Originally Posted by t vago (Post 516254)

It sounds like the MPGuino is being fed a VSS signal that is too fast for it to handle.

Can you tell me what vehicle speed that above o'scope output represents?

Originally Posted by mirkwood (Post 516263)

Thanks for reply :)
Around 50-60km/h /// 31-37mp/h I believe i did at that momment.

Originally Posted by mirkwood (Post 516263)

Yeah, otherwise why would it seem fine and then drop?.

Originally Posted by mirkwood (Post 516263)

But then.. wouldn't my car be strange different than others?, if it makes a signal so fast that the MPGuino can't follow?.

I have been thinking that i could try and tap into the signal before the signal generator/amplifier, if that would make a difference.


Any suggestions for oscilloscope settings and what i should then be seeing are welcome too.

Originally Posted by t vago (Post 516269)

Glad to help. :-)

If this is so, then it would appear that your Alfa is set up to deliver around 18000 pulses per mile, or about 11200 pulses per kilometer. I have not performed a definitive analysis of the upper limit of what the MPGuino can handle, but it seems that you may have found its upper VSS limit, given its parameter settings.



This has a lot to do with digital signal theory. Specifically, your Alfa might have made a VSS signal for the MPGuino, that is higher than what is known as the Nyquist frequency. That is defined as the frequency at which something can be digitally sampled without introducing errors. That is typically 1/2 the sampling frequency.

From what you have described, it appears that the MPGuino, as programmed with default parameters, cannot sample VSS signals that are more than about 187 Hz. That would be the Nyquist frequency for the MPGuino, which means that the MPGuino sampling frequency is about 374 Hz.

What you're seeing is this:

Once your Alfa's VSS goes above 187 Hz, the MPGuino will effectively see a frequency of (374 - VSS) Hz, and that will cause your indicated speed to fall at speeds above 60 kph. At around 120 kph, you'll probably see a speed of 0.

With most cars, they will never see this problem as their VSS signals would not go much above 10000 pulses per mile. However, I am thinking that your Alfa has a VSS that is... 18000 pulses per mile? Is that right?



I think you will just need to adjust a parameter in the MPGuino itself.

There is something called "VSS Debounce" and it is set to a program default of 2. This is for purely mechanical vehicle speed sensors that use a reed switch to generate a VSS signal.

As your Alfa uses an electronically generated VSS signal, you can safely set "VSS Debounce" to zero. Do this, and tell me if your problem goes away.

Originally Posted by mirkwood (Post 516277)

I don't know if it is 18000 pulses per mile (mine also in km i think) .. hmm.. can't read the scope so well, and car technical manual doesn't state.
Someone has made a fine diagram of the signal amplifier if that would be a help to help.
EDIT did you mean what i use in the settings?, got it set at 26280 VSS, it is where it seems ok correct for low speeds.
MPGuino indicates zero speed at exactly 55km/h~ 34mp/h

Originally Posted by mirkwood (Post 516277)

Is "VSS Debounce" the same as VSS delay?, cause i don't recall seeing "VSS Debounce" in settings. Have tried both setting that to 2..1.. and Zero, and still the same.

Originally Posted by t vago (Post 516292)

26280 is really, really high. You will want to place a frequency divider between your VSS signal output and the MPGuino.

Something like this:

http://1.bp.blogspot.com/-C0HaJ6bS1i...IA/s1600/4.png



Yah, they're the same. I was going off an early MPGuino code version. What version of MPGuino code do you have?

Originally Posted by mirkwood (Post 516300)

Hehe :D yeah okay, really really high. :( :)

Yeah, I just noticed this guy had similar problem http://ecomodder.com/forum/showthrea...mit-29787.html
and he was to do a divider aswell, but no conclusion.

Hmm... is this something that can be bought somewhere pre-built?, I tried to do a search, but either i am not using the right words, or then it is rare.

If i am to solder myself then i need a really detailed plan. Soldering no problem, but schematics not my strongest side.

Originally Posted by mirkwood (Post 516300)

In another thread on a Alfa romeo site i found this, but it is an more expensive option.
Universal Speedometer Signal Interface


Where do i see the code version?, on the unit itself?.

Thanks again :).

Originally Posted by t vago (Post 516326)

You could get yourself a 4013 dual D flip-flop CMOS chip. It's a fairly basic piece of 1970s technology that should cost around $0.50 per chip. It will accept a car battery or alternator voltage with no problem.

Here's a datasheet for one -> https://www.fairchildsemi.com/datash...D/CD4013BC.pdf

Get a 14-pin DIP chip, a 14-pin DIP socket for the chip, a 0.1 uf capacitor (preferably not electrolytic), and solder jumper wires as described in the below pin assignments:

10 <-> 8 <-> 7 <-> 6 <-> 4

3 -> 2.2 kohm resistor -> 14
3 -> input Alfa VSS signal
1 -> 11
14 -> V+ (same source wire that powers the MPGuino, for instance)
7 -> ground
14 -> 0.1 uf capacitor -> 7
1 -> 11
13 -> VSS output to MPGuino
2 -> 5
12 -> 9



No problem.

The MPGuino version can be found on the screen, for approximately 2 seconds after it is first powered up.

Originally Posted by mirkwood (Post 516338)

Does this look correct?.

One question, i know the capacitor has a negative side, clearly marked.. but the resistor, to where does the golden strip point?.

Originally Posted by mirkwood (Post 516338)

Originally Posted by mirkwood (Post 516338)

Hmm.. didn't show code version on startup for some reason, tried twice.