A/C ecm wiring for idle
 

Have a 2003 L59 engine and ecm service# 12576106.

Looking to see who has wired in the a/c somehow to raise idle when the compressor kicks on so you dont have idle drop out.

I found some info on the ls1swap website and was wondering if the highlighted wires are the ones needed or if there is a simpler/1 wire way to do it.

Website shows another way where a 12v activation wire going to pin 17 but does not make it clear if this can be used alone of if I would still need the 5v references.

Just hoping someone has dealt with this and has some info.

Re: A/C ecm wiring for idle
 

your a/c is different than the "pin 17 style". the diagram you show a/c was controlled by class 2 data...
is your throttle drive by cable or drive by wire

Re: A/C ecm wiring for idle
 

Drive by wire

Re: A/C ecm wiring for idle
 

On my e67 wire harness provided by PSI I pinned in the three wires for the A/C pressure sensor. Purchased a filter/dryer with provisions for the GM pressure switch and low/high cutout switch. My research found that pressure sensor is all that is needed for the A/C fan control (PWM in my case) and idle adjustment compensation. The HVAC control module will not be used by me as it just controls the compressor. My low/high pressure switches and A/C on/off are independent of the ECM system. Have yet to get the point of testing it. Will see when I get there.

Re: A/C ecm wiring for idle
 

I already have a trinary switch on the drier, I will have to read up some more but I think it gets low and high pressure from the little VA says about it on paperwork.

Going off the lt1 swap and ls1tech forums I think I may just need 3 wires off the senor like you said for my ecm. 5v reference, low reference and pressure sensor. The other wires look like they would turn on compressor and fans from ecm. Thats already done externally, I just need the ecm to know when compressor kicks on or is about to so it can bump idle up a little.

Does fine driving with a/c only when I come to a stop or are moving slow with near idle rpm of 750.


**Have read for a few hours and it looks like my ecm with the hardware# is able to use the 12v compressor power to pin 17 to bump idle up with some adjustments on the tune as well.

Would need to change it from serial to analog. On those they ground pin 55, still reading to see if thats also needed.

Re: A/C ecm wiring for idle
 

Good stuff. Following along for info.

Re: A/C ecm wiring for idle
 

It looks like my ecm# 12576106 with the hardware# 12570558 is internally wired for pin 17 and pin 55 to be used for ac idle activation, like the 99-02 red/blue ecm.


Ordered the hpvi3 and will try it out in the next few weeks after I get credits loaded.

Tomorrow I may pull ecm and measure ohms at pin 55 onto a 5/12v pin (source) and pin 17 to a ground to make sure I have continuity which would show there is a connection inside. And that the needed resistors and trace are already wired inside my ecm. On LS1swap my exact pcm and hdwr are labeled as having this inside but I will need to verify.

If this works I should be able to jump my 12v compressor (on) wire to pin 17 and then pin 55 to a ground connection on cab.

Turn a/c to analog in HP tuners and my idle should adjust when the compressor kicks on. I sure hope its that easy, then later on I may add in the pressure switch for the ecm if there is any other benefits I find while doing research.

Re: A/C ecm wiring for idle
 

I am curious as to which compressor you are using. Forgive me if you have already mentioned that.

Re: A/C ecm wiring for idle
 

Sanden SD7h15

Re: A/C ecm wiring for idle
 

This IS good info then! I am running the same compressor but I have it on a 5.7 with the red and blue ECM.

Re: A/C ecm wiring for idle
 

There's some more info here

https://ls1tech.com/forums/pcm-diagn...green-pcm.html

Re: A/C ecm wiring for idle
 

Trinary switch might read 2 different pressures, but if you understand how ac works you will understand why it does not read both pressures on the same line. If you have high pressure on the low side you got a problem. If you got low pressure on the high side you got problems. It's a generic sensor that can be used in either or locations.

Re: A/C ecm wiring for idle
 

Its a saftey for the compressor I gather, pressure over/under set amount and it cuts compressor off.

From reading ls1swap and ls1tech the sensors when wired into ecm are used for controlling the compressor on/off and when fans kick on while getting feedback from sensors.

There looks to be several ways to wire it and OS to run depending on ecm, so it can be used in multiple configurations. Control ac and fans or only control idle with compressor running, only control fans etc...

I already have everything wired independently to do this and it has been qorking for over a year. I am just needing the idle to rise slightly when the conpressor kicks on and no throttle being applied.

I have my mpvi3 coming in today and should be able to test it next week sometime.

The theory is I only need to wire compressor 12v "on" to pin 17 and ground to pin 55, change to analog in tune and see if I am getting signal to ecm when I send 12v. If it works I will then run wire through firewall to 12v compressor on wire and ground pin 55.

