Adding a high idle switch/knob to a gas engine?

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
I have a 2004 Dodge Ram 1500, 5.7L truck, These trucks have a drive-by-wire system where the gas pedal isn't physically connected to the throttle plate to control it.
Instead it uses a pedal position sensor to tell the PCM what position the pedal is at and adjusts the throttle plate accordingly.

For various reasons I'd like to add in a switch to bump the idle up just slightly, to around 1500 RPM or so. Or have a knob that can adjust the RPM infinitely.

So given the way the system works I'm thinking that if I tie a pot into the APPS (Accelerator Pedal Position Sensor) I would be able to adjust the resistance and control the idle speed with a knob or switched resistor.

The sensor itself is located under my battery, and a cable connects the pedal to the pedal position sensor.

Here is a description of it's operation from the service manual as well as a pinout. I still have to try and find some info about what those sensor wires do and which one of those would be sending the pedal signal to the throttle body. Which means I may try to find a salvage APPS to test and play around with.



The Accelerator Pedal Position Sensor (APPS) is a
linear potentiometer. It provides the Powertrain Control
Module (PCM) with a DC voltage signal proportional
to the angle, or position of the accelerator
pedal. The APPS signal is translated (along with
other sensors) to place the throttle plate (within the
throttle body) to a pre-determined position.
A mechanical cable is used between the accelerator
pedal and the APPS assembly. Although a cable is
used between the pedal and APPS, a mechanical
cable is not used at the throttle body. Throttle plate
position is electrically determined.

APPS Pinout.PNG
 

Tonyr1084

Joined Sep 24, 2015
6,366
Across the two ends of the potentiometer are (likely) power and ground. And power can be 5 volts, or maybe even 3.3 volts - I don't know! But that's something you can determine on your own by reading the voltage across the two ends. The center wiper simply creates a voltage divider which tells the PCM where the pedal is. Idle, half way, or floored. Or anywhere in-between. By adding a resistor to either of the two ends you can affect how it responds. Since I don't know at what position idle is - I can't tell you which side to add a small resistor. And by small I mean very low ohms. Just guessing here, but if the APPS is a 5KΩ Linear pot then adding 100Ω may be close to what you want.

OR if you want to be able to fully adjust the throttle manually (via another Linear Taper Pot) you can but you have to switch between either the foot pedal or the hand throttle. However, you should understand the danger in getting something wrong. My wife drove a "Run-Away" car once. Scared the hell out of her. I was with her and told her to put the transmission in neutral. That stopped the run-away condition and restored full control. We took that car back to the dealership (we were on a test drive and never drove another one of those GM cars). My point is that you can have an unexpected run-away condition and have virtually no time to react. Automotive electronics are held to a very very high standard. I wouldn't recommend you continue this endeavor. But for the sake of answering your question, adding a small resistance to one of the two ends will shift the sensor enough to fool the PCM into thinking you're wanting 10% (for example) power over idle. A switch can either jump out the resistor or let the resistor shift the signal.

I still advise against it though. Especially if you're driving anywhere I drive. Or my family or friends. Or even people I don't even know. Really. Strongly recommend not doing this. But if you have your reasons for doing so then you have your reasons. Good luck. Don't kill yourself. Or anyone else.

One last note - I am willing to bet a small amount that what you want to do is considered illegal in whatever state you live in. If outside the US then I'd think it's probably against the law there as well. Unless you live in some third world country where laws are lax and people bodge cars together - - - . I still say "Don't do it."
 

Alec_t

Joined Sep 17, 2013
12,253
From the APPS pinout data it certainly looks as though there is a double-potentiometer system, possibly for backup if one pot goes haywire.
I'm with Tony on the legality issue. Check Canada's vehicle regulations and also make certain your insurer is happy with the proposed mod to the truck.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
As far as the safety issue is concerned with whoever might be inside or driving the truck, I'm the only person who does, so I know what's been done to the vehicle, what it could affect, and how to handle it should something go wrong.

Someone had mentioned that his work trucks (same model) have a high idle switch installed for offloading cement. So I know there's an aftermarket option somewhere but have been unable to find it and the guy doesn't know either.

Other people who have done it have tapped into their power steering sensor with a relay to simulate turning the wheel which commands the PCM to turn up the RPM..... I don't like this method.

For myself personally I would like to be able to bump up the idle for my sound system at idle, and also during the winter when warming up the truck.

I had the thought that I might be able to just pickup another APP sensor and plug, and tap into the existing system essentially adding a second "pedal" except without an physical pedal.

As for insurance.....well I just don't know, but I'm not jumping into it because I do realize the type of modification that this would be, but I do feel that it would be safer to emulate a second gas pedal than to do anything else.
 

