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PostPosted: Wed Aug 01, 2018 5:50 am 
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Joined: Wed Nov 25, 2015 7:17 pm
Posts: 13
Starting up an old thread, has anyone been able to locate this wire? I'm in the same boat and just about to install an Alpine Halo9 unit and thought while I've got everything apart I may as well tap into this line more as a nice to have rather than complete necessity.


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PostPosted: Wed Aug 01, 2018 6:47 am 
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Joined: Sat Oct 12, 2013 5:36 pm
Posts: 1392
Does the head unit use a vehicle speed signal for some reason? The speed sensor on the transmission feeds it’s signal to the ECU. That info is fed to the speedo for odo and speed.
What function does it serve in a head unit?


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PostPosted: Wed Aug 01, 2018 8:47 am 
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Apparently, it serves to monitor your speed when using GPS so when you enter a tunnel or other areas with no/poor GPS signal the navigation will still be able to locate you based on the speed you are doing even after you've entered the tunnel.


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PostPosted: Wed Aug 01, 2018 12:21 pm 
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Joined: Sat Oct 12, 2013 5:36 pm
Posts: 1392
That would mean the head unit has to be vehicle specific to read a known number of pulses as relative distance covered by the rear wheels. I doubt if the number of pulses in one vehicle is the same for others.

Therefore the head unit would have to have an ability to read pulses sent, compare them to GPS data in normal operation save/remember that info and then when GPS signal is lost, the head unit then switches to and applies the recorded info from it's memory for the duration of the GPS loss.

Sounds like a lot of trouble to go to just to have it, sort of, continue. The GPS receiver will lock on again asap anyway if it is any good.

You may be able to follow the colours of the speed sensor wires on the box, there are Hall effect units most likely three wires OR magnetic pulse type, two wires. Not sure which it is.


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PostPosted: Wed Aug 01, 2018 6:21 pm 
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Joined: Thu Apr 19, 2018 8:45 am
Posts: 20
Location: Canberra
The way this is normally done nowadays for dead reckoning GPS, taxi meters etc is with a separate box that reads the speed from the ECU via the CAN bus, and generates its own speed pulse based on the data it receives from the ECU.

Something like this:

https://hmbe.com.au/can-bus-interface


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PostPosted: Thu Aug 02, 2018 2:27 pm 
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Joined: Sun Dec 13, 2015 11:01 pm
Posts: 9
Location: York UK
I'm guessing the head unit is a Pioneer? I fitted an Avic headunit and got stuck looking for the speed pulse wire. Dropped by my local Isuzu dealer and first mechanic pulled the instrument cluster out at which his mate appeared and said it was actually behind the glove box. Unfortunately I was offered a cup of coffee so never saw them access it but it is in the glove box area. Sorry, not a lot of help. Don't think it particularly matters if it isn't connected.

Edit; Just re read your post and you actually mention it's an Alpine :roll:


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PostPosted: Fri Aug 03, 2018 1:30 pm 
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Joined: Wed Nov 25, 2015 7:17 pm
Posts: 13
Thanks for the replies guys. I ended up ringing around and found the Australian model of this unit doesn't have maps built-in and therefore can't monitor speed. The Australian version of this unit relies on Android Auto or Apple CarPlay for mapping. Only the yanks have the unit with maps in-built. I guess that's just saved me the hassle of installing this wire and the separate GPS module.


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PostPosted: Fri Aug 03, 2018 1:43 pm 
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Joined: Sat Oct 12, 2013 7:25 pm
Posts: 686
Location: Baldivis WA
If the head unit has a GPS, it will work out the speed from that.

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PostPosted: Sat Aug 04, 2018 2:25 pm 
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Joined: Sun Dec 13, 2015 11:01 pm
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Location: York UK
Bob wrote:
If the head unit has a GPS, it will work out the speed from that.


It can and does. I fitted an Avic F70DAB to my previous d-max and it required connection to speed sensing wire. At first it wasn't connected and although the pioneer navigation worked fine it kept knagging about the wire not been connected. Apart from tunnels I seem to think it was also required for adaptive volume control or something.


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