Re: A/C ecm wiring for idle
 

I looked at this for a while and finally figured out that the green/blue PCMs don’t work with the idle bump, only the RED/BLUE pcms have the proper chipset to bump the idle.

Re: A/C ecm wiring for idle
 

What year is the pcm? C1 pin 58 like in post 1 is class2 serial data... i would try and program class2 data to switch or whatever in hptuners for ac request. And give it ground or power and see what hp tuner reads for ac request. But I've also heard that only red/blue pcm works like that.
Clint

Re: A/C ecm wiring for idle
 

What pcm is it, Google i found blue/green p59 solution.

Ls1tech search
Modifying P59 blue / green Pam's for analog 12v a/c request

Could give that a try if its a p59

Re: A/C ecm wiring for idle
 

It is a P59 and has the circuitry for the 2 wire idle ac bump. There is a guide on LT1 swap I found that shows you what to look for on the circuit board and mine has it.

Just haven't gotten around to running the 2 wires in the last few months.

*** From LT1 Swap
A/C Request: This is a signal input to the PCM indicating the drivers request for a/c activation. There are two ways this is done from GM, a physical wire, a 12v+ positive signal on Pin 17 on C2 PCM connector, and a serial data request over databus. RED/BLUE pcms used 1999-02 all support 12v+ positive signal a/c request. BLUE/GREEN 2003-07 the only pcms that will work with physical a/c request are:

Service Numbers: 12576106, 12581565, 12586243, 12589462, 12602801. If your PCM Service number is different from these listed, a/c request input WILL NOT WORK. There is a modification that can be done to enable the a/c request on unsupported service numbers.

Re: A/C ecm wiring for idle
 

Following this thread for doing the same to my '67 c10 with 2002 LS1 and red/blue PCM.
It will stall sometimes if the AC compressor kicks on when idling or moving slow.
I haven't actually looked yet, but I think the PSI harness already has the wiring?

Re: A/C ecm wiring for idle
 

If your harness doesnt have em, and you need terminals to make a sub harness for the ECU or a/c sensor, Im pretty sure I have extras I could send you
clint

Re: A/C ecm wiring for idle
 

I was headed down the intake, same path.
I have a smallish cam, 112 duration @.050.
Before I could act on the information I acquired, I had the truck tuned.
The idle was no longer an issue.
One does have to add slightly more brake pedal pressure than my Silverado, but that was expected.

Re: A/C ecm wiring for idle
 

So here's where I am at with it, think I need to move some wires lol. I do have AC request show yes when compressor kicks on in VCM Scanner now though and shows no when its off so its getting the signal.

Think I need to read some more on the wiring part of it. I have pin 17 going to the wire coming out of compressor, since it gets 12v when its commanded on.

From doing some more reading it looks like this wire needs to go to the low pressure wire on sensor. Since I have a trinary switch it is the same wire used for high pressure, if it goes too high or too low it cuts power to compressor it looks like from reading around the web.

Tried changing analog and analog cycling in tune and no help with idle while AC is on even though ecm knew the ac was on. It does idle fine with AC its just when you give it a little rev and it drops down too low when coming back down.

So I am missing something in the wiring

I will do some more reading and maybe wire moving when it cools off and see if I can get the idle to work out right. Pulled the wires I added to the ecm for now and set back to serial cycling like it was before.

Making some progress as the ecm can now see I am requesting the AC "ON" but just not getting the idle to not drop. Will figure it out one day even if I have to add low/high pressure sensors later on, hoping I don't have to though.

One thing I did fix in the tune was my gear ratio, it was set for 3.43 and I am running 3.73. Made the 1st - 2nd shift much smoother, from comparing old/new tune it lower shift pressures for 1-2 shift about 5-10% as the rpm less change the lower the rpm but about 10% on the higher rpm side.

Re: A/C ecm wiring for idle
 

Well...I got it figured out and working I think.

All I needed was Pin 17 to ac compressor 12V on from relay
Pin 55 to ground

Running scanner it was picking up the request but the AC clutch engaged and AC Active were both showing no even though it was running.

I pulled DTC through VCM Scanner and had one for Refrigerant Sensor A being open. This is for when the AC is low on freon the sensor would stop the compressor from running. IT also does not let the truck idle up with ac on because it thinks its out of freon....

Simply turned that DTC completely off, it was set for "No MIL Light"

I don't need this one b/c my high/low pressure done by the trinary switch and that is what VA sends with it to wire up as a stand alone.

So now when my AC is running and I come to a stop my idle isn't dropping out like before. If I rev it to about 1300 rpm in Park it will come down to 1k hold for half a second and come back down to 800 like it should.

Will test it out some more later this week, went for a drive and it seemed much better.

Re: A/C ecm wiring for idle
 

I'm glad to hear of your progress.
I have the VA with the trinary switch as well (although I had to purchase it separately)
If getting it to work properly involves breaking open the system and adding additional switches, I was just planning on bumping up the idle speed(s) and living with it.
I have everything so neat and routed, I don't have the heart to tear into it.
Still following for more reporting and testing results!
Thanks for posting your progress.

Re: A/C ecm wiring for idle
 

Depending on the ecm you just need 2 wires added but I can definitely understand not wanting tontear it back apart. I used some weed eater wire, pushed it through the loom and firewall to pull my wire through. Did have to undo the loom all the way to compressor too though, that was a pain.

Pin 17 on my green connect I ran to the Compressor 12V signal for when the compressor is commanded on.

Pin 55 was ran to a ground, this one normally runs to the low pressure sensor and would open with low refrigerant. Running it to ground just tricks the ecm that the sensor (mines trinary) is closed and good.

I also bumped up my in gear idle to 800 from 750, this helped alot.

I am also running the BTR Truck Norris cam, after the tune everything has ran beautifully other than AC on and coming to a stop and idling through a parking lot etc..


Just happy I can run AC and not come to a red-light and begin to drop down to 650 rpm with the idle trying to catch up and stumbling. Only ever stalled a handful of times in the months since doing cam with AC on but it's annoying not knowing if it would happen.

Seems to be taken care of for the most part now, will still tinker with it on/off but I am happy I did it.

Took truck for a drive today and the only time it idled down under what I would consider optimal (750 rpm) was I went into reverse to back into my garage. Idle dropped to 650 and stumbled a little and came back up.

Next I am going to make some adjustments to the BAR settings on HP tuners for when I put it into Reverse. It still wants to drop rpm and from reading you can make slight adjustments to this table and counteract that.

I have around 2k mile since the cam install, will see how the next few thousand miles go. Its just now hot enough around here to need to run the AC all the time.

Re: A/C ecm wiring for idle
 

Good info, thanks!

My PCM is mounted on the back of the core support, so there's easy (relatively) access to it. I could easily add the wires and pick up the 12v signal at the compressor connector, ground would be simple as well.

Mine is a DBC (red/blue) but I suppose the PCM would be looking at the same i/o for a signal, and the IAC would make the adjustments to the idle.

I will need to look at my harness to see what wires may be there, and the PCM pinouts that correspond to the AC signal.
I'll hook up the scanner and check some readings first.
Attachment 2425299

Re: A/C ecm wiring for idle
 

From my reading on this the red/blue all have the needed circuitry inside the ecm.

Green/blue like mine only a certain set of them have the needed circuits inside the ecm, lt1swap has a list of them and mine matched.

They also have the step by step to add the needed chips inside blue/green ecm missing the circuitry to make it work but I was happy I did not have to go through all that.

For my compressor SD709 (04709-VMA) from VA I have AC set to Analog cycling (fixed compressor) in tune and it would need to be set to Analog for a variable compressor.

Re: A/C ecm wiring for idle
 

Make sure pin 55 has a good ground, had a few times the past few days where it would stumble and I would see on the scanner that AC Active and AC clutch enabled would turn off with AC request still on.

I have a stud on firewall I am using as a ground for my engine harness and put that ring connector on it from pin 55 on ecm. Found when I tightened the connector it got pulled into the washer I have against the firewall and wasn't making great connection.


The wire is fairly small 22ga and the ring connector is a 5/16- 16ga I have a bunch of. Solved this problem today, I made a new wire to go between ecm and that ground, crimped the connector on for ecm and soldered as well, did the same for the ring terminal. Used a smaller inner diameter washer for firewall so it wouldnt get pulled into the hole when I tightened. May order some 1/4 - 22 ga ring terminals if it becomes an issue later on. But after soldering it in I think it will be fine.

Ran VCM scanner and did not lose those two values at all while I had the AC request.

I also adjusted the Base running airflow in gear up 0.8 g/s for my normal running temp and this looks to have taken care of the stumble in reverse with a/c on and off.

Will take it out later and make sure but it did even just pulling up driveway and backing down I had no issues like I did earlier this week.

The loss of ground (closed) signal to ecm caused it to turn off ac clutch and deactivate ac like it was low on freon.

This also caused the ecm to drop the idle assist I get even though the AC request was still active since that is a standalone 12v wire from VA wiring.