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Old 06-12-2016, 08:39 AM   #1 (permalink)
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VSS problem

Hi

This is not the first time i try to solve this.. had given up, but now saw the MPGuino in the drawer again..

In my first car, a Alfa romeo 155 it worked flawless. Now in my Alfa romeo 75 I cant get a good VSS signal?.

Up until 20-40km/h MPGuino actually shows the speed, but then goes the oppesit way down to Zero beyond that speed??.

----

This is from the lead going directly into the MPGuino, .. I actually have no clue how to operate this handheld device, kinda blindly push buttons.. just, i can see that there actually is a signal, and that i had to put it at "5v" to get it to best fit the screen.



Any ideas?. I have seen many schematics of ways to alter the signal and etc.. But is that something I should try?.
Is the signal too strong?.


A little info: The Transducer speed sensor sits in the gearbox (gearbox in rear of car), and goes to a little box under the backseat, pulse generator/amplifier and then to the speedometer itself.

Update: Have tried to run a shielded cable directly from the pulse generator to the MPGuino, same happens.
And run test procedures on the pulse generator and transducer, all seemingly ok.

Actually the signal seems up to 12v strong in pulses.


Last edited by mirkwood; 06-12-2016 at 10:40 AM..
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Old 06-12-2016, 03:43 PM   #2 (permalink)
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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?
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Old 06-12-2016, 06:11 PM   #3 (permalink)
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Quote:
Originally Posted by t vago View Post
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?
Thanks for reply
Around 50-60km/h /// 31-37mp/h I believe i did at that momment.

Yeah, otherwise why would it seem fine and then drop?.
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.
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Old 06-13-2016, 12:00 AM   #4 (permalink)
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Quote:
Originally Posted by mirkwood View Post
Thanks for reply
Around 50-60km/h /// 31-37mp/h I believe i did at that momment.
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.

Quote:
Originally Posted by mirkwood View Post
Yeah, otherwise why would it seem fine and then drop?.
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?

Quote:
Originally Posted by mirkwood View Post
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.
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.
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Old 06-13-2016, 04:26 AM   #5 (permalink)
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Quote:
Originally Posted by t vago View Post
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.
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


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.

Last edited by mirkwood; 06-13-2016 at 07:40 AM..
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Old 06-13-2016, 09:44 AM   #6 (permalink)
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Quote:
Originally Posted by mirkwood View Post
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
26280 is really, really high. You will want to place a frequency divider between your VSS signal output and the MPGuino.

Something like this:



Quote:
Originally Posted by mirkwood View Post
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.
Yah, they're the same. I was going off an early MPGuino code version. What version of MPGuino code do you have?
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Old 06-13-2016, 12:35 PM   #7 (permalink)
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Quote:
Originally Posted by t vago View Post
26280 is really, really high. You will want to place a frequency divider between your VSS signal output and the MPGuino.

Something like this:





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

Hehe yeah okay, really really high.

Yeah, I just noticed this guy had similar problem VSS limit
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.

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 .
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Old 06-13-2016, 11:28 PM   #8 (permalink)
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Quote:
Originally Posted by mirkwood View Post
Hehe yeah okay, really really high.

Yeah, I just noticed this guy had similar problem VSS limit
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.
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

Quote:
Originally Posted by mirkwood View Post
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 .
No problem.

The MPGuino version can be found on the screen, for approximately 2 seconds after it is first powered up.
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Old 06-14-2016, 07:56 AM   #9 (permalink)
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Quote:
Originally Posted by t vago View Post
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.

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?.




Hmm.. didn't show code version on startup for some reason, tried twice.
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Old 06-14-2016, 01:04 PM   #10 (permalink)
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Quote:
Originally Posted by mirkwood View Post
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?.
The resistor golden strip can point to either pin - it does not matter with resistors.

Quote:
Originally Posted by mirkwood View Post
This drawing looks correct. It should solve your problem.

Quote:
Originally Posted by mirkwood View Post
Hmm.. didn't show code version on startup for some reason, tried twice.
That's a little strange, but as long as your MPGuino works otherwise, it doesn't matter.

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