AC Request

[NC_John]

Will a +12V signal (from the compressor turn-on signal for example) to the AC request pin be enough for the pcm to to consider the ac is on (trigger the higher idle)?

There are numerous inputs and outputs for the AC but since I am controlling through the vintage air I don't want to mess with all that. My tuner has asked me to provide this signal (ac on) to the pcm.

Thanks

[First c10]

Hey john. I looked into this also
I am running an e38 ECM. I will have to use the stock pressure switch and wires from pins 12/13/43 (I think). For me this will control cooling the Fans and idle control. Or at least I hope.
The request wire (59 I think)just turns on a relay that went to the compressor. I am going to use the old air product (my ac) binary switch to control the compressor.
But if anyone knows of a single wire that works I want to know as well.

[First c10]

Check this out

http://67-72chevytrucks.com/vboard/s...d.php?t=776599

[ls1nova71]

Originally Posted by NC_John

Will a +12V signal (from the compressor turn-on signal for example) to the AC request pin be enough for the pcm to to consider the ac is on (trigger the higher idle)?

There are numerous inputs and outputs for the AC but since I am controlling through the vintage air I don't want to mess with all that. My tuner has asked me to provide this signal (ac on) to the pcm.

Thanks

Does your harness even have a 12v A/C request wire? Trucks after 2002 do not, so it's not that simple.

[NC_John]

Originally Posted by ls1nova71

Does your harness even have a 12v A/C request wire? Trucks after 2002 do not, so it's not that simple.

Well, it may not have the wire in the harness but it will have the input. I updated a cruise control thread you were helping me with- we had to reprogram my truck pcm to that of a 2004 corvette z06 to enable my dbw cruise control- if you recall, the truck program could "see" all the correct cruise control inputs as I triggered them while viewing through a tech I, but didn't know what to do with them as the truck was never offered with my configuration of manual transmission and cruise control. We flashed over the Z06 program and the cruise now works perfectly.

Sooooo....now my Z06 brained pcm has some new interest in what the AC is doing. On the green side, wire 17 is now AC request. I haven't looked at the harness but am doubtful that Speartech has a wire there (PSI has one on theirs). I do however have the parts (pins and related pieces) and did run a spare wire with the harness that I can allocate to the task.

So, still not being completely familiar with the logic side of all this I am needing to know that if I do run a 12v hot signal from the vintage air compressor on signal, will it be enough for the pcm to begin the ac ON section of the program - I think there is a second idle map that it reverts to under the added load. I am not seeing any real problems the way it works now, when the compressor engages, the idle recovers, but the tuner is insistent on me providing him his signal so I'll do it.

[NC_John]

Originally Posted by First c10

Hey john. I looked into this also
I am running an e38 ECM. I will have to use the stock pressure switch and wires from pins 12/13/43 (I think). For me this will control cooling the Fans and idle control. Or at least I hope.
The request wire (59 I think)just turns on a relay that went to the compressor. I am going to use the old air product (my ac) binary switch to control the compressor.
But if anyone knows of a single wire that works I want to know as well.

I suspect we have different goals with our projects. I am currently (and successfully) running my vintage air completely independent of the pcm. My fans are controlled by an entropy fan controller setup (which is a pretty slick setup in itself).

My tuner just wants the pcm to know when the ac is on so he can tune around it. Maybe necessary, probably isn't, but if I can do it easily enough and it will make him happy, I'll do it.

He actually wants me to provide the pcm with ALL the factory ac inputs but there is no way I am going through that ordeal. It's completely unnecessary with the vintage air doing the thinking. He's way more versed doing work on the newer stuff and is not as familiar with what's required with a swap.

Sounds like you might be trying to do the ac control through your pcm?

[sprint_9]

What First c10 describes is how gen 4's get the signal you are looking to get with your gen 3.

More or less the same goal but different ways of getting there.

[ls1nova71]

Looking at the air conditioning schematics for a 2004 Corvette, there is no mention of a 12v A/C request, but there is a serial data line from the A/C module to the PCM. However, in the PCM schematics there is one at C2 Pin 17 that says A/C request then in parentheses 'C60 only' so not sure what that means? But, C2-17 is where the request in on the older PCM's that had 12v requests, so it may work. You'll have to keep us updated, because I'd like to know if it works.

[87chevy.com]

Originally Posted by NC_John

I suspect we have different goals with our projects. I am currently (and successfully) running my vintage air completely independent of the pcm. My fans are controlled by an entropy fan controller setup (which is a pretty slick setup in itself).

My tuner just wants the pcm to know when the ac is on so he can tune around it. Maybe necessary, probably isn't, but if I can do it easily enough and it will make him happy, I'll do it.

He actually wants me to provide the pcm with ALL the factory ac inputs but there is no way I am going through that ordeal. It's completely unnecessary with the vintage air doing the thinking. He's way more versed doing work on the newer stuff and is not as familiar with what's required with a swap.

Sounds like you might be trying to do the ac control through your pcm?

Lol, yes you can run a standalone ac but well, the dang pcm just wont know it's on... so your tuner probably does know his stuff. More than likely, he will just tell you, since you couldnt connect the ac system like factory, dont bother me with ac on idle problems.

[ls1nova71]

I have 2 swap cars both with over 100k on them and both have vintage air and never had any issues running the A/C as a stand alone system.

[First c10]

Originally Posted by ls1nova71

I have 2 swap cars both with over 100k on them and both have vintage air and never had any issues running the A/C as a stand alone system.

I have 2 as well , 1 with factory ac and another with old air hurricane system. They run fine but have a little stumble when coming to a stop with the ac on or cooling during idling for an extended period of time with ac on.
My current project I would like to untilize the factory pcm to control cooling fans so I don’t have to add other controllers to turn fans on. Which will also keep the idle issue.
If the pcm can get the info it can regulate idle and cooling fans.
I am trying to keep it simple as my system is matched. I do know segment swaps are a little tricky. Let us know what works.

[NC_John]

Originally Posted by ls1nova71

Looking at the air conditioning schematics for a 2004 Corvette, there is no mention of a 12v A/C request, but there is a serial data line from the A/C module to the PCM. However, in the PCM schematics there is one at C2 Pin 17 that says A/C request then in parentheses 'C60 only' so not sure what that means? But, C2-17 is where the request in on the older PCM's that had 12v requests, so it may work. You'll have to keep us updated, because I'd like to know if it works.

I will. It might be a while- I just had major rotator cuff surgery so it might be a bit before I can crawl under the dash. I'll get that tech II from the dealership again when I do it and see if the pcm can see the input change.

[NC_John]

Originally Posted by 87chevy.com

Lol, yes you can run a standalone ac but well, the dang pcm just wont know it's on... so your tuner probably does know his stuff. More than likely, he will just tell you, since you couldnt connect the ac system like factory, dont bother me with ac on idle problems.

My tuner has been having fits with the driveability of my build. Parking lot (1st gear) was a nightmare with jerking and bucking. Coming down through the gears on decel to a stop was also a problem with the truck almost always stalling at the bottom of first gear. When it didn't stall the idle would instead hunt up and down for a while before it would settle down normally. He finally got it sorted out and its about 95% right now. His biggest complaint has been the limitations of the gen III pcm compared to the newer ones.

They've (builder and tuner) have got a ridiculous amount of time in the tuning already so even though I haven't experienced any idle issues with the ac, I'm happy to provide him with this input.

[NC_John]

Originally Posted by First c10

Hey john. I looked into this also
I am running an e38 ECM. I will have to use the stock pressure switch and wires from pins 12/13/43 (I think). For me this will control cooling the Fans and idle control. Or at least I hope.
The request wire (59 I think)just turns on a relay that went to the compressor. I am going to use the old air product (my ac) binary switch to control the compressor.
But if anyone knows of a single wire that works I want to know as well.

I just circled back to this project last weekend and realized what you were telling me. I wired up a ground signal switched by the vintage air compressor on signal to the A/C request pin and the Tech 2 didn't recognize it (did not show an AC request when the system was running).

I went into the schematics again and saw that the A/C request is an output, not an input and the actual AC request for an 04 corvette programmed PCM has to come from a bcm datalink.

I see the information in the thread about installing another pressure switch and getting the pcm some data that way but don't think the juice is worth the squeeze....

Seems to be running fine when the AC cycles anyway so the tuner gets what he gets.

[Wildkarde]

The later model gen III ecms, the ones with blue and green connector colors do not have the ability to accept an ac request signal any way other than via canbus from the BCM. Or so the experts in how these things work internally have always said.

[NC_John]

Originally Posted by Wildkarde

The later model gen III ecms, the ones with blue and green connector colors do not have the ability to accept an ac request signal any way other than via canbus from the BCM. Or so the experts in how these things work internally have always said.

Seems those experts are right.

I guess the gen III ecm/pcms are extremely limited in capability and thinking power compared to the newer gens. If I do another swap it'll be with a newer brain for sure.

[NC_John]

I just stumbled across this:

Request Input

A 12V signal wire is monitored at pin 17 of the red PCM connector. With no voltage applied, the circuit is open and the PCM does not attempt to turn on the A/C compressor clutch. When 12V is applied to the circuit and based on the pressure in the system, the PCM commands the A/C compressor to turn on.

