't touch that gem of a system!
Blend door and computer affect final temperature..... Based on the feedback from the evaporator core temp thermistor.
 

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Thanks for taking the multiple AC threads (especially the one with a billion pages) and condensing it to a single post. It really helps with sifting out the unnecessary info.

I think it might be a good idea to mention that people should check their Freon levels before attempting this hack. That could be their initial problem.
 
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