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Help with a potentiometer/rheostat question & project

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acdc220

Apr 27, 2014
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Hello: I am brand new to this forum and very much a novice to electronic projects. I am very excited to attempt an auto related modification project but have questions that are keeping me from getting started with actually cutting the wires and attaching my device (which I have already made up) . Seems I should describe the project and purpose along with what I (think) I know. This involves the electronic throttle control of my truck which as I understand is nothing more than a potentiometer. If what I have recently learned about (pots.) is correct they have 3 wires. One is a power supply into what seems to be a resistance coil winding, and the second is out to ground. The third wire is the "wiper" connection which depending on it's contact position along the coil varies it's output voltage. In MY case the supply voltage from the vehicles PCM (power control module) is 5 volts. Sorry if I am just writing stuff you folks already know. I just want to make sure I cover all the details of what I think I know to see if (YOU) have information to correct my understanding of things. The "project" involves creating what is called an AIC (auxiliary idle control) for the purpose of being able to increase the engine RPM while the vehicle sits running. (I will not get into why this is needed for diesel trucks). My (device/project) for doing this is to use a relay to create a bypass circuit through my own set up of pots. which will actually be used as rheostats. (My understanding is the only thing making (it) a pot. or a rheostat is weather you hook up three wires (as the throttle pedal described above works) or Two wires (one a voltage input & the other wire connected to the "wiper" terminal as the varied voltage output). My plan was to cut the wiper OUTPUT on my existing throttle pedal (that's the middle wire sending varied voltage signals back to the vehicle PCM). Then redirect that out to the relay. (I'm not sure of the proper name for the type of relay I am using but its the somewhat typical type...Terminal's 85 & 86 are the coil that make it work. Then there is 87 - 87a - 30. The N/C (normally closed) is between 87a & 30. When the relay is "activated", 87 & 30 make the circuit and the other (87a) is broken (open) so basically it (toggles) between those two terminals (87 &87a) and (30) can be the output. To keep the "NORMAL" operation of the throttle I would SPLIT the output wire I cut at the pedal, and direct one (leg) of the wire to terminal 87a (input and N/C to terminal 30) Then I would direct the terminal 30 wire back up to the other end of the cut wire at the pedal to continue the circuit back to the vehicles PCM. This takes care of keeping it working "normally".
The OTHER leg (from the output wire which I will have "split" would be directed through to the input of (my) pots. (or I guess to be more precise) my rheostats. There will be two of them wired in series. Then from the output of the second pot. I will direct to terminal 87 of the relay.
So it would seem to me that when I "activate" the relay, the signal voltage back to the vehicles PCM is now controlled by MY adjustment of the "Linear Taper Potentiometers through this (bypass) circuit. Now from my studies there have been some Ohms values associated to what I should have for my pots. The first is supposed to be a 10K resistor (But some suggest that it's better to use a pot. so you can "dial in" for a proper IDLE. Otherwise, because of various reasons & different vehicles you would have to have on hand an assortment of resistors of say between 1K and 10K and any number "K" in between to experiment with to get the correct IDLE reading). (The point is that the vehicle PCM does not just use Zero volts or 5 volts as the starting point for the reference signal voltage for IDLE. So the first pot. in (your) bypass circuit is to "dial in" for the IDLE and then you either tape it into position or glue it there or protect it from moving in whatever manner you choose. Now you go OUT this first pots. "wiper" terminal (the middle one) and send it into the second pot. (this is the one (you) would mount through your dashboard with a knob attached to roll through and make the further varied voltage/resistance values and sent back out the "wiper" terminal of this pot. to terminal 87 on the relay (87 is "making" with 30 when the relay is "on"). Terminal 30 is the relay OUTPUT back to the vehicles previously cut wire back to the PCM.
Now, Having planned all this out in my head and actually wired up the device, NOT to the vehicle YET, (I actually use TWO relays , but that's just another switch that has to be bypassed and "activated" and it's just that, "a switch" so no problem there), I thought I might have MISSED SOMETHING that could make this project NOT WORK the way I expected it to.
And FINALLY I will get around to asking the questions..... The FACT that I am using the vehicles throttle pedal pot. "wiper" as the OUTPUT supply voltage to (my) pots. Is this going to (invalidate) the whole thing from working properly? My thought is (and I have not checked this or am not sure how to check it), What is when my vehicle sits with the throttle pedal at IDLE, the potentiometer is "set up" or designed that it is sending out the LOWEST voltage. (I guess that would mean that the "wiper" connection in the pot. is making contact with the winding at a point where the supplied 5 volts has "encountered" it's maximum resistance, and is sending this REDUCED voltage signal out to the PCM. Naturally, if I am using (cutting and redirecting) this output wire to (my) pots./rheostats , then I am only getting a very low voltage as the SUPPLY voltage to my redirected system. Further increasing the Ohms resistance will just continue to lower the voltage signal back to the vehicle PCM to a point where it is BELOW the lowest number the PCM is programmed to receive as a "normal" number. Either causing the PCM to throw a code (check engine light) or possibly just cause it to stall out.
Now (If) the vehicle is set up to START at the higher voltage at IDLE (say the full 5 volts), then I don't think any of this would be a problem.
So the questions are....Do I need to cut the 5 volt supply line and send that as the redirected bypass line and INPUT THAT into (my) pots. and then "adjust that 5 volts (via. my pots.) and send that back to terminal 87 on the relay then out terminal 30 line back to vehicle PCM?
OR do I only know how to wire it AFTER I find out what the IDLE voltage is out the "wiper" wire from the original pedal pot. ???
Seems if the IDLE voltage is the HIGH output voltage, I could use the "wiper" (center wire) from the throttle as the input to my pots. for supply voltage for the (bypass) circuit. Otherwise I NEED to "tap" that 5 volt supply line (sorry, I think I said I was going to cut it above somewhere) I meant "tap" it.
Gee wizzzzz I sure hope this makes scene to someone!
ANY HELP or response sure would help out here. I just don't seem to know where else to get this kind of information. Thanks so much if you have read this........ Bob
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Hello and welcome to Electronics Point :)

