Electric Fan Conversion AC Issue

[wstefan21]

So here's what I've found.

Serial = The 05+ truck uses the transducer for high pressure cutoff through the pcm, then the low pressure switch is actually wired into the hvac module which controls the a/c signal through serial.

Analog = f-body uses the transducer for high pressure cutoff through pcm, there is no low pressure switch.

Analog Cycling = 99-03ish truck uses high pressure cutoff on 12V request signal and low pressure cycling switch. This one never controls cooling fans because this design never came with them.

The key detail with all of this is that both the f-body and (most of) the 99+ gm trucks all used variable displacement compressors, which for the most part eliminate the need for a low pressure cycling switch by automatically adjusting their displacement.

The concern is that since I am running an older r4 ac compressor for my application that with no low pressure cutoff I could burn out my compressor. Most vintage air compressors aren't variable speed either.

I was thinking about a low pressure switch on the request signal, but I've never seen this wired like this before. I will definitely test to see if the low pressure switch still works on "analog" mode, but I have my doubts. Not really sure of the best option at this point...

05SilveradoAC.pdf
03ChevyExpressAC.pdf
99CamaroAC.pdf

[wstefan21]

So went waaay back to before computers and turns out they used to use a cycling switch with no relay. Modern switches are definitely NOT going to hold up to that kind of current.

Therefore, the options to keep a non variable displacement compressor safe are to:

1. put inline the a/c request signal

2. put inline the a/c clutch signal

My issue with 2 is that the computer will attempt to engage the clutch even if the switch won't allow it, which will turn on the fan when not needed (which isn't a huge deal).

Ideally, I think 1 is the way to go even though this means the high-pressure cutoff switch will be inline the cycling switch (which I found some older cars that started implementing high pressure switches are configured exactly this way).

This might cause a bit of delay for compressor engagement, but that's a small price to pay for correct a/c operation without risking the compressor.

Going back to the bypass method, I've heard that there is a threshold that switches between high and low fans. If this is true, the only behavior this will change vs transducer sensor is fan high-low operation, so this becomes a safer option if used in conjunction with the cycling switch inline high pressure switch.

[Life is an Enigma]

Okay this is how my low pressure cycling switch is hooked up in my 04 Chevy Blazer with a p59 PCM. Put the green wire from the cycling switch to pin 55 other red / green PCM connector and the black with white stripe to ground that will let the PCM control the AC cycling. Then go in to your tune for your vehicle go to system click on AC and set type filter to serial cycling. That's how mine is setup up in my 04 Blazer and my AC cycling works. The only real difference I know between the PCM you're using and the PCM that I'm using that mine can do drive-by-wire yours can't.

Just looked at your AC wiring schematic for your 1988 it looks like your high pressure switch and cycling switch are series together to turn on the compressor relay. To make it work with the newer PCM some rewiring needs to happen. You can clone the S10 AC wiring in your setup and make your AC system work correctly it's pretty simple.

1988 Chevy C1500 A/C wiring
Screenshot (763).png

04 S10 Blazer A/C wiring
Screenshot (764).png

[wstefan21]

Okay this is how my low pressure cycling switch is hooked up in my 04 Chevy Blazer with a p59 PCM. Put the green wire from the cycling switch to pin 55 other red / green PCM connector and the black with white stripe to ground that will let the PCM control the AC cycling. Then go in to your tune for your vehicle go to system click on AC and set type filter to serial cycling. That's how mine is setup up in my 04 Blazer and my AC cycling works. The only real difference I know between the PCM you're using and the PCM that I'm using that mine can do drive-by-wire yours can't.

Just looked at your AC wiring schematic for your 1988 it looks like your high pressure switch and cycling switch are series together to turn on the compressor relay. To make it work with the newer PCM some rewiring needs to happen. You can clone the S10 AC wiring in your setup and make your AC system work correctly it's pretty simple.

Wait... mine isn't hooked up like 88 (completely re-did the wiring for ls swap), so that's exactly how mine is hooked up now! I thought serial meant the turn on signal had to be over class 2 data?? If that's not the case then I'm set! I'll definitely give it a go and let you now how it goes.

[Fast4.7]

Okay this is how my low pressure cycling switch is hooked up in my 04 Chevy Blazer with a p59 PCM. Put the green wire from the cycling switch to pin 55 other red / green PCM connector and the black with white stripe to ground that will let the PCM control the AC cycling. Then go in to your tune for your vehicle go to system click on AC and set type filter to serial cycling. That's how mine is setup up in my 04 Blazer and my AC cycling works. The only real difference I know between the PCM you're using and the PCM that I'm using that mine can do drive-by-wire yours can't.

Just looked at your AC wiring schematic for your 1988 it looks like your high pressure switch and cycling switch are series together to turn on the compressor relay. To make it work with the newer PCM some rewiring needs to happen. You can clone the S10 AC wiring in your setup and make your AC system work correctly it's pretty simple.

