How to Debounce a Geo Metro VSS

[Coyote X]

If it is not steady with a steady throttle it probably could use the debounce resistors to get it to read smoother. You can connect the resistors on the wires before it plugs in to the mpguino. Or really anywhere that it is convenient to do.

[turbobob_01]

Ok, I keep reading this over and over... "So my solution involved a couple of 1k resistors. I ran one from switched 12V to the vss wire. And another from the VSS wire to ground.", ...but am not clear on it.

I wish I could draw you a diagram, but I am not that computer smart either. Do I connect both of these resistors in parallel directly onto the VSS wire? It makes me a bit nervous to add voltage to a circuit. I am trying my best to understand what this will do. Thanks for your patience with me.

[moorecomp]

Has anyone besides me thought that the bouncing up and down could be caused by the normal function of closed loop operation. I look at my mpguino when I am running down the road at a steady throttle setting (I don't have a cruise control) and the instant mpg's will read 36, 40, 38, 41, 35, 39,.... It kind of looks like what you would expect see with the ECU responding to the O2 sensor. I don't mind the bounce as it shows me I am running closed loop. Or not?

[Coyote X]

The mpg bouncing up and down on the instant mpg is normal. If the speed is bouncing up and down at a steady state speed then it is not reading correctly and needs debounced on a Metro. A lot of other cars won't be able to be debounced because they are not mechanical switches generating the VSS signal.

Something like my Astro has an electronic module generating the VSS and there is nothing that can be done to clean up the signal from it if it was noisy. As far as I can tell the Metro is about the only car that uses a mechanical switch to generate the VSS signal.

So to do it simply get a resistor, it doesn't have to be exactly 1k it can be 1k to 10k and still work so if you have one laying around use it. Connect one side of the resistor to the VSS wire going to the mpguino and the other side to the ground wire going to the mpguino. That will most likely be all that is needed to get the signal clean. If it is still bouncing around some then you can add a second resistor one side connected to a switched 12V source like the radio or heater blower fuse the other to the VSS wire going to the mpguino. You can't use the power wire going to the mpguino since it is 'always on' 12V.

[turbobob_01]

That is how mine acts to moorecomp. Coyotye, can you explain in electronic terms how adding that resistor cleans up the signal?

[pentium0]

I have a debounce question. With the latest version of the MPGuino, what is the preferred method to debounce a geo metro? If the double resistor method is not working well, should we consider using a capacitor instead of a resistor between ground and VSS? (see below)

What follows is what I've tried with my '96 geo metro 1.0L. For reference, I just got my MPGuino, so it is version 1.7 or something. I tapped into INJ underneath the airfilter, like someone has posted pictures of, for their '98 Metro. For VSS I went behind the glove box.

Without any debounce resistors, instant speed was bouncing around pretty bad while driving 30 MPH and up.

I tried adding a 1k resistor from ground to VSS (no additional resistors) and it wasn't getting any VSS reading at all.
I tried switching to a 10k resistor from ground to VSS and it worked again, with somewhat less bouncing.
Right now I'm doing 5.6k between VSS and Ground, and 5.6k between VSS and switched 12V. It still bounces some while driving 40MPH+.

The MPGuino is a great little device! It would be great to have a more definitive way to debounce the Metro VSS for it. More resistance? Less resistance? Keep the Ground and 12V resistance equal? etc.

Has anyone tried a Metro VSS debounce circuit with a capacitor?
It is explained at ikalogic.com/debouncing.php
If I understand the schematic right, for this design you would keep a resister between 12V and VSS, but instead of a resistor between VSS and ground, you'd use a 100nf ceramic capacitor. Any reason to not try this? Could it fry the VSS unit?

[Coyote X]

There is no VSS unit on a Metro. It is just a mechanical switch that is activated from the back of the speedometer. The VSS on a metro is only used for that idiotic door locking thing on the 93/94 and going to the computer to let it know how fast the car is going. I guess it could also be used for cruise control if someone is lucky enough to find a car with cruise. It is just switching 12v on and off on the wire. So everything connected to that wire can handle 0-12v.

So you can't really hurt it unless you just burn up the switch by grounding it out or something, even that would probably just blow a fuse. Just as an educated guess I would probably just go with a slightly higher value resistor on the ground side than the 12v side. If that didn't clean it up I would use a capacitor on the ground side with the 12v to vss resistor still connected. As these cars get older the reed switch is just going to get worse. So ultimately a new switch might need to be found and put in the cluster if it can't get cleaned up any better.

I have my cluster out right now doing some random work on my car so if I get time I will see if I can figure out where to order a new part from and see how big of a job it would be to change. From what I can remember it was a pretty simple design so it should be easy to put a new reed switch in the cluster. As these cars start passing 250k miles and 20 years that switch is probably not going to make it a lot longer than that and will need replacing to keep the mpguino working.

[dcb]

You can also experiment with the "VSS Delay ms" configuration item. It is the number of microseconds that it sits quiet before looking for another vss edge. Increasing it will reduce the top speed however, try one unit at a time, default is 2.

[pentium0]

Thank you Coyote X for the encouragement to try the capacitor, and dcb for the VSS Delay idea. I will try upping the VSS Delay on the mpguino if I need to. The capacitor seems to work!

A Successful Debouncing of 1996 Geo Metro 1.0L w/ 140,000 miles:

• 100nF (same as .1 μF in radioshack units) ceramic disc capacitor connected from VSS to ground.
• 56k resistor connected from switched 12V to VSS

Now the speed is stable at 40+ MPH. Before, using only resistors, I was still getting bouncing of the speed in 40+ MPH range.

[mjspiess]

Would a polarized cylinder style capacitor work to debounce the VSS? I have a handfull I can choose from. What voltage & uF would be recommended? I'm tired of this noisy wire! Not only is it affecting my MPGuino, but now that I have a cruise control installed, it's affecting that as well.