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-   -   How to Debounce a Geo Metro VSS (

Coyote X 11-25-2008 05:13 PM

How to Debounce a Geo Metro VSS
I got my MPGuino installed and working ok but I noticed that it would jitter a lot at certain speeds. I took the speedometer apart to see how it worked and they are really pretty simple. The housing has a magnet inside that spins and makes a reed switch contact. The yellow/green wire is the vss output wire and the other side of the reed switch goes to ground. On my car there was no pullup on the vss side of the reed switch but the cruise control module was pulling it up enough to make it barely work. The automatic door lock thing has been removed from the car and that is probably what normally pulls up the yellow/green VSS wire to 12V. On a scope the signal was pretty noisy.

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. A bit more than a normal debounce but I wanted a clean signal and without the automatic door locking thing I didn't have a clean 12V on the VSS wire. I recommend to do all Metros the same way just to make sure it has a clean signal.

On my site I finally posted a complete list of wiring diagrams for 89-01 Metros. On mine I went from a black/white wire on the same connector as the vss on the back of the gauge cluster for 12V and black/blue for ground. Optionally on the back of the cluster there are clearly marked + and - places so the resistors could be soldered to the back of the cluster as well.

That pretty much cleaned up the signal on the scope and it should work on all Metros as long as you verify the wiring. I spun the tires up to 65 and it read nice and smooth with no jitters in the display at all.

MetroMPG 11-26-2008 10:07 AM

Nice work, Coyote.

I'm surprised to read there was an auto door lock feature in the Metro. I've never seen one with power locks.

Coyote X 11-26-2008 11:00 AM

It would lock the doors at 15mph but it did not unlock them. The 93 models was the first year for it and I am pretty sure it was also the last. It was really a stupid system. I can't think of any reason any one would want their doors to lock automatically so I killed that feature and removed the solenoids a few days after I started driving the car.

I wonder if the debounce resistors could be added right at the MPGuino instead of taking the cluster apart to add them to the back of it? That would make it a whole lot easier for people to add if needed.

dcb 11-26-2008 12:11 PM

re: add to guino, probably. Was thinking how to use a pin for configurable pullup or pulldown. Do you know what firmware version your guino came with?

Coyote X 11-26-2008 02:37 PM

I am using the latest firmware, I copied it off the project page when I reloaded the AVR.

The pin could be configured to pullup 5V but that is on the wrong side of the zener and current limiting resistor so it would maybe not have that much of an effect on reducing the jitter coming off the 10-20 year old reed switches. Adding a pullup resistor from the 12V line in to the VSS and one from ground to VSS on the 7805 would probably clean up any Metro as good as doing it on the back of the cluster.

Most cars probably use a better setup than a Metro to generate the VSS pulse so I am not sure if adding it to the standard board would be a good idea. It might confuse some ECUs possibly. I guess the first step would be a standard 1K debounce resistor from VSS to ground. And if that doesn't clear up the signal then adding the 12V to VSS resistor. But so far it looks like the Metros are really the only ones having issues and adding both resistors won't hurt a Metro so it seems ok to try.

I followed the VSS wire in the diagram and it goes to the door interlock, swift automatic seat belts, cruise control, and the ECU. My car doesn't have any of those things anymore so that is why my wire was not getting the full 12V. The audiovox cc that is in it now was slightly putting some voltage out on the line and that was what was letting my MPGuino read anything but I suspect that the 12V pullup on that VSS wire actually comes from the ECU and not the door lock module like I originally thought so anyone running the stock ECU will probably only need the VSS to ground debounce resistor. But it will take a car that is a bit less modified to test out what is actually required.

go2guy 05-17-2009 03:05 PM


Originally Posted by Coyote X (Post 75007)
I guess the first step would be a standard 1K debounce resistor from VSS to ground. .

Forgive my noobiness, but where is the easiest/best place for this resistor, and just a basic 1k resistor?

MetroMPG 05-17-2009 06:41 PM

And I'll throw another question in here: dcb, did you debounce your Metro's VSS?

dcb 05-17-2009 07:12 PM

Not on the 98, pretty sure it is a reed switch. When I went to the "average the last three readings" approach it got a lot smoother, and when I added the speed updates in .75 it got smoother yet.

MetroMPG 05-17-2009 09:11 PM

Ah. OK, thanks. I'm embarrassed to say I haven't yet re-installed my Guino (I didn't bring it to the east coast for the winter) or updated the firmware. It's on the to-do list.

turbobob_01 03-05-2010 04:22 PM

Is debouncing defined as: erratic readings for instant MPG? My 95 Geo has great readings for the Tank Mileage and the Trip Mileage, but the instant mileage feature is so up and down that it is really of little use to me.

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