Originally Posted by JF01

Got it figured out, I had to disable the VATS and it worked.

Originally Posted by nitrouspowrdss

Ok, first off I would like to thank 1998Chevy1500 for this thread. It got me believing that I wanted to tackle this project. I recently replaced the AC system on my 1999 Silverado along with the radiator and did the Efan swap with a stock wiring harness from an 06 Silverado and pressure transducer kit from BP Automotive. My original OS was 9373372 and from reading here it was determined that it would not operate the cooling fans properly so I found a suitable engine calibration under OS 12212156 and did a segment swap on it to accommodate my 4L80E. I wired in the fans and the pressure transducer, changed the System settings to Analog so that it would see the transducer, charged up the AC system and found everything to be running well. AC was ice cold, fans operated by both temp and AC head pressure.

The one thing I found that I did not like was that the low side pressure/cycling switch was no longer recognized. Sitting in the driveway with the gauges on checking the AC charge I was somewhat comforted by the fact that the low side pressure hovered around 22 psi. I was somewhat ok with this as it wasnt freezing up and the fans were operating as they should. The next day I took the truck for a drive on some errands and when I got home I popped the hood just to check on things and that is when I saw the low side line and the line after the orifice tube completely frozen. I half way expected this seeing the pressures the previous day while sitting in the driveway.

So I dug out the factory service manuals and I started looking at the wiring diagrams to see if I could figure out a way to integrate the factory low pressure switch back into the system. I also looked at the diagrams for a 2006 and noticed that they indeed do have a low pressure/cycling switch.
The solution I came up with is as follows.

1. Remove the wire/terminal at C2-17, cut the terminal off of it and strip back some insulation in preparation for a crimp terminal.
2. Remove the wire/terminal at C2-55 and move it to C2-17.
3. Take the wire that used to be at C2-17 and connect it to the black wire on the low pressure switch. I went to the junkyard and got all of the wire from PCM to switch off of another truck so that I could do this without disturbing my original wiring and just ran the new wire inside the factory split loom.

What this accomplishes is putting the low pressure switch in series with the high pressure switch on the AC request circuit.

Bingo bango, you're done. Now you have an AC system and cooling fans that function exactly the same as a factory 05-06 truck which is necessary for long compressor life due to the fact that the trucks have a fixed orifice as opposed to the thermal expansion valve that the F-Body cars have. I believe that I thoroughly researched this as much as I could to the point that I dont think this could have been done by changing any settings in the pcm. The pinouts for the later trucks low pressure/cycling switch are different.

Originally Posted by 1998chevy1500

Here are some screen shots from HPTuners software showing the setting that needs changed to convert the 1999-2002 trucks from using a high pressure switch to a high pressure sensor. One screenshot is from a mostly stock 2002 6.0, the other is from a 2002 Camaro (which use a pressure sensor)
Truck
Attachment 152113

Camaro
Attachment 152112

As was stated in the EFI Live thread you guys posted, this setting is not mapped out in EFI Live. It is mapped in TunerCats and HP Tuners.

I appreciate the scaling info for the A/C high pressure sensor. In case the link breaks in the future, I'm posting a quote from that thread here.

Originally Posted by 1998chevy1500

Physical Installation:

Well, the fan relay harness I used was an OEM one from a 2005-2006 Suburban which fit right up like it belonged there just like the previously mentioned inspirational link. When routing the harness under the battery tray and along the bottom of the radiator the existing 1999-2002 split loom is too small to fit in the additional cooling fan wires. Also the clips which secure the split loom aren't able to take any larger split loom anyway so I just did like most people have done and zip tied my new fan wiring to the old wiring. Oh, I should point out that to keep the harness looking nice I sprung a couple dollars on Amazon for some "harness tape" this is a roll of approx 1" wide black PVC tape that doesn't have any adhesive. You use it to wrap the wires up nicely to keep them together and then secure the end with electrical tape. This is the same type of stuff the factory harnesses use and it results in a harness that isn't full of sticky electrical tape adhesive goo whenever you need to get back into the wiring. Well worth it. I don't think I need to go much more into the fan harness portion of the installation as it has been covered numerous times throughout the internet. My ultimate goal here was to make this harness installation as close to stock as possible. I even got the underhood fuse panel cover from a 2005-2006 truck as it has a little panel above the fan relays to allow access to the fuses and relays without having to remove the entire fuse panel cover.
As for the A/C high pressure sensor, the sensor will thread right on in place of the old switch. The port on the A/C system has a little schrader valve on it so all you get is a little puff of r135a when you remove the switch, no need to evacuate the A/C system. On the 2003+ trucks the sensor sits right next to the AC compressor and the wiring is part of the engine harness, on the 1999-2002 it is up by the radiator support and the wiring looks to be part of the body wiring. What I did find is that even with the different routing, the wire length turns out to be about the same so I didn't need to solder or crimp any PCM or sensor pins, just remove the wires from the junkyard harness and route them into my truck. I abandoned the old pressure switch wiring in place. I routed the new wiring from the sensor down to the harness running underneath the radiator (same harness the cooling fan routes along side). I wrapped these wires with the "harness tape" to keep them together, and since these wires were so small I was able tuck them into the existing split loom along the bottom of the radiator. When I got over by the battery tray it was kind of a bitch to fit this inside the existing loom so I just ran a chunk of new loom through there that had the fan wiring and A/C high pressure sensor wiring in it. Once I got up by the fuse box, I routed the sensor wiring and the two fan control wires over to the PCM doing the best I could to keep the wiring inside the split loom and wrapped up to keep it looking nice and together.
Get the 145A alternator. I put this on my truck years ago when putting electric fans on was the big thing. At the time the availability of the fans was limited and the prices were too rich for my blood. Still I was able to get the alternator for cheap back then so I started with that upgrade and never got any further (life and funds and such). You will need a slightly different length serpentine belt for the 145A unit as well. Don't cheap out here on the alternator, these fans draw 10A at stage 1 and a whopping 40+A at stage 2. The fans I have now are the older design where one fan is 5 blades and the other 7 blades. This is due to a mixup with the ebay vendor who was supposed to be sending the higher end ones which run 7 and 9 blades respectively. By my math the 5&7 blade units draw 500-600 Watts depending on what voltage you use to do the math, the 7&9 blade units are mentioned numerous times on the internet to be 700 Watts. I'm in discussions with the ebay vendor to either return these fans for the fancier ones or just discount what I have and call it good. The 5&7 blade units still move a lot of air.


