discomania said:

A PWM signal is however very simple to read and takes no decoding as such, it is simply a pulse, count the pulses and the software knows what to do.

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Pulse Width Modulation (PWM) is as it's name suggests, "encodes" or modulates the width of a pulse to represent the value of the input, in this case, the speed of the vehicle. That implies that the faster the vehicle goes, the wider the pulse and the slower it goes, the narrower the pulse. In such circuits, the repetition rate of the pulses usually remains the same, however some PWM circuits do vary the pulse width or the mark while the space has a pre-set width. It's just the width of the pulse varies. Counting the pulses is meaningless.

brian47 said:

Pulse Width Modulation (PWM) is as it's name suggests, "encodes" or modulates the width of a pulse to represent the value of the input, in this case, the speed of the vehicle. That implies that the faster the vehicle goes, the wider the pulse and the slower it goes, the narrower the pulse. In such circuits, the repetition rate of the pulses usually remains the same, however some PWM circuits do vary the pulse width or the mark while the space has a pre-set width. It's just the width of the pulse varies. Counting the pulses is meaningless.

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I am hearing you but we are talking about the TD5 odometer which as far as I know works on frequency, 2 points:

You can use a simple 555 timer circuit (or similar) to switch a transistor bloody fast to create the high frequency required to wind an odometer forward, dead simple switched supply, no modulation and it works fine, vary the program to reduce the switching frequency and the odometer speed drops - sadly I don't have a odometer lying about or I would post a video of this.

Secondly as you can drive a TD5 speedo using a transducer from a transfer case which only creates a pulse each time it goes round using the hall effect, yes the pulse width will vary in length depending on speed but due to the way these work I would very much doubt they can rely on the pulse width to monitor speed, the ECU in the Disco and the Defender is more or less the same and I don't see them making the Disco odometer different to the Defender and then having different control from the ECU depending on vehicle.

Now I will gladly be proven wrong as I am always keen to learn new things but I am fairly certain these work simply on pulse frequency.

Alpinejim said:

Thanks for all of the options everyone.

I think the Pi interrogating the ECU is way more complex than I need - although I admit that having an air op diff is more complex than necessary anyway. However, if you could do this, you could easily have an auto locking diff if the ECU can tell you which wheels are losing traction.

The desire for in interlock in the first place was really due to the placement of the switch which would be on the centre console between the window switches. This is easy to get to without leaning but also very easy to switch the difflock instead of the window.
The missile cover would solve that I suppose. But, how long until that gets broken?

I like the lo range interlock option so far.

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I have a "missile" switch to arm my winch before I can actually apply power to it. Despite a lot of abuse it's not fallen apart yet - but in fairness I did get it from a surplus sale when they decommissioned some missile silos so it is the real thing and wonderfully chunky.

Let's talk about that a bit cos it's interesting and i'm with Brian in this PWM thing

discomania said:

You can use a simple 555 timer circuit (or similar) to switch a transistor bloody fast to create the high frequency required to wind an odometer forward, dead simple switched supply, no modulation and it works fine, vary the program to reduce the switching frequency and the odometer speed drops - sadly I don't have a odometer lying about or I would post a video of this.

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Do you confirm that you have tried that ^^^ with a D2 odometer and it worked?

discomania said:

Secondly as you can drive a TD5 speedo using a transducer from a transfer case which only creates a pulse each time it goes round using the hall effect, yes the pulse width will vary in length depending on speed but due to the way these work I would very much doubt they can rely on the pulse width to monitor speed, the ECU in the Disco and the Defender is more or less the same and I don't see them making the Disco odometer different to the Defender and then having different control from the ECU depending on vehicle.

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That's not quite so IMO, you can't compare a D2 speedo(actually the instrument pack) with a Defender one just cos the engines are both Td5, the signal to the ECU is one thing and to the odometer/instrument pack is another, these are different between the D2 and Defender cos the D2 odometer works with digital PWM(8000pulses/mile) VSS delivered by the SLABS while the Defender's instrument with that analogue signal from the transducer, the D2 ECU gets the same PWM VSS from SLABS to pin 13 black plug while the Defender's Td5 ECU doesnt get the analogue signal from the transducer but from the instrument pack after it was converted to suitable PWM, this is not described in any document but visible in the diagrams and i presume that there's a converter within the Defender's instrument pack, if i had a Defender within reach i'd put the oscilloscope on that input to pin 13 to see but i can't

though if you confirm that you tried this with a D2 instrument pack i'll not contest that

Can I ask what kind of sensing arrangement we have in the hubs of the D2, a basket type thing with spokes that go past the speed sensor thus creating pulses?

If SLABS receives this kind of signal, why wouldn't it just pass it on to the speedo where there could be a frequency to voltage converter to power the needle?

SLABS would only have to pass the speed of the slowest wheel to show correct speed, in case some are without traction.

Or is this idea complete nonsense?

Someone will have to put a scope on pin 13 to find out what's really happening.

Alpinejim said:

If SLABS receives this kind of signal, why wouldn't it just pass it on the the speedo where there could be a frequency to voltage converter to power the needle?

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Because the D2 speedo wasnt built this way maybe to use the same VSS which is used by the engine management too as part of the addaptive strategy and that's compulsory to be PWM for accuracy... what you say is actually how the defender's speedo works based on the transducer's input