Mustang EFI – Commanded Fast Idle

xcntrk · Mar 4, 2015

EFI gurus; any thoughts on how I can accomplish a commanded fast-idle to be used in certain off-road situations?

I have a manual 5-spd transmission and would like to have an ability to bump the engine idle up in order to 2-foot drive the brake/clutch and have enough RPM to maintain momentum over an obstacle (those of you off-road guys know what I’m talking about). I know there’s secondary throttle cable options out there, but what I’m exploring is how to make this happen at the flip of a switch. Is there a way to manipulate the IAC in order to bump throttle? What about leveraging the AC circuit to fool the EEC into thinking the AC compressor is running and therefore bump the idle?

Here’s my configuration:

302 motor
Mustang 5.0 EFI (mostly all stock)
A9L (variant) computer
Explorer serp & accessories (no AC)
Painless EFI and chassis harness

Anybody actually done this or have a proven approach?

xcntrk · Mar 4, 2015

I was doing some research with the following materials:

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

It appears I can simply supply 12V to EEC pin 10 in order to simulate a maximum AC setting. A few more questions with this:

Is this information accurate?
Anybody doing this today?
Can the AC idle speed be manipulated with a tuner?

;D

skerrybronco · Mar 4, 2015

xcntrk said:
I was doing some research with the following materials:

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

It appears I can simply supply 12V to EEC pin 10 in order to simulate a maximum AC setting. A few more questions with this:

Is this information accurate?

Anybody doing this today?

Can the AC idle speed be manipulated with a tuner?

;D

following this
you could go the manual throttle cable route

ransil · Mar 4, 2015

xcntrk said:
I was doing some research with the following materials:

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

It appears I can simply supply 12V to EEC pin 10 in order to simulate a maximum AC setting. A few more questions with this:

Is this information accurate?

Anybody doing this today?

Can the AC idle speed be manipulated with a tuner?

;D

AC pin is not useful for this, I don't see much of a difference.

I built a PWM controller to switch in to control the IAC, its been in the garage for 2 years its on my list to test out.

surfer-b · Mar 4, 2015

If you were running the later model Explorer you could use the AC and set the RPM at whatever you desire. Contact EFI Guy he may be able set the RPM on the Mustang PCM, not sure

xcntrk · Mar 4, 2015

Just guessing here, but there's got to be a an alternate idle table for the AC setting that either Moates Quarterhorse or TwEECer can modify.

I plan to run a tuner anyway and this would be just another item to customize. I suppose I should wonder over to corral.net or stangnet.com and start poking around...

xcntrk · Mar 4, 2015

Well that didn't take long.. ;D

Turns out there's a few methods within the EEC to customize the AC RPM idle with the most obvious being the "ISC A/C RPM Adder" scaler. You simply specify what additional RPM to bump when 12v on Pin-10 is present. So I think I'll wire this up with a dash switch (Fast Idle) supplying 12v to pin-10. Then later when I start messing with a tuner I can customize this AC value. Would be sweet to get about 1400 RPM at the flip of a switch when rock crawlin.

jw0747 · Mar 4, 2015

here's hoping you don't fry something...

xcntrk · Mar 4, 2015

jw0747 said:
here's hoping you don't fry something...

I second that one ;D

Here's the perfect rocker for the switch as well. I have a 6-bank of Contura switches where this will fit nicely.

EFI Guy · Mar 4, 2015

Nice find on the rocker switch!

Your method is sound, that's how I do it on both the Mustangs and the Explorers.

With multi-position chips I sometimes enable high idle with just a second tune on the Mustang stuff. That way I can use the A/C wire to bump idle slightly for electric fans cycling or A/C if it's actually being used.

70_Steve · Mar 5, 2015

xcntrk said:
Turns out there's a few methods within the EEC to customize the AC RPM idle with the most obvious being the "ISC A/C RPM Adder" scaler. You simply specify what additional RPM to bump when 12v on Pin-10 is present. So I think I'll wire this up with a dash switch (Fast Idle) supplying 12v to pin-10. Then later when I start messing with a tuner I can customize this AC value. Would be sweet to get about 1400 RPM at the flip of a switch when rock crawlin.

