False VSS triggering while not moving FIXED

[Vortec350s10]

I am finishing my MS3 install in a '88 Camaro. When the engine is not running I can spin the speedo cable with a drill and it works perfectly. I belive the way the VSS signal works on my car is this: Speedo cable turns a reflective drum inside the speedo gauge which is looked at by an optical circuit. The optical circuit consists of a VSS "buffer box" which has 3 connections. 12v power, ground, and signal output. I believe this would be a square wave signal to ground. I have the ground and signal wires from the buffer box ran directly to the MS3,through a shielded cable.
I have read about adding a resistor inline with the 12V supply, or adding a diode to the signal wire, or installing a resistor between the 12V and ground for the sensor. I'm not sure what my first step should be to try to alleviate these false readings. When idling it will periodically and randomly show speed when I'm not moving.
I have noticed during the periods of the highest false VSS spikes, the AFR signal seems to be somewhat effected as well.
I've attached a log.
I'd appreciate any help or advice!
Thanks.

[Vortec350s10]

I attached the schematic page showing the VSS if it helps.

I have the signal wire connecting to JS11 on the main board, and I built the little input circuit to protect the chip.

[Matt Cramer]

Exactly what input pin is being used for VSS input on the MS3?

[Vortec350s10]

Matt, maybe you missed my second post since we kind if posted at the same time. I'm pretty sure I have it connected to JS11. I can double check tonight but I believe that's how I have it set up.
Thanks.

[Matt Cramer]

Please post a schematic of what "little input circuit" you are using.

[Vortec350s10]

Sorry I haven't been more clear. I was replying from my phone. Here is the input circuit I have installed in the proto area. Taken from the MS3 base hardware manual.

[Vortec350s10]

Hey guys, I still haven't tried anything to see if I can eliminate the noise. What approach should I take? I'm thinking of adding a diode inline with the trigger wire, then maybe a resistor instead. I'm not in a real rush to figure this out since I won't be driving the car for a few more months, but if someone has experience with this I'd rather take their advice instead of spending a bunch of time starting up the car and trying random stuff.

[Matt Cramer]

I'm kind of stumped as to why that circuit would false trigger, as it doesn't really have much room for that sort of thing. If it were my project, I'd put an oscilloscope on the VSS signal.

[Vortec350s10]

Hmm yeah I can borrow a scope and see what it's doing. That's a good idea. Another thing I noticed that could be related is there is some electrical noise somehow passing through my blinker indicator lights in the dash cluster. I have LED's installed for the indicators and the lights will flash at a high frequency, and at a low intensity. They don't seem to mimic any pattern either. They kind of dance randomly and changing rpm or anything else doesn't seem to affect the flickering. When they are flashing brighter/faster it seems this is when the false speed input reads higher. Obviously no one knows how my car is wired, but there is something a little weird going on. Imy first thought would be a grounding issue of some sort but who knows.
I have noticed a similar thing in my other car with MS2 where I have an LED indicator wired to a programmable output on the MS2. The LED will flash dimly when the output is off to the pattern of the injector drivers. I'm guessing that's normal, but with the randomness of the LED blinker indicators it seems to be caused by something entirely different.
Does this trigger any ideas for anyone? Maybe it's a clue?

Thanks for the help Matt. I just bought your book -Performance Fuel Injection Systems. It's great.

[Vortec350s10]

I've had my MS3 running in my Camaro for about 2 months now and most everything is working awesome. I switched to EFI after 4 years with a blow through carb (single turbo). The one thing that I couldn't figure out was how to get the VSS input to stop spiking with the engine running. I've got an '88 Camaro with a 700r4 utilizing the stock VSS "buffer box" as the manual calls it which is located on the back of the gauge cluster. With it off I could spin the speedo cable with drill and the speedo input worked perfectly in TunerStudios.
I ended up just randomly trying stuff (I'm not an electrical genius) till I found a solution. I picked up some random capacitors and installed one between the actual wire connected to JS11 (my VSS input) and ground on the proto board. The VSS signal actually has a quick 12V spike with it as it switched between gnd and open. It actually never pulls the voltage to ground when triggering. Only goes as low as about 1V. I also switched the pull up resistor from a 2.2K to a 4.6K to try to help the signal. I tried this first and it made no difference.
Here's what I found with the capacitors:

10uf - VSS had no spiking while stopped but only worked under 20mph.

22uf - VSS had no spiking while stopped but only worked up under 8mph

561pf - VSS had no spiking while stopped and works up to at least 130 mph. It spiked once on me when I was backing up slowly, but that was it. So far I've only gone on one test drive but it seems good. If I do see any spikes, I will try a little bigger capacitor to increase the filtering.

Hope this helps someone.