How to pushbutton start cars hold IGN on?

Discussion in 'The Projects Forum' started by Steve C, Nov 22, 2009.

  1. Steve C

    Thread Starter Active Member

    Nov 29, 2008
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    I can't edit titles, but it should read "How do 'pushbutton-start cars' hold IGN on?"

    Simple question, complicated answer.

    It seems to me, that a relay is not what the OEMs would use. They aren't the most reliable component money can buy, and I don't imagine it would take more than a couple relay failures when the car is going 80 on the freeway to end in a couple high profile lawsuits against car makers.

    So what do you guys think? I'm not privy to what the OEMs do, so I don't know. DO they simply use relays? What component could be used to hold the ignition on that would be reliable?
     
  2. ke5nnt

    Active Member

    Mar 1, 2009
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    I am not an expert in this field at all... so take my response as pure speculation.

    Vehicle "ignition" systems are VERY basically broken down into 3 things...

    Starting the car... power is applied to several systems so that 1. the starter motor starts cranking the engine; 2. the fuel pump is delivering fuel to the engine; 3. the vacuum system is delivering air to the engine. When you're thinking of push button ignition systems, this is the only role it would play. Basically to tell the battery to run the starter motor, once the engine is running, it cranks itself via combustion.

    Engine is running... Alternator is supplying the power to all the systems needed to keep the vehicle running, i.e. the fuel pump, vacuum system, cooling system, etc. The push button wouldn't be used here, it would be in "standby" again just like if the vehicle was off.

    Shutting the car off... In a traditional key ignition, when you turn the key to "off" the engine loses air, fuel, and heat. The 3 things required for something to burn. Air and fuel are obvious, your heat source is the spark plugs, which would stop sparking at this point.

    Even in push-button ignition vehicles, a key is still required to allow ignition to occur, so the button at this point would be useless as well. However, I suppose you could design a similar system that used a button to instruct the engine to stop, because essentially, that's exactly what you're doing when you turn the key to "off".

    That's my thought...
     
  3. Steve C

    Thread Starter Active Member

    Nov 29, 2008
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    I'm sorry, from the basic answer it is clear I've given a poorer impression than I had hoped.

    To be clear, what I am interested in is information about how auto manufacturers hold IGN on in the ignition harness. This is traditionally done through a mechanical key switch.

    But since the standard ignition switch has been removed in modern pushbutton start cars, (mazda anyways, not nissan or BMW), IGN must be held on electronically. presumably, some sort of contactor.

    But I struggle with that notion because of potential lifetime issues with contactors. Plus, simple failures in what holds that contactor on can fail, unlike mechancal ignitions with ignition locks. If that contactor drops out, the car dies. If that happens on the freeway, injury can result. It seems like there is much more risk of legal action against an OEM because of higher risk of failure.

    But I am not privy to the all of the knowledge and experience the folks here have access to concerning component reliability, or other methods of accomplishing the task of energizing the IGN circuit in a reliable way. Nor do I have access to actual schematics of particular cars (2009 Mazda 6 for example) that both: do not use mechanical keyswitches to energize IGN and are also not a hybrid.

    That was the information I was hoping to get from the folks here. Either how the OEMs DO do it, or how it might be done reliably, either using contactors or another method.

    Is that more clear?
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Any modern car has a half dozen uControllers imbedded in its various subsystems(ECU). It is trivial to add an "ignition ON" state to an internal variable in software. The actual powering ON/OFF of the ignition module is likely done by a FET which is controlled by an output from the ECU, (maybe even via a CAN network).
     
  5. Steve C

    Thread Starter Active Member

    Nov 29, 2008
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    Isn't that a really big FET?
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The starting contactor (relay capable of switching >1000A) is likely one of the last remaining relays in the latest model cars. Everything else is switched either directly by switches, or indirectly by FETs. The ignition module for cars draws only a few Amps.
     
  7. Dan-O

    Active Member

    Jul 26, 2009
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    Current automobiles and trucks still use relays, lots of 'em! :)
    Relays do fail, but it's rare.

    Most OEM pushbutton start cars still use a relay to start, for accessory power and for ignition power supply as well.

    Some cars still use high current ignition switches, and some use relays to supply the power using a low power switch on the steering column. (Mine uses relays)
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How current? My Toyota and my GMC pick-up use FETs to turn on/off the headlights, dome light, turn flashers, etc.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Cars/trucks that stop running are NOT a problem/hazard.

    It's the cars/trucks that cannot STOP that are a problem/hazard.
     
  10. ke5nnt

    Active Member

    Mar 1, 2009
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    :D lol, I agree. luckily cars continue to coast to a gentle rolling stop if the engine quits. unless your gentle stop is made sudden by a telephone pole because you started concentrating too hard on what caused the failure and too little on where you were going.
     
  11. Dan-O

    Active Member

    Jul 26, 2009
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    Agreed, my car has a body module that does all that stuff too, but the start and ignition circuits still use relays controlled by the ignition switch.

    As with anything, I'm sure there are cars that use FETs for everything, the cars in my lab (newest is an '08) still rely on relays for the critical circuits mentioned above.
     
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