AC Request: How does it work
AC Request: How does it work Something is going on with my AC request Process. When I turn on the AC the car wants to stall. I have the white AC request wire hooked up to my compressor on circuit as directed by PSI and verified that it is getting 12 volts DC when I turn on the AC. I believe I have the AC request circuit turned on in the software of the ECM but I have no way of checking it.
So my question is; when the AC request circuit is working properly, exactly what is supposed to happen?
So my question is; when the AC request circuit is working properly, exactly what is supposed to happen?
I did a little more research and as Steve says the idle air stepper motor (IAC) should be controlling this. But what happens when the AC is turned on? Should the IAC signal change? Can I watch this on a scan tool?
Staging Lane
Joined: Oct 2015
Posts: 89
Likes: 3
From: Northern Idaho
I would think the ECM would detect the low idle and bring the idle up.
You might also get low idle/stalling when turning the power steering at parking speeds.
Doesn't the ECM compensate for this?
You might also get low idle/stalling when turning the power steering at parking speeds.
Doesn't the ECM compensate for this?
TECH Enthusiast
Joined: Feb 2008
Posts: 503
Likes: 1
From: Ventura, Ca
If you have it wired with the ac request going through the PCM and programmed like the factory depending on the compressor type you should be good to go. There are tables to compensate for the ac at idle when it gets turned on. If you have modified the motor with headers, cam, etc then those tables need to be modified for the ac. Large cam motors without ac can even want to stall if not compensated for.
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Verdad Gallardo No idle problems at all with the AC off. It doesn't want to die immediately when the AC comes on, there is a few second delay to answer LS1Nova71. Another wrinkle is that as the car runs with the AC on, the idle problem corrects itself. 69 Ghost may be on to something. My idle with AC off is 800 RPM. I would think the AC commended idle would be higher than that, but how can I check it? DO I have to go back to the tuner or can I check it with a scan tool. I have an Actron 9145.
On The Tree
Joined: Sep 2007
Posts: 196
Likes: 0
Howdy, this is is Justin at Speartech. Without going into much detail the only way to utilize the LS1 Red/Blue PCM with full A/C functionality is to use the relay and 3-pin A/C hi-pressure setup to bump the idle. Normally this isn't needed or done during a swap. What you may be experiencing is an overcharging issue on your A/C. Make sure that you have the proper amount of refrigerant installed.
Hope this helps some.
Hope this helps some.
Justin, thanks for commenting even though I have a competitors harness (PSI). If what you are saying is correct, why would I even have the AC Request wire on my harness? Is it just to control the fans?
TECH Enthusiast
Joined: Feb 2008
Posts: 503
Likes: 1
From: Ventura, Ca
Your Idle tables command idle and idle with A/C and without. There are constants that compensate for the A/C by adding some airflow on top of that. Part of your tuning should be for idle. If it wants to die I would get the idle tuned. Your little cam will have a completely different idle airflow than a stock motor. I am sure it probably needs at least 2g/s added to the idle tables pretty much across the board. BTW Justin is completely wrong. As stated before the pressure sensor is used on variable compressors which are not made to turn off. You need to program your PCM for a fixed compressor with vintage unless you are using the stock compressor. Vintage setups use fixed compressors. GM used both setups and the differences are in the wiring and OS programming.
Staging Lane
Joined: Dec 2011
Posts: 95
Likes: 1
Your Idle tables command idle and idle with A/C and without. There are constants that compensate for the A/C by adding some airflow on top of that. Part of your tuning should be for idle. If it wants to die I would get the idle tuned. Your little cam will have a completely different idle airflow than a stock motor. I am sure it probably needs at least 2g/s added to the idle tables pretty much across the board. BTW Justin is completely wrong. As stated before the pressure sensor is used on variable compressors which are not made to turn off. You need to program your PCM for a fixed compressor with vintage unless you are using the stock compressor. Vintage setups use fixed compressors. GM used both setups and the differences are in the wiring and OS programming.
I'd rather have configurable control than a set binary or trinary switch.
Teching In
Joined: Apr 2004
Posts: 4
Likes: 0
From: Missouri
Would be interested in the outcome of this also because i have the same issue with my 442 . Using the factory olds a/c system & wiring & triggering the a/c request to kick the fans on . Also have a PSI harness .
TECH Enthusiast
Joined: Feb 2008
Posts: 503
Likes: 1
From: Ventura, Ca
The gm service manual for the corvette clearly states that the ecm disengages the ac clutch when the pressure is below 30 psi and above 410, much like the binary switch supplied with the vintage air system. Justin also stated that if you want FULL control of the ac, which includes proper fan control for ac pressure and vehicle speed, you need the sensor.
I'd rather have configurable control than a set binary or trinary switch.
I'd rather have configurable control than a set binary or trinary switch.
Staging Lane
Joined: Dec 2011
Posts: 95
Likes: 1
I would like to see a single diagram of any GM or for that fact any manufacturer including the aftermarket systems that use a pressure sensor with a fixed compressor. You have no more control with that than a switch. Fixed compressors run at lower pressure than 30 when on. Variables run at 30 minimium. They are made to run continuously they do that by the low side pressure. If you try that with a fixed compressor the evaporator will freeze. Don't believe me check to see what the low pressure switch is set to or call vintage yourself. My vintage air ran between 20-24 on the low side. Than means the sensor would turn it off all the time because it is to low. The PCM still controls the fan when the a/c is turned on and vehicle speed. Trinary switches are not needed. Oh and by the way the Vettes like the Camaros use a variable compressor. If you don't believe me look up a 03 S10 4 cylinder and a 6 cylinder. The 4 uses a variable compressor the 6 uses a fixed. The setup is totally different.
The compressor control outside the va control module for a vintage air system is simple. The fixed compressor is either on or off based on the request from the module. The binary switch installed on the high side to protect the compressor since the va control module does not have pressure input.
I did call VA about using the Fbody/Ybody compressor control with my VA Gen 4 system and was told that many of their customers are doing so.
If the ECM is properly calibrated with the code from a vehicle with the pressure sensor input, the pressure sensor is properly wired and the VA system is properly charged, the LS1 ECM can replace the binary or trinary switch. This gives improved control over the fans as well as allowing the ECM to compensate for the load of the compressor something the trinary switch cannot do.
Sorry I missed some of this conversation, but I feel like you guys are getting in a lot deeper than required.
I am running the complete Vintage Air System including their compressor the only link between my engine ECM and the Vintage Air control is one white wire that PSI supplied as "AC Command".
As I understand, when that wire receives a 12 volt signal, it is supposed to;
1. Turn on the fans
2. Command AC idle speed.
I think 69 Ghost has been the closest to answering my question about the Idle speed. I will talk to my tuner about that.
A related question, why would my idle speed be OK with AC on after a few minutes of running with the AC on?
I am running the complete Vintage Air System including their compressor the only link between my engine ECM and the Vintage Air control is one white wire that PSI supplied as "AC Command".
As I understand, when that wire receives a 12 volt signal, it is supposed to;
1. Turn on the fans
2. Command AC idle speed.
I think 69 Ghost has been the closest to answering my question about the Idle speed. I will talk to my tuner about that.
A related question, why would my idle speed be OK with AC on after a few minutes of running with the AC on?