Tonyr1084

Joined Sep 24, 2015
6,366
OK, I'm making some assumptions here, but this is only intended to send you in the right direction. We know from your printout that we're dealing with 5 volts. So my rig is so labeled with voltages as the "APPS NO. 1 SIGNAL" and "APPS NO. 2 SIGNAL" output. Your system likely has more than one APPS pot so you'll have to figure out which is which and which side to put the resistor on. But if 0.25 volts on the SIGNAL line - remember, I'm making assumptions - and your idle is 700 RPM, switching off S1 (allowing the resistor to be in the circuit) your idle may (again, assuming) jump to 1680 RPM. Or there about. Close S1 and the resistor is jumped out of the circuit and the idle returns to normal. Again, yours is probably a dual sense system, so you'll have to put one the way it's drawn and the other will have the resistor on the opposite side (the high side, or the 5 volt side) so that the PCM sees a consistent signal. And you'll want precision resistors and find two that are virtually identical in resistance or you may throw a code.

The "5" and the "0" are in circles representing the voltages at those points (potentially). Again, this is a lot of guesswork on my part.

High Idle Switch.jpg
 
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shortbus

Joined Sep 30, 2009
8,880
Don't know about the Dodge, but most cars/trucks that have air conditioning step up the idle when it is on. Wouldn't it be safer to tie into the air conditioning compressor signals than doing it with the throttle/gas pedal? What I am saying is to fool the engine that the air is on when it's not.

But on second thought, doesn't a modern alternator regulate the charge rate by demand? Not by engine speed?
 

Tonyr1084

Joined Sep 24, 2015
6,366
most cars/trucks that have air conditioning step up the idle when it is on.
I believe the AC sensing is done inside the PCM, so there wouldn't be any readily accessible way to fool the computer. The PCM turns on the signal to the compressor, so it knows when the AC is on and off. No outside sensing. At least that's my understanding of it. I could be wrong because I've been wrong before. Like it says below - I am as accomplished as I am. No more - no less.

[edit] The 5 in the circle is the "5 VOLT SUPPLY" and the 0 in the circle is the "APPS NO.1 (or 2) RETURN". The resistor and switch needs to be placed between the pot and the return. (been examining the data sheet a little closer) And we still don't know the resistance value of the pots. But if the data shows anything significant is that a resistor that is 10% should give you a 2.4 times bump in idle speed. AGAIN - ASSUMING HERE!
 
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shortbus

Joined Sep 30, 2009
8,880
I believe the AC sensing is done inside the PCM, so there wouldn't be any readily accessible way to fool the computer. The PCM turns on the signal to the compressor, so it knows when the AC is on and off. No outside sensing. At least that's my understanding of it. I could be wrong because I've been wrong before. Like it says below - I am as accomplished as I am. No more - no less.
I too have been wrong many times. And things change so fast now on cars, and how one company does things isn't how other do them. That said, it would be as simple as putting a switch in the compressor clutch coil wiring, one that would let the air conditioning be turned on at the dash and not at the compressor.

But most modern alternators don't use speed now to increase output, they are capable of putting out a pretty high output at idle.
 

Tonyr1084

Joined Sep 24, 2015
6,366
it would be as simple as putting a switch in the compressor clutch coil wiring, one that would let the air conditioning be turned on at the dash and not at the compressor.
Excellent point.

The question now is "How much of an increase in idle speed does the thread starter want?" And would the change in idle speed be sufficient when blocking the AC clutch?
 

Tonyr1084

Joined Sep 24, 2015
6,366
Just realized the TS wants to up the current for his stereo. @GRNDPNDR Do you understand that your battery has more current capabilities than your alternator? If you're after a more booming base - they sell capacitors for that purpose. I think this might be a better way to pursue your desire for greater current availability at the amp than modifying your truck to do something your battery can already do. Unless your battery voltage is dropping below 13.6 volts at idle when you're blasting your stereo, I wouldn't worry about it.

Oh, and on a totally unrelated to electronics comment - I used to play rock and roll drums. That was 40 years ago and my ears have been ringing since then. Tinnitus is a condition brought on by prolonged exposure to loud noises and loud music. Here's some advice that if you take it you'll never thank me, but if you don't you'll wish you did: "PROTECT YOUR HEARING!" Failure to do so will result in years of misery. Also due to tinnitus, brain matter can be lost due to atrophy, which can start in the hearing center due to a lack of full stimulation. In other words, I may soon become the village idiot because of not protecting my hearing.

