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Anyone made A/C AND Fans work with Blue/ Green PCM?
I'm swapping a 4.8 from an '04 Tahoe into an '89 Firebird, and I want to control fans (GM three-relay setup) and A/C compressor with the PCM.
I've read through several threads here and on thirdgen.org, and basically a few guys say that they got it to work, and a lot of other guys say that it never worked.
The PCM I'm using is # 12586242.
I understand that the PCMs with IAC drivers from vans control the PCM with an analog signal and high/ low pressure switches, but cannot control fans.
The Silverado/ Tahoe systems use serial data and a three-wire sensor, and I'm not installing an HVAC module in my car, and I don't want to use a trinary switch; I want it to all be the PCM.
I'm in the wiring stage of my swap, so I'd like to know if I have to wire in pressure switches, or a pressure sensor.
Yes the Green/Blue P59 will control fans properly if you get everything setup correctly. Im running one on a 04 extreme blazer with LS1 fans and a 3 relay harness and it has worked flawlessly for years. Need to know if you are going to be running a cycling fixed displacement compressor or a non-cycling variable displacement compressor.
With an analog cycling setup, the PCM monitors the low pressure switch signal. The AC request signal is sent from the HVAC controls, through the high pressure switch, and then to the PCM. There is no pressure sensor on a cycling setup. When the PCM is setup for cycling, it will not command the fans based on AC refrigerant pressure, but it will command the fans for engine coolant temp.
With a non-cycling setup, the PCM monitors a pressure sensor mounted on the high side. It does not use any pressure switches. When setup as non-cycling, the PCM will command the fans on based on AC refrigerant pressure as well as engine coolant temp.
You will need HPT or something similar to make the necessary changes in the OS. I use HPT.
Under System / AC, Set Recirculation Fitted to "Fan #2". Set Type Fitted to "Analog". This sets up the PCM to use the 12v input for AC activation and to use the Low and High speed control for your efans. In the Fans section, make sure the Fan Type is set to "2 Fans" and then set the other fan settings to your preference.
You will need to install a pressure sensor on the high side and wire it to the PCM.
If you are going to be using a cycling fixed displacement compressor, you need to install a low pressure switch on the low side. Do not wire the low pressure switch up like the factory schematic shows. Since the PCM is now setup for non-cycling, it will ignore the low pressure switch signal and your compressor will not cycle. Instead, run your AC request signal through the low pressure switch and then to the PCM. This will allow you compressor to cycle. When the low pressure switch opens the request circuit, the PCM will then command the compressor off. When the low pressure switch closes the request ciruit, the PCM will command the compressor back on.
If running a variable displacement compressor, you just need to make the necessary changes to the OS as stated above and install the pressure sensor.
Yes the Green/Blue P59 will control fans properly if you get everything setup correctly. Im running one on a 04 extreme blazer with LS1 fans and a 3 relay harness and it has worked flawlessly for years. Need to know if you are going to be running a cycling fixed displacement compressor or a non-cycling variable displacement compressor.
With an analog cycling setup, the PCM monitors the low pressure switch signal. The AC request signal is sent from the HVAC controls, through the high pressure switch, and then to the PCM. There is no pressure sensor on a cycling setup. When the PCM is setup for cycling, it will not command the fans based on AC refrigerant pressure, but it will command the fans for engine coolant temp.
With a non-cycling setup, the PCM monitors a pressure sensor mounted on the high side. It does not use any pressure switches. When setup as non-cycling, the PCM will command the fans on based on AC refrigerant pressure as well as engine coolant temp.
You will need HPT or something similar to make the necessary changes in the OS. I use HPT.
Under System / AC, Set Recirculation Fitted to "Fan #2". Set Type Fitted to "Analog". This sets up the PCM to use the 12v input for AC activation and to use the Low and High speed control for your efans. In the Fans section, make sure the Fan Type is set to "2 Fans" and then set the other fan settings to your preference.
You will need to install a pressure sensor on the high side and wire it to the PCM.
If you are going to be using a cycling fixed displacement compressor, you need to install a low pressure switch on the low side. Do not wire the low pressure switch up like the factory schematic shows. Since the PCM is now setup for non-cycling, it will ignore the low pressure switch signal and your compressor will not cycle. Instead, run your AC request signal through the low pressure switch and then to the PCM. This will allow you compressor to cycle. When the low pressure switch opens the request circuit, the PCM will then command the compressor off. When the low pressure switch closes the request ciruit, the PCM will command the compressor back on.
If running a variable displacement compressor, you just need to make the necessary changes to the OS as stated above and install the pressure sensor.
Thanks for that great writeup!
I'll be using a Denso 10S17F fixed-displacement compressor.
Does this wiring look right to you (you gave me a bunch of options up there!)?
EDIT:
Originally Posted by LT4T56Monte
Pick up a copy of Mike Noonan's "GM Gen IIILS-Series Powertrain Control Systems". It reveals many of the 411's secrets.
