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PostPosted: Wed Dec 24, 2014 5:24 pm 
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Location: Dunfermline
Car Model: 11 cars
You could do what I used on my Omega B234 conversion to get speedo working..
It's what Vauxhall use for corsa's etc without ABS.. you can purchase an orange speed signal relay from gm or get one from scrapyard part number : 09185826 and connect the 5 pin connectors..:
15 - Ignition
31 - Ground
IN1 - one side of ABS sensor
IN2 - other side of ABS sensor
OUT - Speed signal to blue/red wire


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File comment: speed signal relay
relay.jpg
relay.jpg [ 87.06 KiB | Viewed 914 times ]

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PostPosted: Fri Dec 26, 2014 9:14 am 
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Car Model: 9000
Update: Rainy christmas days, so managed to find some time to test the speedometer. As it turns out the 2458 pulses per km are indeed correct for the speedo signal used by the 9k's electronics. However the speedo sensor in the gearbox actually delivers a signal with 4 times as much pulses per km. To be exactly, my speedometer needed 268 Hz to show 100km/h, which translates to 268*3600/100 = 9828pulses/km.

So what's going on here... Well the speedometer has 2 connectors. The speedo sensor is basically a magnet/coil system and produces a sine wave signal. This is connected to the 2-wire connector at the bottom. A BA6712N IC on the speedometer’s PCB processes it into a clean square wave signal and also produces a divided by 4 version of the same signal. The undivided signal (9828pulses/km) is used internally for the tachometer stepper motor, while the divided signal (2458 pulses/km) drives the other speedomotor electronics and is also fed to the centre pin of the second 3-wire connector which is the speedometer output connection. From there on the 2458p/km signal is distributed to all other electronics in the car (EDU, cruise control, trionic, clock). Wire colour is green.

So all we need is a 9828 pulses per km signal and feed that to the speedo sensor connector, the speedometer electronics will do the rest for us.

Now things start to get a bit more complicated. The ABS system produces 46 pulses per rev * 1000/1.993 revs/km = 23080 pulses per km, so we need to divide this by 2.35 to get 2458. That is not an integer number so not something a simple digital divider can do. What are the options:

Division by 2 is dead easy but results in readings that are almost 15% too high. While this might make you feel good (car seems so much faster and more economical and the whole world seems to be speeding all day :D ) I think this fault is just too large to be acceptable.

Division by 2.5 is not too hard either. That will result in speedometer readings that are 6.5% to low. The result is closer but the fault is still rather large and low readings might result in a lot of speeding tickets :mrgreen:

Best option would be to construct some circuit that actually produces the correct # of pulses per km. The old school way would be to use a frequency to voltage converter circuit to transform the ABS signal in a voltage linear with the speed and feed that control voltage to a VCO that produces the correct output frequency. Not extremely difficult but requires a more complicated circuit.
The modern way would be to use a tiny micro-controller (PIC/AVR) to basically do the same: let the controller measure the input frequency and calculate the required output frequency, and have it produce that frequency on one of it's outputs. The circuit would consist of little more than the controller, a crystal and probably some decoupling and protection circuitry. It would probably be a 30 minute programming job to write the controller software for someone who knows his way around those things, unfortunately I am not very experienced with micro-controllers at this moment. If anyone here is, please jump in?

Allow for some time, I will look into the old school option 8)

@car55: that "relay" will convert the abs signal into a square wave but will not provide frequency conversion. Could be useful though if the Teves ABS ecu does not provide an usable output signal.


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PostPosted: Fri Dec 26, 2014 10:04 am 
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Joined: Mon Apr 02, 2007 11:52 am
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Location: not in a SAAB.
Car Model: One of the first RHD 9-5s
Would be be simple and cheap to use a Rasperry Pi type device? Loads of GPIO available on it so would just need a bit of code to read the input and then output it again at the right frequency?

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PostPosted: Fri Dec 26, 2014 12:13 pm 
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Joined: Wed May 12, 2004 9:53 pm
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Location: Devon
Not too hard to do a non-integer frequency divider with a little PIC. You sound like you have some circuit design experience. If you can arrange a square input at a sensible voltage (I'll look into voltage requirements) and assuming a square output is acceptable then I could try to provide a programmed PIC for the job. I don't think you would even need a crystal - PICs (at least the small ones) can use an RC clock and that would be stable enough for this.

Raspberry Pi would be vast overkill and trying to time GPIO outputs accurately from a user-mode application on top of a heavyweight operating system such as the Pi is designed to run would be practically impossible. You'd be into writing device drivers and interrupt handlers, I think.

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PostPosted: Fri Dec 26, 2014 5:54 pm 
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Joined: Sun Jan 26, 2014 10:28 am
Posts: 153
Car Model: 9000
BillJ, that would be great.

As for input, easiest would be to use an output signal from the abs ecu. I will try to find a moment this week to hook up a scope to my ABS ecu to verify if and what signals are present on the output pins (9-12). Don't know for sure if all ABS ecu's provide those signals, it is something that was used in the 93-94 models for the TCS system so could be the later ecu's don't have this at all. My car is MY96.

But even if those signals are not usable a simple opamp circuit is all it takes to convert the abs sensor signal into a clean square wave. Might be even easier as the abs ecu connector might not have wiring installed on pin 9-12 at all. Other possibility is to use one of those speed signal relays mentioned above by car55. Anyway, providing a square wave input at 5 or 12V is no big deal. Don't know what duty-cycle to expect though so best to use only one flank of the signal to determine the frequency.