Free advice. Take it and never find out if I'm right or wrong, or ignore it and find out the hard way.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
Just realized the TS wants to up the current for his stereo. @GRNDPNDR Do you understand that your battery has more current capabilities than your alternator? If you're after a more booming base - they sell capacitors for that purpose. I think this might be a better way to pursue your desire for greater current availability at the amp than modifying your truck to do something your battery can already do. Unless your battery voltage is dropping below 13.6 volts at idle when you're blasting your stereo, I wouldn't worry about it.

Oh, and on a totally unrelated to electronics comment - I used to play rock and roll drums. That was 40 years ago and my ears have been ringing since then. Tinnitus is a condition brought on by prolonged exposure to loud noises and loud music. Here's some advice that if you take it you'll never thank me, but if you don't you'll wish you did: "PROTECT YOUR HEARING!" Failure to do so will result in years of misery. Also due to tinnitus, brain matter can be lost due to atrophy, which can start in the hearing center due to a lack of full stimulation. In other words, I may soon become the village idiot because of not protecting my hearing.

Free advice. Take it and never find out if I'm right or wrong, or ignore it and find out the hard way.
audio is not the only reason, just one, also, capacitors are crap. they're snake oil. If the alternator doesn't have the capacity to keep up with the system then adding in another item that needs to be charged isn't helping. There is never a need for a cap.

Anyway, running it a little higher in the winter to warm up is another reason. Typically when I'm in the vehicle warming it up I give it a few mins then idle it at around 1500 until heat starts blowing.
And another reason is to do Seafoam treatments, as I do a couple of times a year. sucking it into the engine bogs the engine down unless you can bump up the rpm to about 1500.
Normally this can be done just by manually moving the throttle control or sticking a clothespin in it......but that can't be done on our trucks because there's no linkage between the pedal and the throttle body.

The APPS is a single unit. It may contain 2 pots, but this is the only unit that controls the throttle. The cable runs from the pedal into the APPS. then the APPS determines pedal position and sets the throttle position to match, so at the very least there isn't a second pot floating around in some unknown location in the truck.

I'm going to grab an APPS to test from the wreckers.....cause the god damn things are $600 brand new. So I can use a salvage APPS to test this theory, and even break into it to see what's inside all for a whopping $10 if that.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
the second pot may actually be a failsafe type device, or someway to compare each pot against the other to determine if one is failing or if they're both in nominal ranges.
 

bwilliams60

Joined Nov 18, 2012
1,419
The APS on this truck is a dual redundancy pedal. It is basically a dual potentiometer and the PCM is looking for the two sensors to be reading the same at all times. If one of them fails, the other one is there as a backup. At this point it will set a code to notify the driver that there is a failure in the APS. This is leading into autonomous vehicles and we will need to have redundancy on a few of our sensors to make sure we are safe at all times. The APS sensor was probably the second sensor to take on this role after the vehicle speed sensor.
As for adding a resistor in line with the potentiometer, this is a bad idea and will set a code. The PCM is looking for specific voltages for idle and high idle and everything in between. The PCM uses a pull-up or pull-down resistor to set the base voltage on the circuit. Changing that resistance will change everything.
The AC system will have pressure sensors or thermistors or both in it to monitor system pressure or temperature which tells the AC system when to turn the AC compressor clutch on. Does the engine speed up when the AC is turned on? If so, it may only be 75-100 rpm.
With your sound system, I would beef up the alternator amperage output or put a smaller pulley on it. Doing this will make it turn faster and change the charge curve to a more suitable one for your stereo. Adding a second battery will also aid in your charging systems ability to compensate for the stereo.
As for winter startup, understanding oil viscosity makes a difference here. Having it idle higher than it is spec'd for will also create more wear on a cold engine. Oil when cold becomes thicker and does not flow as well. You can go to a lighter oil but that will change the properties as it warms up.
What you want to do can be done. They do it in garbage trucks all the time. I won't tell you how to do it but I have given you enough clues. There is a liability here as mentioned in a previous post and I will not be responsible for that. I would leave it as is and work around your other issues.
Cheers and good luck.
 

Tonyr1084

Joined Sep 24, 2015
6,366
There is never a need for a cap.
I've never found a need to have that much base in a car stereo. If MY windows are rattling to pieces every time I pull up next to someone with those stupid base systems then I can only imagine what they are doing to their own car. All the time.

The theory behind the caps is that they can give a sudden surge of energy, and as long as the automotive regenerative system is up to the task it can replenish it during those quieter moments - "Quieter" being a relative term here. 150 decibels can be quiet when compared to 180 decibels. Both can destroy your hearing, which I have some hearing loss myself.

So on the subject of caps - I can't speak with any experience. I only understand the theory behind them. High power with low ESR. But I'm not the expert on this. B.Williams mentioned a higher capacity alternator. But you're throwing a lot of money at a non-existent problem (my opinion; and opinions are like azz holes; everybody has one and most stink). As for warming the vehicle - idle versus slightly higher idle - I've never investigated the difference. But I've found that nearly every vehicle I've ever owned warmed up quick enough. But since you're up in Canada, I suppose it's colder there and warming the car before you get in may be something I'd want to do myself. Years ago I built a rig using a couple relays and a bunch of diodes. Start the car, push a button, turn the key off but the engine keeps running. Step on the brake pedal and shut everything down - unless the key is in the ignition and in the "Run" position. That way I could start the car and remove the key, thus locking the wheel and the shifter so nobody could hop in and drive away. Also, the moment you step on the brake pedal - you kill the engine (without the key). So warming the car in the cold is something I've done before myself. And I'd stay in the house long enough to let things thoroughly warm up. Anyway - - - .

As for adding a resistor in line with the potentiometer, this is a bad idea and will set a code. The PCM is looking for specific voltages for idle and high idle and everything in between. The PCM uses a pull-up or pull-down resistor to set the base voltage on the circuit. Changing that resistance will change everything.
Sounds like someone who speaks on the subject with authority. Only I don't care for the smaller pulley idea personally. At higher RPM's you're possibly over-spinning the alternator. Again, another one of those "Opinions". The higher capacity alternator should suffice as far as the stereo goes. And as for pre-warming the car - they sell heaters for the car you plug in. I've seen heated dip-sticks and magnetic heaters that stick to the oil pan. Seen the kind that goes in the radiator hose line. Warmer water rises and circulates into the engine while colder water circulates out. Start the car and it's already warm. And a lot cheaper than $600.

I only came up with the resistor diagram thinking of it as a large voltage divider circuit. I have no idea how sensitive the PCM is to such changes. But since B says so - I'd opt to follow his advice. Find another way to warm the car and play louder music.
 

Tonyr1084

Joined Sep 24, 2015
6,366
another reason is to do Seafoam treatments
Put a stick between the pedal and the seat. Build an adjustable rig so you can dial in the speed you want. Come to think of it - maybe I'll do that this winter for MY truck. I've heard of seafoam but didn't think it needed to be done twice a year. In fact, over 40 years driving cars - I've NEVER had a need for SeaFoam.

Good luck. Hope you find a quick and cheap solution. I'm thinking rather highly of the stick idea right now. Easy to install, easy to remove before trade-in. And if you DID "Mod" the throttle system then sold the car then someone died because of a failure of your work - well, at least I won't be held responsible.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
I think it's best that we all just fixate completely on the reasons WHY I want to do this, and focus entirely on how those reasons affect the person who's talking about it and their preferences and the conditions under which they use their vehicle.

good thing you guys were here to clarify things though cause I've been redlining my vehicles on a cold start in the dead of winter, and listening to my music at ear piercing levels every time I'm in it.

Someone should start some kind of loud music based competition, I can't be the only one who likes loud music and big sound systems. They could call it the musical quarter mile.....no wait, I've got it, dB Drag Race. ya that sounds better.
 

Tonyr1084

Joined Sep 24, 2015
6,366
OK, everybody focus on GRNDPNDR and what he wants.

Well, I gave you my answer to your desires. If it doesn't work for you then I got nothing else. We all know that you will do what you will do and none of us can stop you. Or talk some good sense into you. Not sure I mentioned my tinnitus from rock n roll drums. My ears ring constantly and without letup. And I'm more sensitive to loud noises. Life sucks (hearing wise speaking). But you will do what you will do. So have at it. You're smarter and more invincible than the rest of us. Just one thing I sincerely ask - don't drive behind me please. Ever. In front of me - I can control what happens to me better than if you're behind me.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
OK, everybody focus on GRNDPNDR and what he wants.

Well, I gave you my answer to your desires. If it doesn't work for you then I got nothing else. We all know that you will do what you will do and none of us can stop you. Or talk some good sense into you. Not sure I mentioned my tinnitus from rock n roll drums. My ears ring constantly and without letup. And I'm more sensitive to loud noises. Life sucks (hearing wise speaking). But you will do what you will do. So have at it. You're smarter and more invincible than the rest of us. Just one thing I sincerely ask - don't drive behind me please. Ever. In front of me - I can control what happens to me better than if you're behind me.
I'm not sure what you're getting at here, if you know that I'll do whatever I want to do and nobody can stop me then why are you trying?
Yes, you've mentioned tinnitus, but I'm not really following along with why it's a problem. You see YOU did that to yourself, I never once said that I needed ear piercing music and that I had to listen to it that loud, you all just assumed that based on what I said..... but this goes back to fixating on these things.

the real fact of the matter is that I want a high idle switch. That's it. It doesn't matter why but everyone just keeps going back to the why.
 
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