Thread: VSS problem
View Single Post
Old 06-13-2016, 12:00 AM   #4 (permalink)
t vago
MPGuino Supporter
t vago's Avatar
Join Date: Oct 2010
Location: Cedar Rapids, IA
Posts: 1,766

The Karen-Mobile - '05 Dodge Magnum SXT
Team Dodge
90 day: 26.72 mpg (US)

Fiat Dakota - '00 Dodge Dakota SLT RWD Quad Cab
90 day: 16.67 mpg (US)

The Red Sled - '01 Dodge Durango SLT 4WD
90 day: 16.96 mpg (US)
Thanks: 799
Thanked 681 Times in 437 Posts
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.

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?

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.
The Fiat Dakota

The Karen-mobile

The Red Sled
  Reply With Quote