rocketmench

Hi all, I hope this helps somebody. I am going to be transplanting a 2004 LS LM7 5.3 and 4l60e into my 1968 Chevy K20 truck. I want to keep the stock A/C system on the truck and use the truck's original A/C controls and hardware while using the 2004 A/C pressure sensor system. As outlined in various forums and over at lt1swap.com (https://lt1swap.com/airconditioning.htm), Green/Blue P59 PCMs with service numbers 12576106, 12581565, 12586243, 12589462, 12602801 will accept a 12V analog A/C request signal on pin C2 17, but all other P59's will not. If you don't have a PCM with one of these service numbers, any attempt to get analog A/C request working will fail. You can program the PCM to switch from Serial to Analog, but it will not work.

Here's why. The PCMs that fail are missing two resistors and a capacitor that connect pin C2 17 to a chip on the motherboard (pin 8 on the chip labelled 20366). These components act as pull-down and de-bounce (low-pass filter) circuitry. I was able to trace out the circuit on a good PCM (Service number 12576106) and compare it to my original 12586242 PCM to determine the difference.

Once I determined the component values, I added the missing pieces to the board. I then set up a bench-top setup to test my fix to verify it actually worked. To do this, I setup the PCM such that it was "tricked" into thinking the engine was running. This was done with an Arduino Nano running the Speeduino Ardustim code. I also had to provide the A/C pressure and engine coolant (ECT) signals, (2.5 V or about 80 kOhm resistor for the pressure sensor) and a potentiometer to provide an ECT (engine coolant) signal.

Here's a picture of my very messy bench-top setup:



To test my setup, I started with a baseline configuration as the PCM would have been setup from factory with the HVAC module from the truck. I connected the HVAC module class-2 serial data line to the PCM and confirmed that I could request the A/C relay control to come on. I did this by measuring the voltage drop across a resistor to verify the PCM setting pin C2 43 to ground. I did the same for the Fan #1 and Fan #2 controls. Here's my bench-top circuit schematic:


Using lsdroid on my Android phone and Tunerpro, I configured an 12587603 OS to accept Analog A/C, and I setup the two electric fans to come on as well. At this point I was able to test the A/C analog 12V control and verified that it would work with my 12586242 PCM, which it did. But ONLY after I added those missing resistors and capacitor.
I'm adding some pictures to show how you can do this too.

Step 1: Add missing 3 kohm 1/2 watt 1210 SMD resistor. Part no. CRCW12103K00FKEA from Mouser or possibly Digikey.




Step 2: flip board over and remove 10 kohm pull-down resistor.


Step 3: Add 1/2 Watt 0805 SMD 100 kohm resistor. Part number CHP0805AFX-1003ELF.


Step 4: Add 0805 SMD 1.5 nF capacitor. Part number C0805C152J5HACAUTO.


The circuit you just added looks like this:


Good luck. I hope this helps somebody.

willehca

I joined LS1tech.com just to come here and say thanks for doing the work on this! This is great, I had planned on using a van PCM to get my a/c to work in my swapped C10. But the OS were different so I was going to have to figure that out plus the $100 in credits. I think $5 in components and firing up the old solder iron with your hard work and I'm in business! Your PDF from the other thread should be posted here.

rocketmench

Well thank you, and please let us know how it goes. I'd be happy to answer any questions.

willehca

I used a test light instead of activating the clutch directly, bypassed my cycling switch (I don't have a charge in my A/C system yet), set the tune up for analog(i tried analog cycling first but that didn't work) AC type, fired up the truck it worked. I can't PM you because I'm a new member but I would like to ask permission to share this information, I feel like it should be out there more. When trying to understand what my options were to make the AC work, I found so many threads where people ran up against this issue.

rocketmench

Good to hear it is working! You are the second person (other than myself) who has confirmed that this mod works. Feel free to disseminate this information!

Fast355

Considered adding the IAC drivers to one but found 4 in the wrecking yard recently.

_angeldawwg

I added circuit to my 03 pcm this past Wednesday it wasn’t so bad i was a bit scared at first Tomorrow morning im programming it to have a 12v request signal from it hopefully i can get it to work before the hot weather here in texas

Terreur350

Thanks for the info, I did that and it works !

rocketmench

Great! Thanks for letting us know. I'm happy to hear this works for other folks too.

350SS

iTrader: (38)

appreciate you taking the time to figure this out and write it up to share with others . that's some badass work here...

astate37

I would like to do this but one of the part numbers listed (CRCW12103K00FKEA) is out of stock everywhere I've looked. Would this be a viable alternative?

https://www.mouser.com/ProductDetail...%252Bv8g%3D%3D

rocketmench

That part should work just fine. It's rated for 1/2 watt which is way more than the resistor should ever see (Power = V^2/R = 12^2/3000 = 0.048 Watt).

jborecky

I haven't gotten excited about a post in awhile. I've done a ls swap into a Jeep Comanche and want to get the AC going long term. I'm also playing around with creating a digital shift indicator using a arduino uno and the range signal coming out of the mlps. I know that these are two different work streams, but gives me hope that I might pull this off without causing the PCM too much grief.

Rust-o-matic

I just joined this forum specifically to say how freakin cool this is! Thanks so much for taking the time to document and share your work with us. I have researched the subject of AC request on blue/green pcms extensively, and thought it would not be possible with my pcm. This is awesome news.

I have a couple of questions for those who have done this. And they may be stupid questions, as I'm working on my first swap ('03 4.8 into an '84 C10), so please bear with me:

Does adding the analog request circuit allow the pcm to bump the idle rpms up when AC is requested? I understand it triggers the fan relays, but was curious what all else it accomplishes.

Would I tie into the AC compressor clutch power wire for the analog source wire that would run to green pin 17?




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rocketmench

Thanks Rust-o-matic for the kind words. Yes, generally the PCM should bump the idle any time the AC has been engaged. I should think any 12 volt source would work for your AC request switch/lever/**** to C2 17.

Good luck with your project.

Rust-o-matic

Great, thank you for confirming. Keep up the good work.

Kjenevein

i have a ls swapped squarbody that I am running into this exact aggravation, looks like i may have stumbled onto the right thread! soo adding these components will make my computer take the physical 12v input on pin 17 then out pin 43 gnd to ac relay?? or does the computer also have to have to be running on a certain OS??

rocketmench

My belief is that you have to specifically select AC request in the OS for this mod to work. If that option exists, you should be good to go.

Kjenevein

Thank you again awesome info!

jmunson

Thanks for the info! my question is I have a 12602802 P59, is there anyway to verify before i start adding components that the above will work on mine too? Im assuming by opening it up and following the traces and comparing to your pictures, or just send it !!! LOL