1988 Chevy C1500 A/C wiring
Screenshot (763).png

04 S10 Blazer A/C wiring
Screenshot (764).png

The PCM has to be one compatible with analog ac input to do that. What operating system is your Blazer?

[wstefan21]

So, not sure what's going on, but "serial cycling" somehow still works with cycling switch and regular turn on command, but still doesn't command fans on. I'm going to try adding the pressure sensor to see if that does anything. I'm wondering if "serial cycling" isn't referring to the fact it's "serial" communication to turn on a/c, but rather it's a mashup of the transducer fan functionality from the 05+ "serial" and the 99-03 cycling clutch input from the "analog cycling" so they just called it "serial cycling". Only one way to find out!

[Life is an Enigma]

The PCM has to be one compatible with analog ac input to do that. What operating system is your Blazer?

My 04 Blazer operating system is 12587603 and that one is set on serial cycling. And my 2003 GMC Sonoma operating system 12579405 that one set up on analog cycling. I believe either setting will work because both vehicles use the same wiring harness and engine trans combo and both of them were mechanical fan.

[wstefan21]

My 04 Blazer operating system is 12587603 and that one is set on serial cycling. And my 2003 GMC Sonoma operating system 12579405 that one set up on analog cycling. I believe either setting will work because both vehicles use the same wiring harness and engine trans combo and both of them were mechanical fan.

So you're saying your 03 has analog cycling set and no pressure sensor and the a/c and electric fans work? This could just be the pcm hardware I'm using then. Still not sure why the serial cycling works. I'm hoping that adding the sensor will make the fans work.

Here's my current tune. Hopefully this is set up similar?

88chevy1500afterUpgrade.hpt

[Life is an Enigma]

So you're saying your 03 has analog cycling set and no pressure sensor and the a/c and electric fans work? This could just be the pcm hardware I'm using then. Still not sure why the serial cycling works. I'm hoping that adding the sensor will make the fans work.

Here's my current tune. Hopefully this is set up similar?

88chevy1500afterUpgrade.hpt

You can do the resistor bypass as I stated earlier to get the fans to work when A/C on. I had the same issue the fans would not come on with the AC I finally came across the solution. A lot of people I talk to you that do swaps don't even put a high pressure switch in the system they just run with the cycling switch and bypass the high pressure. Which that is risky if you hit too high of a head pressure something's going to give and it's not going to be cheap it's not ever with AC. One guy I talk to that has a 5.7 swap in his S10 has been running this AC without a high pressure switch for over 5 years without any issues. I have not personally tried the bypass yet using the resistor. I am too busy this week coming but, the week after that's when I'm going to try it out and I will post my results but I'm 96% sure that it will work. Also will post what variable resistor I will be using. I'm lucky my wife's car uses the same pressure switch so I'm going to take readings off of that one so I can find the right variable resistor to use.

[Life is an Enigma]

looked at your tune I can't believe how high GM sets the turn on for the fans. I found a tune file for 06 Silverado with a 4.3 it was the same temp settings. I found GM settings are not that great, the 4.3 runs better when the temperatures stays under 210 when it gets over 210 it starts struggling knocking and gets extremely poor gas mileage. My MPG sucked I was getting about 7 mpg. After I lowered the fan on temps and I changed to a 180 thermostat in working on fuel trims & timing I got it up to 19 MPG and that's a mix of in-town and freeway driving.

[wstefan21]

looked at your tune I can't believe how high GM sets the turn on for the fans. I found a tune file for 06 Silverado with a 4.3 it was the same temp settings. I found GM settings are not that great, the 4.3 runs better when the temperatures stays under 210 when it gets over 210 it starts struggling knocking and gets extremely poor gas mileage. My MPG sucked I was getting about 7 mpg. After I lowered the fan on temps and I changed to a 180 thermostat in working on fuel trims & timing I got it up to 19 MPG and that's a mix of in-town and freeway driving.

Yeah. I agree. I've never got around to messing with it because I'm in Florida so I'd rather not drive without the a/c so I haven't driven it hardly at all. That and due to my giant aftermarket radiator I've never even seen it get hot enough to turn on the fans since I've been either highway or short drives. Definitely knocking those temps down at least 10 degrees though for sure.

[wstefan21]

You can do the resistor bypass as I stated earlier to get the fans to work when A/C on. I had the same issue the fans would not come on with the AC I finally came across the solution. A lot of people I talk to you that do swaps don't even put a high pressure switch in the system they just run with the cycling switch and bypass the high pressure. Which that is risky if you hit too high of a head pressure something's going to give and it's not going to be cheap it's not ever with AC. One guy I talk to that has a 5.7 swap in his S10 has been running this AC without a high pressure switch for over 5 years without any issues. I have not personally tried the bypass yet using the resistor. I am too busy this week coming but, the week after that's when I'm going to try it out and I will post my results but I'm 96% sure that it will work. Also will post what variable resistor I will be using. I'm lucky my wife's car uses the same pressure switch so I'm going to take readings off of that one so I can find the right variable resistor to use.