As for output signal, I found out the speedometer will accept any signal above approx 1Vtt. Sine, square, it eats it all. Duty-cycle has no influence either. So a 5V tt square wave output from a PIC will do just fine and can be hooked up straight away to the speedo sensor connector in the engine bay.

Btw keep in mind for automotive applications you need to cope with a temperature range of -30 to +50C at least, don't know how stable the RC oscillator of a PIC will be over that range. Could be use of good quality R/C is enough, could be we need a crystal anyway.


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PostPosted: Fri Dec 26, 2014 6:29 pm 
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Joined: Wed May 12, 2004 9:46 pm
Posts: 14949
Location: Mulbarton,Norfolk
Car Model: TTid's only now :-(
90000006 wrote:
Update: Rainy christmas days, so managed to find some time to test the speedometer. As it turns out the 2458 pulses per km are indeed correct for the speedo signal used by the 9k's electronics. However the speedo sensor in the gearbox actually delivers a signal with 4 times as much pulses per km. To be exactly, my speedometer needed 268 Hz to show 100km/h, which translates to 268*3600/100 = 9828pulses/km.

So what's going on here... Well the speedometer has 2 connectors. The speedo sensor is basically a magnet/coil system and produces a sine wave signal. This is connected to the 2-wire connector at the bottom. A BA6712N IC on the speedometer’s PCB processes it into a clean square wave signal and also produces a divided by 4 version of the same signal. The undivided signal (9828pulses/km) is used internally for the tachometer stepper motor, while the divided signal (2458 pulses/km) drives the other speedomotor electronics and is also fed to the centre pin of the second 3-wire connector which is the speedometer output connection. From there on the 2458p/km signal is distributed to all other electronics in the car (EDU, cruise control, trionic, clock). Wire colour is green.

So all we need is a 9828 pulses per km signal and feed that to the speedo sensor connector, the speedometer electronics will do the rest for us.

Now things start to get a bit more complicated. The ABS system produces 46 pulses per rev * 1000/1.993 revs/km = 23080 pulses per km, so we need to divide this by 2.35 to get 2458. That is not an integer number so not something a simple digital divider can do. What are the options:

Division by 2 is dead easy but results in readings that are almost 15% too high. While this might make you feel good (car seems so much faster and more economical and the whole world seems to be speeding all day :D ) I think this fault is just too large to be acceptable.

Division by 2.5 is not too hard either. That will result in speedometer readings that are 6.5% to low. The result is closer but the fault is still rather large and low readings might result in a lot of speeding tickets :mrgreen:

Best option would be to construct some circuit that actually produces the correct # of pulses per km. The old school way would be to use a frequency to voltage converter circuit to transform the ABS signal in a voltage linear with the speed and feed that control voltage to a VCO that produces the correct output frequency. Not extremely difficult but requires a more complicated circuit.
The modern way would be to use a tiny micro-controller (PIC/AVR) to basically do the same: let the controller measure the input frequency and calculate the required output frequency, and have it produce that frequency on one of it's outputs. The circuit would consist of little more than the controller, a crystal and probably some decoupling and protection circuitry. It would probably be a 30 minute programming job to write the controller software for someone who knows his way around those things, unfortunately I am not very experienced with micro-controllers at this moment. If anyone here is, please jump in?

Allow for some time, I will look into the old school option 8)

@car55: that "relay" will convert the abs signal into a square wave but will not provide frequency conversion. Could be useful though if the Teves ABS ecu does not provide an usable output signal.


This is pretty much the conversation BillJ, MarkE and myself had over on Sabscene 10 + years ago before Rasp Pi's and the like were easily available.

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PostPosted: Fri Dec 26, 2014 11:44 pm 
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Joined: Wed May 12, 2004 9:53 pm
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Location: Devon
90000006 wrote:
for automotive applications you need to cope with a temperature range of -30 to +50C at least

Strictly speaking, yes. However, if it is mounted outside the engine bay it's unlikely to need to work over that entire temperature range.

And if it's only dividing a frequency, then as long as any clock drift is over a few seconds rather than over a few milliseconds it won't matter - the same clock will be used for measuring the input as for generating the output.

The output is indeed likely to be 5V. I'll check the input requirements but as you say, it won't be hard to sort out.
Just need to drag my programmer out of the loft and have a bit of a play with it.

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PostPosted: Sat Dec 27, 2014 12:33 am 
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Joined: Thu Jun 29, 2006 10:47 am
Posts: 606
Location: Sudbury/Cambridge
Goodness me, you guys - I am well out of my depth here but mightily impressed. It seems like there is enough mileage in this discussion to keep me away from the electronics specialist's doors for a little while. I can soulder and not much more so suspect that I won't be much use in the design stage. Needless to say, with a car whose speedo is reading 0 I would be happy to provide the guinea pig!

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PostPosted: Mon Jan 12, 2015 1:52 am 
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Joined: Thu Jun 29, 2006 10:47 am
Posts: 606
Location: Sudbury/Cambridge
I have found someone who may be able to help with design and build. I don't expect to sell hundreds but would plan to market it as a plug-in solution if we could get something working. will keep you posted.

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