Really Stupid or Really Creative?
Thread Starter
TECH Junkie
Joined: Dec 2012
Posts: 3,614
Likes: 1,326
From: Iowa
Really Stupid or Really Creative? Ok, First I'd like to ask if anyone has ever done this (I doubt it...but we will see)
Second...Will it even work?
I was looking at the idle tables and recognized that they have different target idle speeds for A/C on vs A/C off. And the table maxxes out at 8k RPM!!
My Foxbody LS swap doesn't have AC...so could I wire in a switch to tell the ecu that the A/C is on and set that part of the table to a higher rpm to use a a poor man's way to make launching more consistent. For example if I want to launch at exactly 3000 rpm every time, could I set that idle table to 3k when the AC is on? then I would just pull to the line, stage, flip my "launch control" switch and hold the brakes. Then have the ECU do the hard work of keeping the car right at 3000rpm for me to launch.
Second...Will it even work?
I was looking at the idle tables and recognized that they have different target idle speeds for A/C on vs A/C off. And the table maxxes out at 8k RPM!!
My Foxbody LS swap doesn't have AC...so could I wire in a switch to tell the ecu that the A/C is on and set that part of the table to a higher rpm to use a a poor man's way to make launching more consistent. For example if I want to launch at exactly 3000 rpm every time, could I set that idle table to 3k when the AC is on? then I would just pull to the line, stage, flip my "launch control" switch and hold the brakes. Then have the ECU do the hard work of keeping the car right at 3000rpm for me to launch.
Thread Starter
TECH Junkie
Joined: Dec 2012
Posts: 3,614
Likes: 1,326
From: Iowa
Thanks for the honest feedback guys..i had a feeling that would be the verdict. Can someone explain why it wouldn't work just so I can better understand.
It was mentioned that as soon as I touched the throttle the timing tables would close, which would be true and I understand that...but why couldn't my VE table just take over from there and already be into the meat of the table at 3k?
I just want to understand better the "ehy" behind it
It was mentioned that as soon as I touched the throttle the timing tables would close, which would be true and I understand that...but why couldn't my VE table just take over from there and already be into the meat of the table at 3k?
I just want to understand better the "ehy" behind it
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Restricted User
Joined: Mar 2014
Posts: 7,192
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From: Ohio
Use a 5 pin relay. Pass your VSS signal through it in the normally closed circuit. Put the ground for the relay on a momentary button, which will disconnect the VSS signal when pressed. Disable all of your DTCs for VSS so it doesn't force your transmission into limp mode.
Go to the Fuel -> Cutoff, DFCO tab. Top left, under Bad VSS Cutoff/Resume, put your RPM where you would normally put it with a 2 step. Do your burnout, and then roll up to stage holding down the momentary. It'll force a rev limit to whatever you put there.
Let go of the button, launch.
Free fuel cut 2-step rev limiter. Works fine N/A. Things get sketchy on a turbo setup.
One day I'll get around to uploading the 10+ hours of videos to YouTube that I made for stock PCM tricks.
This was a freebie. You're welcome.
Go to the Fuel -> Cutoff, DFCO tab. Top left, under Bad VSS Cutoff/Resume, put your RPM where you would normally put it with a 2 step. Do your burnout, and then roll up to stage holding down the momentary. It'll force a rev limit to whatever you put there.
Let go of the button, launch.
Free fuel cut 2-step rev limiter. Works fine N/A. Things get sketchy on a turbo setup.
One day I'll get around to uploading the 10+ hours of videos to YouTube that I made for stock PCM tricks.
This was a freebie. You're welcome.
Thread Starter
TECH Junkie
Joined: Dec 2012
Posts: 3,614
Likes: 1,326
From: Iowa
Thanks Joe! That sounds like exactly what I'm going for! Stock ecu tricks are the best!
When you open the relay, does the ecu see the open circuit and that is how it knows the sensor is bad?
I'll have to give this a shot! Could I use pin 21 on the red connector?
When you open the relay, does the ecu see the open circuit and that is how it knows the sensor is bad?
I'll have to give this a shot! Could I use pin 21 on the red connector?
Restricted User
Joined: Mar 2014
Posts: 7,192
Likes: 109
From: Ohio
I should probably make up a diagram at some point. It was trial and error and took me a few tries, and I can't remember off of the top of my head exactly how it was wired. I keep a notebook of all of this stuff at home, maybe I'll get around to checking it when I get home.
I do remember wiring the relay backwards, so that pin 30 on the relay for the common circuit went to the PCM and not the VSS sensor, that way by powering the relay I could switch what was actually being used as the input that was being sent to the PCM. A single 5-pin relay and a momentary took care of the whole thing.
I do remember wiring the relay backwards, so that pin 30 on the relay for the common circuit went to the PCM and not the VSS sensor, that way by powering the relay I could switch what was actually being used as the input that was being sent to the PCM. A single 5-pin relay and a momentary took care of the whole thing.
On The Tree
Joined: Aug 2004
Posts: 104
Likes: 2
From: Central, IL
I know that I really enjoy learning about these little tricks that can be done with stock PCMs. Thanks for all that you have shared over the years. 
Years back I tested using the fuel cut rev limiter as a 2 step in the GTO. The results were wholly unimpressive. The fuel cut just does not hold the rpm steady like a spark cut does.
Restricted User
Joined: Mar 2014
Posts: 7,192
Likes: 109
From: Ohio
It works about as well as a standard rev limiter.
It keeps you from going over a certain RPM but doesn't do the greatest job of preventing you from falling below a certain RPM.
Set the high/low to only be 20 RPM different and it will react faster. I haven't tried anything lower than 20 and was satisfied with it. It will keep you in a somewhat steady RPM range for a launch.
It keeps you from going over a certain RPM but doesn't do the greatest job of preventing you from falling below a certain RPM.
Set the high/low to only be 20 RPM different and it will react faster. I haven't tried anything lower than 20 and was satisfied with it. It will keep you in a somewhat steady RPM range for a launch.