MPGuino and power door lock annoyance

[poindexter]

I recently got my MPGuino put together and I found that my power door locks cause it to register a VSS signal if the car is off. It is really just an annoyance because when I unlock my car to get in it, it causes the current mpg to go to all 9s and I cannot read what the last trip mpg was. Other than that it works great.

I looked at the VSS voltage with my cheap digital voltmeter and I cannot tell how much the voltage is swinging from the door locks, just that it is. It seems to happen about 70% of the time I hit the locks. I can tell right away because the backlight comes on.

Any ideas? Maybe a high pass filter? Maybe a pulldown resistor on the input? I tried enabling the built in pullup 20k resistor in the code but it prevented the MPGuino from registering any VSS signals.

[wagonman76]

Does your car also automatically lock the doors once you start moving? I wonder if it is wired so any pulse from the VSS will trip the door locks. Probably not much you can do unless you get a wiring diagram for the car and can maybe put in a diode so the locks dont backfeed to the VSS. But Im not super talented with electronics so there may be another solution.

[poindexter]

Originally Posted by wagonman76

Does your car also automatically lock the doors once you start moving? I wonder if it is wired so any pulse from the VSS will trip the door locks. Probably not much you can do unless you get a wiring diagram for the car and can maybe put in a diode so the locks dont backfeed to the VSS. But Im not super talented with electronics so there may be another solution.

That is a good idea, but no they don't automatically lock.

Yeah, I'll probably look into the electrical diagrams. The diode idea sounds good. I used the electrical diagrams at serviceexpress.honda.com to hook up the MPGuino. It is only $10 for a 3-day subscription, but I failed to save the VSS circuit diagram to my hard drive, so maybe I'll look again.

[MetroMPG]

That's really interesting. I'm sure I've read at least one account of the ScanGauge doing the same thing. But you've poinpointed it to a VSS signal, where they were still in the dark.

[wagonman76]

Another thought is if the wiring for the VSS runs very close to the door lock relay, the electromagnet in the relay could be sending a pulse down the wire. Or I have seen on a lot of relay sockets, they put a diode across two terminals to stop backfeed when the relay trips. Perhaps if there is not a diode there, it might be causing some issues.