How cool!!! I've been running a hand throttle mounted to my shifter, and it's actually kind of a PIA to use, so I haven't been using it much. But so simple to mount a momentary switch to the shifter to get a couple hundred extra RPMs.

Anyone know what the default value of ISC A/C RPM Adder is?

xcntrk · Mar 5, 2015

70_Steve said:
How cool!!! I've been running a hand throttle mounted to my shifter, and it's actually kind of a PIA to use, so I haven't been using it much. But so simple to mount a momentary switch to the shifter to get a couple hundred extra RPMs.

Anyone know what the default value of ISC A/C RPM Adder is?

Here's a couple threads I found about it:
http://eectuning.org/forums/viewtopic.php?p=108630&sid=584f4905b9123f2856ad3cfb2f3e11bd
http://forums.corral.net/forums/94-95-tech/1212379-1994-mustang-how-does-idle-control-vs-c-work.html

Looks like that stock value varies depending on computer. That seems to make sense since some models have more low-end power then others (aka Cobra computer, etc.), but the two values I've seen is 24-48 rpm bump when AC is enabled. That's not very much additional for a rock crawling fast-idle. Not sure if there's value maximum on that scaler. I think at least an additional 400 rpm would be required to 2-foot MT wheelin ;D

Also, it wouldn't be a momentary switch, the computer will need constant power on that pin whenever the function (A/C) is enabled.

70_Steve · Mar 5, 2015

xcntrk said:
... but the two values I've seen is 24-48 rpm bump when AC is enabled. That's not very much additional for a rock crawling fast-idle.

Hmm... That is a little disappointing.. Was hoping for at least 150 rpm or so... And I don't have the tools to, nor do I want to pay someone to, edit the EEC.

xcntrk said:
Also, it wouldn't be a momentary switch, the computer will need constant power on that pin whenever the function (A/C) is enabled.

My thought was to hold the switch ON while two-footing it, then letting it go to drop to normal idle, rather than a normal SPST switch that you have to (remember to) turn off.

ransil · Mar 5, 2015

I had a bronco II with an Idle solenoid on it, look like this or close to it

http://shop.advanceautoparts.com/p/wells-vehicle-electronics-idle-stop-solenoid-e4000/10302080-P

lonesouth · Mar 5, 2015

what about a cable operated cam hooked to the pedal or the throttle body that would change the throttle body stop?

something like the fast idle linkage on a carb, or yea, that...^^^^^^^^

KyleQ · Mar 5, 2015

EFI Guy said:
Nice find on the rocker switch!

Your method is sound, that's how I do it on both the Mustangs and the Explorers.

With multi-position chips I sometimes enable high idle with just a second tune on the Mustang stuff. That way I can use the A/C wire to bump idle slightly for electric fans cycling or A/C if it's actually being used.

Cool - might look into something like that

It would be good for jumping vehicles too!

xcntrk · Mar 5, 2015

KyleQ said:
Cool - might look into something like that It would be good for jumping vehicles too!

Good point! I can see high-idle at the flip of a switch useful for:

Charging an OBA tank (belt driven compressor)
Additional power for jump-starting
Maybe onboard welder - additional power
Poor man's cruise-control ;D

broncoitis · Mar 6, 2015

Personally I think it is a good idea. Hopefully it can be pulled off. Why not just get a simple old hand throttle setup that used to be on the trucks? Cant get much simpler than that I would think. you could even use a 10 sp bike shifter which would give you adjustable throttle if you need a bit more then give it one more click. etc. etc.

KyleQ · Mar 6, 2015

I'd rather have a switch over running a new cable with a lever - simply because it can be done via electronics. I've got enough crap going on - when I switch to an auto I plan on having a button on the shifter for crawling/jumping/welding.

sykanr0ng · Mar 6, 2015

How much more complicated would it become to have two different fast idle settings?

Like a 300 to 500 rpm higher setting and a 600 to 1000 rpm higher setting.

Mustang EFI – Commanded Fast Idle

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