Source is https://www.lsenginediy.com/upgradin...tioning-guide/

This contradicts the other stuff I've seen and heard though. I have a switched ground going to pin 17 on my green connector. I wonder what the PCM will see/show/do if I send 12V+ instead of the ground...... I'd sure hate to hurt the PCM.

[First c10]

After much research the genIV stuff has to have ac request via canbus. No other way for the PCM to know the ac is on.
I found a modular for 300$ for ac request for gen4s. NOT.
Dakota digital should offer a BIM for it. I am going to call and ask about it.

[NC_John]

Originally Posted by First c10

After much research the genIV stuff has to have ac request via canbus. No other way for the PCM to know the ac is on.
I found a modular for 300$ for ac request for gen4s. NOT.
Dakota digital should offer a BIM for it. I am going to call and ask about it.

I don't think the gen III is smart enough to recognize anything more than +12V or ground. That article I posted the link to has me wondering if I should try switching positive 12V to the pcm for the AC request signal instead of the switched ground I tried that the pcm didn't see (through a Tech 2). I'm just nervous about sending 12V that it doesn't want to see and hurting the pcm. The schematic I am using as a reference shows ground but its a 3rd party schematic and not from GM. I can't seem to get a definitive answer anywhere.

[First c10]

Originally Posted by NC_John

I don't think the gen III is smart enough to recognize anything more than +12V or ground. That article I posted the link to has me wondering if I should try switching positive 12V to the pcm for the AC request signal instead of the switched ground I tried that the pcm didn't see (through a Tech 2). I'm just nervous about sending 12V that it doesn't want to see and hurting the pcm. The schematic I am using as a reference shows ground but its a 3rd party schematic and not from GM. I can't seem to get a definitive answer anywhere.

Tough call. But all we have is 3rd party information until we try it.
The link you posted does say 12+ positive. But for me it’s the wrong generation.

[Wgesnerjr]

This is expensive but may solve the serial data issue.
http://www.rpmextreme.com/Product/23...st-Module.aspx

This one says you have to specify 12volt or ground.

[First c10]

Originally Posted by Wgesnerjr

This is expensive but may solve the serial data issue.
http://www.rpmextreme.com/Product/23...st-Module.aspx

Yes this what I found. I just can’t see paying that. It has to be just a few diodes and resisters. I never looked into what the canbus actually is or how it communicates.

[Wgesnerjr]

From my basic understanding, canbus sends many different signals down the same wire. I think about it like an interstate highway. People in a common area funnel on to the highway, go to another area then exit and go to different destinations. Everyone dosent get their own road to where they want to go, but must share a common route.

If you think about it, cars used to be wired with a direct connection between a switch and a light or motor. Now with canbus each item can use a common wire to get their signal to where it needs to go. That's how I understand it. Someone correct me if I'm wrong.

However, my buddy and I wired in a factory bluetooth phone rearview mirror into his wife's Grand Cherokee so she could have hands free through the audio system. We just spliced the data wire into the main serial line and it all worked. How the signals know where to "exit" the common wire is still a mystery to me. But once connected, it found the stereo as its destination.

I hope this little jewel of a module gets cheaper over time. $300+ is a little steep when you "can" go without it.

[NC_John]

Originally Posted by NC_John

I don't think the gen III is smart enough to recognize anything more than +12V or ground. That article I posted the link to has me wondering if I should try switching positive 12V to the pcm for the AC request signal instead of the switched ground I tried that the pcm didn't see (through a Tech 2). I'm just nervous about sending 12V that it doesn't want to see and hurting the pcm. The schematic I am using as a reference shows ground but its a 3rd party schematic and not from GM. I can't seem to get a definitive answer anywhere.

Circling back, again.

I went through the hassle yesterday of changing the switched ground to AC request to a switched +12V signal. I confirmed its operation through multiple tests with a multimeter. When the ac compressor engaged, the pcm got a 12V signal.

Unfortunately, as seen though a GM Tech 2, the pcm did NOT see (or acknowledge) it as an AC request.

Maybe something has to be turned on through HP Tuners but I'm giving up on this for now. I wish I understood the logic in how the system turns on the AC (how all the inputs and outputs work together) but there are too many gaps in the information available for me to figure it out.

[87chevy.com]

Is this a 2002 and lower pcm? If yes, 12v to the ecu is will then the compressor on IF the pressure high pressure switch is installed.. or of the signal wire is grounded.

If it's the 03 and newer think that's can bus and for some reason the aftermarket is being lame on solutions.
Clint