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Need help with simple (I hope) auto circuit

Discussion in 'Automotive Electronics' started by jeepin jeff, Jan 3, 2015.

  1. jeepin jeff

    jeepin jeff New Member

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    Hello, I have been trying to find a solution to a simple issue I have and found my way here with hopes of getting a little help.
    I have swapped in a modern electronically controlled GM engine and transmission into my old jeep truck. Harness has been modified and the PCM flashed, it all works well and I have driven the truck.

    I now have a working tow/haul mode button that simply grounds a pin on the PCM using a simple push mutton switch. Push the button and its in tow mode and the transmission shifts differently, push it again and its out of tow mode, turn off the engine and it resets to tow mode off. Problem is that I have no way to know if its in tow mode or not. What I would like to add is a simple circuit that turns on a light or LED that would indicate that its in tow. There is no output on the PCM that would have gone to a the original trucks dash light, not sure how they did it yet but I think it involved components I didn't need in my setup.

    I think with the help of a relay or two I should be able to push the button, pin 71 on the PCM is momentarily grounded and a dash light (that I install) is on. Push it again and pin 71 is grounded again disabling tow/haul and the light goes out.

    Can anyone help me with a simple relay design for this? I understand the basics and do my own work but I need help with this.
    Thank you,

    Jeff
     
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    The trick is synchronizing the light with the transmission state. Certainly you can use a simple circuit to switch a light off and on but how will you insure that the light indicates the proper state?
    You could conceivable add a push-button that would change the state of the light without changing the transmission to get the two in sync, as long as the transmission doesn't change states on its own. When the engine is tuned off and back on does the transmission come up in the state it was previously in, or is it always the same state?
     
  3. jeepin jeff

    jeepin jeff New Member

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    It comes back to its default state of tow mode off. Tow/haul mode is normally off after stating the truck. Ground the pin with the button and tow mode is enabled, push it again and its off, etc. This works fine today, I just can't see it! The original truck used a body control module and extra circuitry that I don't need for anything else and never added.

    Is there such a thing as a push button back lit switch that will light when pushed yet still act as just a momentary switch?
     
  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    The added circuit can reset whenever the key is turn off/on, so it's state should match that of the transmission.
     
  6. jeepin jeff

    jeepin jeff New Member

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    In stock form pushing the button grounds a pin on the PCM which tells the trans to shift at different rpms and the dash indicator lamp is on. Push it again and the pin on the PCM is again grounded and the light goes out - tow mode off.

    Its really all about momentarily grounding that pin and making the light (that I will have to add) come on and go off each time its pushed. I hope that make sense.
     
  7. jeepin jeff

    jeepin jeff New Member

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  8. shortbus=

    shortbus= Well-Known Member

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    Maybe the light should be tied into the transmission input from the ECM. That would sink it to the transmission. Is the tow mode a high output from the ECM or a low output?
     
  9. crutschow

    crutschow Well-Known Member Most Helpful Member

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    You could use a CMOS CD4013 or CD4027 flip-flop and a few additional parts to do what you want.
    Is that something you could build?
     
    Last edited: Jan 3, 2015
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  10. shortbus=

    shortbus= Well-Known Member

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    But that would only tell you if the switch is on, not what mode tranny is in, right?
     
  11. jeepin jeff

    jeepin jeff New Member

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    The PCM (or ECU) controls the transmission mode internally by programming. All it takes to function is grounding pin 71 on the PCM, which tells the computer its in tow mode and changes shift points. There is no interface between the button and the transmission. I just need a light to come on when that pin is grounded via the push button. It would be easy if they had a pin that was "out to indicator lamp" on the PCM but they don't. It was handled with a Body Control Module in the dash of the Chevy or GMC truck and I didn't add any of that to my project.
    I do appreciate every ones help.
    Thanks
     
  12. jeepin jeff

    jeepin jeff New Member

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    I don't care what mode the trans is in. Its by default not in tow, if I push the button pin 71 is grounded and a light would glow. I push it again grounding pin 71 and it takes it out of tow and the lights out. Push it again 71 is grounded, light is on trans in tow mode.

    I attended a basic electronics program over 20 years ago and did well as a John Deere tech for 10 years before that but have been in the computer/network field the last 15 and cant remember a lot of the stuff I should know, its frustrating! I keep thinking I should be able to come up with a simple way to do this. Most people that do these swaps don't worry about the light, I think it would top off the project. I have tied the cruise control, A/C, 4wd, vss and everything else into the factory jeep dash but this one stupid light is killing me! :arghh:
     
  13. jeepin jeff

    jeepin jeff New Member

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    If you can show me a basic schematic I can build it. :happy:
     
  14. jeepin jeff

    jeepin jeff New Member

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    I might clear up what I probably haven't made clear. Right now I have 1 leg of a momentary switch grounded. The other goes to pin 71 on the PCM or main computer. Grounding that pin changes the tow mode state every time its pushed. The default is off and it goes back to default if the ignition is shut off. I think you guys with late model GM trucks will understand.
    Again, I do appreciate everyones input.
     
  15. jeepin jeff

    jeepin jeff New Member

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    Maybe I just need to turn a lamp on with a push button switch while momentarily grounding another circuit at the same time......
    might be an easier way of attacking it. lol!
     
  16. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Can you measure the voltage at the PCM input when the switch is open?
     
  17. jeepin jeff

    jeepin jeff New Member

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    I'm pretty sure its a low voltage but I'll go out and take a measurement tomorrow if I get a chance.
    Thank you
     
  18. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    I predict it will be 5V.
     
  19. jeepin jeff

    jeepin jeff New Member

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  20. shortbus=

    shortbus= Well-Known Member

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    If you don't care what mode the trans is in why the light? You say there is no connection, ECM to the trans. If theres no connection how does it change shift points in the trans? Just knowing whether the switch has been pushed doesn't mak much sense to me. The body control module told the dash light what mode the trany was in, and pretty sure it was the same circuit that controls the shift mode. without the body module you need to link back to the trany input for shift points.

    1. start engine and it is in default mode -no light
    2. push button for tow mode - ECM tells trany to go to tow mode - light comes on.
    3. shut off engine and on next restart - trany in default mode no light, push button and light come on and trany goes in to tow mode.

    An 'and' logic chip with output triggering a mosfet or transistor to ground the light is whats needed, it's what happens in the body module. Doing it the way you are saying the light will only come on when the push button is held in, as soon as you let the button go there is no longer a connection to ground for the light. Pin 71 is only an input to the ECM unless the push button locks on when you push it how will it control the light?
     
  21. jeepin jeff

    jeepin jeff New Member

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    Its only in tow mode if I push the button momentarily, just like on your new Chevy pickup, which has the BCM circuitry to light a lamp. I don't have that lamp, but it would be nice to have. Which is why asked for ideas.

    All the shift changes are made in the PCM by software/programing. There is no "going between" the PCM and trans.
    Basically a bit is flipped (in PCM) when the pin grounds telling the software to change shift points and pressures.

    I don't know exactly what I need, that's why I started this whole discussion. I have the simple circuit now that grounds the pin and puts it in tow and it works, I just have no visual confirmation. I'm looking for a clever way to add an indicator light that functions off/on every time the pin is grounded, that's all!

    I know I said I don't care if its in tow but what I was getting at was that if the light could somehow be turned on when that pin is grounded and then turned back off again when its grounded the next time it would be the indictor that I'm in tow and what I'm looking for.
    I'm sorry, I probably haven't explained it very well and after a day of Googling the heck out of things nobody else has figured out yet. But somebody will
     
    Last edited: Jan 4, 2015

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