I have that book, but I was having trouble making sense out of that section: he just glosses over the truck PCMs.
Last edited by dixiebandit69; 05-03-2020 at 04:14 PM.
The pressure sensor wiring is correct. The wire going from the low pressure switch to the pcm needs to be connected to C2-17 not C2-43. C2-17 is the 12v input for AC request. C2-43 applies ground to the coil of the AC Compressor clutch relay to energize the relay. It is not necessary to wire up the high pressure switch, as the pressure sensor will provide protection against high pressure, but you can wire it up if you want. I have mine wired up because the wiring and connector were already there.
The pressure sensor wiring is correct. The wire going from the low pressure switch to the pcm needs to be connected to C2-17 not C2-43. C2-17 is the 12v input for AC request. C2-43 applies ground to the coil of the AC Compressor clutch relay to energize the relay. It is not necessary to wire up the high pressure switch, as the pressure sensor will provide protection against high pressure, but you can wire it up if you want. I have mine wired up because the wiring and connector were already there.
Hey, I just wanted to know that if the pcm doesn't have a wire in pin 17 do i just install one and it work or does the pcm need to be flashed? i bought an 05 with Blue/Green connectors that the harness was swap ready and there is no wire in pin 17. any help would be appreciated.
Last edited by Camarojoe70; 04-10-2023 at 11:57 AM.
Reason: I wanted to add more info
The pressure sensor wiring is correct. The wire going from the low pressure switch to the pcm needs to be connected to C2-17 not C2-43. C2-17 is the 12v input for AC request. C2-43 applies ground to the coil of the AC Compressor clutch relay to energize the relay. It is not necessary to wire up the high pressure switch, as the pressure sensor will provide protection against high pressure, but you can wire it up if you want. I have mine wired up because the wiring and connector were already there.
@branndonw Do you not use C2-55? I currently have a similar setup except for the low pressure switch going to c2-55(ground) and high pressure switch going c2-17(12v) . The ac turns on just fine but my e-fans do not turn on no matter what I do. But I think your guide here might solve my e fan problems. Before I just want to confirm that you do not use c2-55 at all.
C2-55 is no longer used. When the system is set to "cycling", it will monitor the low pressure switch connected to c2-55. When the system is set to "analog" (non-cycling), the pcm will monitor the pressure sensor and ignore pin c2-55.
That makes perfect sense. I don't know how you were able to figure that out but I appreciate your help and quick response.
I am going to copy your wiring diagram since I have both low/high pressure switches and pressure sensor connected. Did you make any fan configuration changes for the AC?
Last edited by homermadoff; 07-23-2023 at 11:31 PM.
"When the PCM is setup for cycling, it will not command the fans based on AC refrigerant pressure, but it will command the fans for engine coolant temp."
The '05 1500 trucks were fitted with e fans stock, and yet they are set up for AC system type "serial cycling". It seems as if the fans are programmed to come on with A/C pressure so - is there just something different about how the '05 operating system works? Or am i missing some other hidden setting?
ok, that OS definitely got used for v8's, i have a 2003 L59 tune that uses it and a few others i think.
very interesting. truth be told i have not tried to use this on a swap with AC myself, but i did have a customer mention something about the fans not working with AC, and this could explain it - he is using OS 12579405. But i literally baked my noodle on it because i copied over all the FAN and AC settings from an 05 that had efans by default, and the settings were exactly the same. AC system type - 1 - serial cycling.
on the flip side, i had another customer for whom i turned on efans on the '04 operating system - 12587603 - and he says they are working as expected (Admittedly i have not verified, but he is pretty good at troubleshooting and sends me detailed information). AC system type is set to 1 - serial cycling.
not sure what gives. if i had a blue/green DBC ecu kicking around i would experiment with this myself but i dont at the moment.
sorry to bug you guys, but I have the same problem (and the tuner doesn't care apparently to fix it).Is a P59 on a 2003 truck and I converted to electric fans. Fans work flawlessly as the ect goes up, no problem. A/C operation is the issue. On A/C request, compressor clutch activates like normal, HP sensor reads around 1.5V and starts to go up (close to 4V), condenser gets super hot, but no fan kicks on. It just doesn't respond to A/C HP sensor. Recirculation fitted is set to "Fan # 2" and Type fitted is set to "serial cycling". is this correct? truck has factory 3 wire pressure sensor on high side.
I contacted the tuner and ask for a new file, they emailed me one, but they only changed ON/OFF temps under fan setting (which seems irrelevant to the issue) and I can't even write to ECM, keep asking for a license. Is this sounds right? Tuner "theoretically" used one or two credit for this VIN, why asking for license again? Appreciate any input.
05-02-2020 | 04:30 PM
Anyone made A/C AND Fans work with Blue/ Green PCM? 