Good luck! I happen to have the port for a high pressure, cycling, and pressure transducer on my a/c setup already so I'm going to give that a try. Electrically speaking I can't imagine why it wouldn't work.

[wstefan21]

Still need to do some troubleshooting, but this might just be a limitation of the pcm hardware I have. I added the switch exactly like the 05+ with:

low reference - C2 80 - factory 6.0l low reference
5V reference - C1 45 - factory fuel tank 5V reference
a/c ref press sens - C2 14 - factory n/a

I'm going to test for 5V reference and resistance value in range tomorrow as well as the direct pressure reading if I can view that on hptuners. Just got too dark and too many dang mosquitos!

Test with "serial cycling" still allowed 12V signal to cycle clutch, but still no electric fan. I'm also going to test with "analog" tomorrow as well and if that works I'll see if the cycling switch input is still doing anything.

[Fast4.7]

You can do the resistor bypass as I stated earlier to get the fans to work when A/C on. I had the same issue the fans would not come on with the AC I finally came across the solution. A lot of people I talk to you that do swaps don't even put a high pressure switch in the system they just run with the cycling switch and bypass the high pressure. Which that is risky if you hit too high of a head pressure something's going to give and it's not going to be cheap it's not ever with AC. One guy I talk to that has a 5.7 swap in his S10 has been running this AC without a high pressure switch for over 5 years without any issues. I have not personally tried the bypass yet using the resistor. I am too busy this week coming but, the week after that's when I'm going to try it out and I will post my results but I'm 96% sure that it will work. Also will post what variable resistor I will be using. I'm lucky my wife's car uses the same pressure switch so I'm going to take readings off of that one so I can find the right variable resistor to use.

Whats going to give is most likely its going to vent alot of your refrigerant. That being said the GMT400 trucks generally had 3 switches. There was one to enable recirculation, one for the low pressure cycling and a high side cutout. The recirculate switch closes at 250 psi.

[Fast4.7]

looked at your tune I can't believe how high GM sets the turn on for the fans. I found a tune file for 06 Silverado with a 4.3 it was the same temp settings. I found GM settings are not that great, the 4.3 runs better when the temperatures stays under 210 when it gets over 210 it starts struggling knocking and gets extremely poor gas mileage. My MPG sucked I was getting about 7 mpg. After I lowered the fan on temps and I changed to a 180 thermostat in working on fuel trims & timing I got it up to 19 MPG and that's a mix of in-town and freeway driving.

I have had people argue against running my Express van that cool for years. ~190F is where it starts to fall off in hot weather. Mine runs 176-182F going down the road with a duramax clutch fan and a 170F thermostat.

[Life is an Enigma]

Good luck! I happen to have the port for a high pressure, cycling, and pressure transducer on my a/c setup already so I'm going to give that a try. Electrically speaking I can't imagine why it wouldn't work.

Here a ac wiring diagram
https://portal-diagnostov.com/en/202...et-tahoe-2004/

[wstefan21]

Got around to messing with it this morning and I think I can pretty definitively say that my pcm model doesn't support the transducer or serial for a/c request at all...

Doesn't matter what my a/c request type is, it always triggers off of 12V input.

Pressure sensor voltage always reads 0V even when shorted to 5V running or stationary (thus why fans don't turn on). I've measured back to the pcm pin to ensure it's actually getting voltage.

The fix is relatively straightforward; I need to replace the pcm with a newer model. The concern is how do I know what models work and which are like mine?

[Life is an Enigma]

Is your PCM the blue red connector or the blue clear connector? Maybe going to the 411 PCM instead of the p01 that you're currently using.

[wstefan21]

Is your PCM the blue red connector or the blue clear connector? Maybe going to the 411 PCM instead of the p01 that you're currently using.

It's a custom wiring harness, but it's wired like blue red. And I think you're right. Mine is missing the sticker, but it's a mystery pcm from a v6 4.3 which original OS was 99.

Really wish there was another way. I've already spent $200 since I had to upgrade the OS and unlock once, but sounds like there's not really other options at this point.

[Life is an Enigma]

Why did you spend so much? You didn't need to upgrade the OS you could have found a similar OS and just did a segment swap between OS. That's what I do so I only spent two credits when I license my PCM and OS then I found other two files with the same OS and swapped over the segments I needed. Here's an example of what I did I used my original OS which was a 4.3 with 4l60e I did a engine segment swap with an 04 Tahoe with a 5.3 then I did a segment swap from a 2004 SSR for the 4l60e and it didn't cost me any extra credits.

The p01 more likely don't have the drivers inside for the AC high pressure switch switch try to find a 411 PCM out of 02 with blue and red connector. I would say go to a P59 then you would have to rewire your red connector because the green connector is pined slightly different from Red connector.