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Speedo Transducer to ABS Sensor for VSS

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Member - Posts: 2
Member spacespace
Joined: October 18, 2014
Location: Australia
Posted: October 18, 2014 at 5:26 PM / IP Logged  
Hi guys,
I have a 2005 BA Ford Falcon . I'm in the final stages of my trans swap and now I need is to find a way of getting VSS.
My old trans is Tremec t5 which had a 3 wire Speedo transducer . The new trans is a t56 which has no provision for a speed sensor at all .
This wouldn't normally be a problem as 98% of BA, BF, FG series Falcons get speed signal from the ABS module . Sadly my vehicle is one of the very few few without ABS, haha .
I have purchased a ABS sensor and a electronic Speedo corrector kit which detects the ABS sensor and puts out a signal to my ECU and to the instrument cluster via canbus.
This is where it all gets a little complicated .
The Speedo reads erraticly and then the car goes into Limp Home Mode or it dies instantly .
I have discovered that the ABS sensors generate a tiny tiny voltage, which increases in range as the wheel speeds up - at about 10km/h it's like .02 or .2V or something IIRC .
Would amping the ABS signal to gain more adjustment on Speedo corrector kit fix this problem ?
Any help would be greatly appreciated .
- Chang
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: October 18, 2014 at 6:10 PM / IP Logged  
Not that I know anything about BA or Ford electronics, but speed sensors usually require the correct ratio - ie pulses per km. Tho they can be easily subdivided by whole numbers (eg, 2-10 or 2 - 100 etc using CD4017 chips), multiplication is more complex.
I'd assume - and by your voltage proportional to speed comment - that the ABS uses reluctor sensing. These typically use zero-crossing detection which should sense close to zero volts (ie, within 0.1 - 0.2 V of zero/GND). Probably any reluctor type ignition/distributor ignitor will do that - simply connect a pull up resistor (say 1k - 10k) to its output. The ignitor would be placed on one wheel's output.
Maybe a separate Hall (Effect) sensor could be fitted instead.   
There are also speedo cable splitters tho I'm only familiar with the common Japanese threaded speedo/gearbox types. Maybe the old sensor could thus be added...
For Jap cars many use Nissan N13 speedo sensors (10 else 20 pulses per cable rev from memory, hence usually excessive pulses that can be divided down) as the 2nd sender.   
There are also thru-transducer types - ie, plug the transducer/sender into the gearbox & the existing speedo cable plugs into the back of that.
Of course the whole set up depends on aforementioned ratios and the ECU - gearbox interactions.
Have you searched Ford forums for information or similar works?
Platinum - Posts: 1,366
Platinum spacespace
Joined: August 01, 2004
Location: Illinois, United States
Posted: October 18, 2014 at 7:00 PM / IP Logged  
Doesn't a T56 have an electrical speedometer hookup?
The electric sensor was a reluctor 2-wire type.
Member - Posts: 2
Member spacespace
Joined: October 18, 2014
Location: Australia
Posted: October 19, 2014 at 2:22 AM / IP Logged  
Hi Oldspark,
From what I have have read on the forums (AFF, Fordmods . . . ect) The speedo transducer signal is a square wave, from 0 to 8.2V. It's frequency changes with speed and the number of pulses per Km is 6250.  
The corrector module won't read any signal from the ABS sensor below speeds 40 Km/h . Once I reach 40 Km/h the Speedo goes from 0 to 70 Km/h and as I accelerate faster 80 Km/h (GPS) my Speedo reads 190Km/h - ish .
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: October 19, 2014 at 2:59 AM / IP Logged  
In that case the signal is from a "module" (ie, reluctor or Hall sensor with relevant squaring circuitry). Sorry - I read it as if the peak voltage increased...
It should be easy enough to amplify that square wave - eg thru a resistor to an NPN transistor (or N-ch FET; logic type) with open collector output (like an ignitor, it connects to GND, ie, transistor Emitter to GND) with a pull up resistor to IGN +12V. The actual circuit depends on the output to be amplified (eg, if Open Collector its pull up resistor must be small enough to turn on the transistor, otherwise a PNP tranny else FET or other circuit is required.
Make sure the following circuit does not require a 5V or similar voltage limit, tho if it's not sensing 8V it probably isn't.
But the problem could be frequency... The receiving circuit may expect a higher (or lower??) frequency and have an input filter to reject noise...
The 6250/km sounds familiar (I used to know common speedo etc rotations or pulses per km) and is ~4 pulses per wheel rotation. IMO that means some sort of divider because ABS uses probably 20 or more teeth/slots per hub. Of course that's from those I have seen and the little I know...
Make sure you find out the ratio information before you start - ratio changing circuits can probably include all the signal/voltage matching required.

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