I haven't read your whole post, but I can tell you that you're unlikely to get any help, or even encouragement, if you want to modify the accelerator control of a diesel truck!

Just Google "Toyota unintended acceleration lawsuits" to see one reason. Although the cause of that problem is totally different, the warning is clear. Another reason would be that any modification of that kind would be illegal in any country that has a responsible road transport safety organisation! And it would nullify your insurance.

Also, I would expect that the ECU would take care of any minimum idle speed requirement. Perhaps there are ECU parameters that can be changed by a qualified automotive electrician.
 

acdc220

Apr 27, 2014
3
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Apr 27, 2014
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Wow!! Hello Kris....I'm a little bit disappointed to see that the first reply to my "project" had so much NEGATIVITY towards it. And I might also add...I disagree with much of what you have said. First of all, MANY people are "modifying" their diesel trucks. At least here in the states. Removing mufflers, adding cruise control, Removing the "guts" of the CAT's...The list goes on & on. Now I do realize the "idea" of doing such things is (frightening) to (some) people. Just as climbing trees or telephone poles or even radio/TV antenna towers is to some. Others are terrified to remove the cover plate on a wall electric outlet, let alone change the switch or plug...This list also goes on & on. To suggest that a lawsuit might result from someone being SO DARING as to respond to my post with information (to me) is just a joke. What do you envision? My project results in something (similar) to other throttle malfunction events but it's because of my modification, and after this happens, I am to seek out the electronicspoint.com person from ???(east west Chaskestazzzeanamm Russia U.S.S.R of GUAM?) to file my claim against him for the bad advice he gave me on this forum/Thread? Come on now.....
Also, on insurance. Do you figure I am going to pick up the phone and call my insurance company as soon as I make this modification to (of all things) the THROTTLE CONTROL and ask them if they will still keep me on their billing cycle? Again, Please.....
MANY vehicles NEED to have the idle RPM increased so when the vehicle is parked and running it can maintain the alternator and battery charhe to power electronics. Also others for the supplied oil pressure and PTO's that drive other attached equipment... This is another list that goes on & on.
My Truck didn't have Cruise Control when I acquired it. Now it DOES! And guess what....It didn't cost the$1500 plus the Stealership said they figured it would cost if they could even do it! It cost me about $75 for the Steering column I got at a local wrecking yard and the Positive friendly advice from some other forums on how to do it. And I did NOT call my insurance company to tell them that my vehicle is now equipped with Cruise control that was NOT Factory installed! I also have an alarm system and electronic door locks. Guess who DID NOT install those? Yep, a dealership or a (so called) "professional". Just little old Law breaking, Insurance scamming, environmental nightmare, National Highway department violating ME! LOL
So now that everyone knows me a little bit better....I hope we might be able to get back to some EDUCATIONAL & POSITIVE Can Do , think outside the box type of responses that might actually address the questions I am asking in this thread. I know I wrote a fairly long post and a photo (schematic) is worth a thousand words, but I wanted to cover as much of what I think I know about how it works as well as what I "think I know" about electronic devices like pots/rheostats... And it is my very first post so I really don't know all the rules.
All I DO KNOW is that this "project" IS being done to diesel trucks and most of the boys doing it (and POSTING about how to do it), just seem to leave out a lot of details about wire colors and that/how they change from year to year (let alone month to month). Generally they either just don't have the time to write it all out or they just are not detail oriented or possibly just not good at expressing in words...
Again, Help and information is being sought out here about an Electronics project. The fact that it is a throttle control (ewwww, very frightening!!!)
is really TOTALLY IRRELEVANT! HELP , PLEASE............. Bob
 

acdc220

Apr 27, 2014
3
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Apr 27, 2014
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It's obvious I need help because I cannot even figure out how to log out of this forum!!!!! LOL
 

davenn

Moderator
Sep 5, 2009
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hi Kris

he was just rude and obnoxious

I banned ee2am till the end of the year as well ... we all are sick of his company advertising etc

Dave
 

Kiwi

Jan 28, 2013
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I am an automotive electrician who deals with electronic diesels every day. Thinking that I could help him, I started reading his post but had to give up part way through.
Those posts should be shown to anyone who says that punctuation is not necessary.

He was correct saying that the throttle control on most electronic diesels is just a 5v potentiometer.
 
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