Wiring Connections:

1. The Low Speed fan wire (Dark Green) was connected to C1-42.
2. The High Speed fan wire (Dark Blue) replaced the re-circulation pin in position C2-33. I just taped up the old pin and tucked it inside the factory harness.
3. The A/C high pressure sensor signal wire (Red/Black) connected to C2-14.
4. For the +5V sensor reference wire (Gray) you can't use the same pin as the F-body as that pin is occupied by the +5V feed to the fuel pressure sensor. After quite a bit of digging, I couldn't seem to come up with a spare +5V pin anywhere in the PCM. I could have just spliced my wiring onto another pin, but I didn't even know if this science experiment was going to work so I wanted something that was going to be easy to remove when it all went to crap. For this I chose to rob the +5V feed to the EGR (I removed my EGR years ago so these pins weren't feeding anything but a couple dangling wires anyway). The +5V signal for the EGR position is C1-48.
5. For the low reference signal wire (Black) I was in the same boat as the +5V reference, I couldn't find a damn spare pin anywhere. Once again, the EGR gave up his pin location for the greater good, this time it was position C1-23.


Final Comments:

I haven't had much time to do any exhaustive testing of the configuration so there is a possibility that there are other issues I have not discovered. Thus far all I have done is start the truck, fire up the A/C and make sure it causes the fans to come on. With the fans running on low with the A/C on full blast I disconnected the command to the fans for a little bit so the compressor would build up a bit of pressure on the high side and then plugged them back in. The computer ran the fans right to high speed and kept them there for the minimum on-time duration before dropping back to low speed. I scanned the truck with an old AutoTap I had laying around and it reported a value for the A/C high pressure sensor which seemed to vary depending on air flow. The AutoTap is kind of a piece of crap because it only displays the A/C high pressure sensor reading as 0-5v with no actual pressure scale, hell I can figure out that with a damn multimeter, but what else can you expect from 15+ year old software. The good news here is that the PCM reports a voltage number suggesting that it is taking in the sensor reading and using it to control the fans. I have not driven the truck nor have I even brought it up to operating temp since doing this install, however I do not anticipate any issues in that regard as that part of the installation is well proven territory.


Appendix:
A -- A/C Type Descriptions:

In the HP Tuners software under System->A/C there is a setting called Type Fitted. This setting configures the PCM for the type of A/C system being used. Based on my limited research these are the following settings and descriptions:

Serial - Used when the command to turn on the A/C compressor comes over the class 2 data bus and there is a A/C hihg pressure sensor being used. This is what the 2003+ trucks use.
Serial Cycling - Same as Serial except that a A/C high pressure switch is used.
Analog - This is what the F-body cars use (also various years of the Savannah vans). This is where the command to turn on the compressor comes from a hard wired PCM input and there is a A/C high pressure sensor being used.
Analog Cycling - This is what the 1999-2002 trucks use. This is where the command to turn on the compressor is hard wired to the PCM and there is a A/C high pressure switch used.
Passive - No idea WTF this is used for.
Not Fitted - Same here, no idea.

B -- Attached Schematics:

I have attached some of the GM schematics I have marked up to represent my installation. My truck has the automatic climate control, thus I used those schematics to document my work, but for this application the manual climate control differences are of no consequence.

Questions/comments? I'll try and answer what I can, but I am not on the forums much so I can't guarantee how good my followups will be. Also sorry for no pictures, never been much good at taking pictures to document my work.