Page 1 of 2 12 LastLast
Results 1 to 10 of 17

Thread: Key fob

  1. #1
    Join Date
    Nov 2013
    Location
    Central Scotland
    Posts
    428

    Key fob

    Just replaced my 451 key fob with a new CR2016 small button battery, as the side lights were fast flashing with a lot of flashes, when opening or closing button pressed. Its fine now.

    This is the signal to tell that you need a new battery, which i have now sorted, but my question is this....

    My question is....How does the car itself know that the key fob battery needs replaced just before it runs out of battery power, and creates this fast flashing?

    PS......for anyone wanting to know how to replace key fob battery, its very simple, just insert a 10 pence coin in the slot at the back of the key fob, twist the coin, and the two halves of the fob open up, then remove battery, put in the new one and click the two halves closed. sorted. Takes about a minute to do.
    Last edited by colin2x2; 01-12-2017 at 05:05 PM.

  2. #2
    Join Date
    Mar 2012
    Location
    Burnley
    Posts
    1,270
    I replaced mine yesterday for the same reason We will have to see which lasts longest. I use Renata batteries.

    The key transmits the fob battery status every time you use it. Once it is marked as low a bit changes so the car knows it's time to change it.

    It's so easy to write how to do it but it took me over 20 minutes to clip the goddam thing back together.

  3. #3
    Join Date
    Jan 2011
    Location
    uk
    Posts
    774
    The car works it out because as the battery voltage decreases , the output oscillator frequency of the key fob shifts a little ... this is how the car works out the battery is on the way out.

    The key doesnít transmit its status , other than sending the function command and the next code sequence generated by a pseudo-random number generator.
    The car can work out itís a known key because it knows the last pseudo-random number spat out from it... as itís pseudo-random it knows what the next code sequence will be.
    When you program the car , youíre just telling it the next number received will be a genuine number, after that the receiver know the next number from that fob. In a likewise way it can store another key.
    Last edited by reinaldo_is_spam; 01-12-2017 at 05:50 PM.
    You're so money supermarket and you don't even know it!

  4. #4
    Join Date
    Nov 2007
    Location
    BANCHORY
    Posts
    3,149
    Interesting explanation Reinaldo.

    Question for you: Why have my key fob batteries not yet expired? They are all original from 2002 and most likely made in 2001 or earlier since car was first registered in the UK in April 2002. Smart 450 Pulse Cabrio Cdi.
    Drives a Smart Cdi - 65 to 85 MPG

  5. #5
    Join Date
    Jan 2011
    Location
    uk
    Posts
    774
    Quote Originally Posted by tolsen View Post
    Interesting explanation Reinaldo.

    Question for you: Why have my key fob batteries not yet expired? They are all original from 2002 and most likely made in 2001 or earlier since car was first registered in the UK in April 2002. Smart 450 Pulse Cabrio Cdi.
    Youíve been lucky , extremely so ... itís a wonder they havenít at least started to bulge

    Are they alkaline or lithium ?

    Why not just replace them
    You're so money supermarket and you don't even know it!

  6. #6
    Join Date
    Nov 2007
    Location
    BANCHORY
    Posts
    3,149
    Quote Originally Posted by reinaldo_is_spam View Post
    You’ve been lucky , extremely so ... it’s a wonder they haven’t at least started to bulge

    Are they alkaline or lithium ?

    Why not just replace them
    They are all Varta. My original spares are also Varta.
    Drives a Smart Cdi - 65 to 85 MPG

  7. #7
    Join Date
    Mar 2012
    Location
    Burnley
    Posts
    1,270
    Quote Originally Posted by reinaldo_is_spam View Post
    The car works it out because as the battery voltage decreases , the output oscillator frequency of the key fob shifts a little ... this is how the car works out the battery is on the way out.
    Do you have any links about that because I can't see why that would happen or work. I would like to read about it.

  8. #8
    Join Date
    Jan 2011
    Location
    uk
    Posts
    774
    Quote Originally Posted by Mjolinor View Post
    Do you have any links about that because I can't see why that would happen or work. I would like to read about it.
    Can’t find any just this minute , but i’m Busy chatting lol

    It’s not that hard to believe that one could base the thing on a voltage controlled oscillator is it ?
    They’re just utilising an inevitable end-game to their advantage to add a ‘feature’
    The receiver can be set up to receive a range of incoming frequencies and when it gets to a certain threshold it triggers an alert
    You're so money supermarket and you don't even know it!

  9. #9
    Join Date
    Mar 2012
    Location
    Burnley
    Posts
    1,270
    Well I just powered a key up from a power supply and frequency does not drift at all. At around 2 volts it stops working.

  10. #10
    Join Date
    Mar 2012
    Location
    Burnley
    Posts
    1,270
    As far as I know the TX uses an NRZ protocol with an inherent clock. The RX locks to the frequency and it will not know if that frequency is correct or not.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •